TypeCheckAttr.cpp

//===--- TypeCheckAttr.cpp - Type Checking for Attributes -----------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
//
// This file implements semantic analysis for attributes.
//
//===----------------------------------------------------------------------===//

#include "MiscDiagnostics.h"
#include "TypeCheckAvailability.h"
#include "TypeCheckConcurrency.h"
#include "TypeCheckDistributed.h"
#include "TypeCheckObjC.h"
#include "TypeCheckType.h"
#include "TypeChecker.h"
#include "swift/AST/ASTVisitor.h"
#include "swift/AST/ClangModuleLoader.h"
#include "swift/AST/Decl.h"
#include "swift/AST/DiagnosticsParse.h"
#include "swift/AST/DiagnosticsSema.h"
#include "swift/AST/Effects.h"
#include "swift/AST/GenericEnvironment.h"
#include "swift/AST/ImportCache.h"
#include "swift/AST/ModuleNameLookup.h"
#include "swift/AST/NameLookup.h"
#include "swift/AST/NameLookupRequests.h"
#include "swift/AST/ParameterList.h"
#include "swift/AST/PropertyWrappers.h"
#include "swift/AST/SourceFile.h"
#include "swift/AST/StorageImpl.h"
#include "swift/AST/SwiftNameTranslation.h"
#include "swift/AST/TypeCheckRequests.h"
#include "swift/AST/Types.h"
#include "swift/Parse/Lexer.h"
#include "swift/Parse/Parser.h"
#include "swift/Sema/IDETypeChecking.h"
#include "clang/Basic/CharInfo.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/Support/Debug.h"

using namespace swift;

namespace {
/// This visits each attribute on a decl.  The visitor should return true if
/// the attribute is invalid and should be marked as such.
class AttributeChecker : public AttributeVisitor<AttributeChecker> {
  ASTContext &Ctx;
  Decl *D;

public:
  AttributeChecker(Decl *D) : Ctx(D->getASTContext()), D(D) {}

  /// This emits a diagnostic with a fixit to remove the attribute.
  template<typename ...ArgTypes>
  InFlightDiagnostic diagnoseAndRemoveAttr(DeclAttribute *attr,
                                           ArgTypes &&...Args) {
    return swift::diagnoseAndRemoveAttr(D, attr,
                                        std::forward<ArgTypes>(Args)...);
  }

  template <typename... ArgTypes>
  InFlightDiagnostic diagnose(ArgTypes &&... Args) const {
    return Ctx.Diags.diagnose(std::forward<ArgTypes>(Args)...);
  }

  /// Deleting this ensures that all attributes are covered by the visitor
  /// below.
  bool visitDeclAttribute(DeclAttribute *A) = delete;

#define IGNORED_ATTR(X) void visit##X##Attr(X##Attr *) {}
  IGNORED_ATTR(AlwaysEmitIntoClient)
  IGNORED_ATTR(HasInitialValue)
  IGNORED_ATTR(ClangImporterSynthesizedType)
  IGNORED_ATTR(Convenience)
  IGNORED_ATTR(Effects)
  IGNORED_ATTR(Exported)
  IGNORED_ATTR(ForbidSerializingReference)
  IGNORED_ATTR(HasStorage)
  IGNORED_ATTR(HasMissingDesignatedInitializers)
  IGNORED_ATTR(InheritsConvenienceInitializers)
  IGNORED_ATTR(Inline)
  IGNORED_ATTR(ObjCBridged)
  IGNORED_ATTR(ObjCNonLazyRealization)
  IGNORED_ATTR(ObjCRuntimeName)
  IGNORED_ATTR(RawDocComment)
  IGNORED_ATTR(RequiresStoredPropertyInits)
  IGNORED_ATTR(RestatedObjCConformance)
  IGNORED_ATTR(Semantics)
  IGNORED_ATTR(NoLocks)
  IGNORED_ATTR(NoAllocation)
  IGNORED_ATTR(EmitAssemblyVisionRemarks)
  IGNORED_ATTR(ShowInInterface)
  IGNORED_ATTR(SILGenName)
  IGNORED_ATTR(StaticInitializeObjCMetadata)
  IGNORED_ATTR(SynthesizedProtocol)
  IGNORED_ATTR(Testable)
  IGNORED_ATTR(WeakLinked)
  IGNORED_ATTR(PrivateImport)
  IGNORED_ATTR(DisfavoredOverload)
  IGNORED_ATTR(ProjectedValueProperty)
  IGNORED_ATTR(ReferenceOwnership)
  IGNORED_ATTR(OriginallyDefinedIn)
  IGNORED_ATTR(NoDerivative)
  IGNORED_ATTR(SpecializeExtension)
  IGNORED_ATTR(Sendable)
  IGNORED_ATTR(NonSendable)
  IGNORED_ATTR(AtRethrows)
  IGNORED_ATTR(AtReasync)
  IGNORED_ATTR(ImplicitSelfCapture)
  IGNORED_ATTR(InheritActorContext)
  IGNORED_ATTR(Isolated)
  IGNORED_ATTR(PredatesConcurrency)
#undef IGNORED_ATTR

  void visitAlignmentAttr(AlignmentAttr *attr) {
    // Alignment must be a power of two.
    auto value = attr->getValue();
    if (value == 0 || (value & (value - 1)) != 0)
      diagnose(attr->getLocation(), diag::alignment_not_power_of_two);
  }

  void visitBorrowedAttr(BorrowedAttr *attr) {
    // These criteria are the same preconditions laid out by
    // AbstractStorageDecl::requiresOpaqueModifyCoroutine().

    assert(!D->hasClangNode() && "@_borrowed on imported declaration?");

    if (D->getAttrs().hasAttribute<DynamicAttr>()) {
      diagnose(attr->getLocation(), diag::borrowed_with_objc_dynamic,
               D->getDescriptiveKind())
        .fixItRemove(attr->getRange());
      D->getAttrs().removeAttribute(attr);
      return;
    }

    auto dc = D->getDeclContext();
    auto protoDecl = dyn_cast<ProtocolDecl>(dc);
    if (protoDecl && protoDecl->isObjC()) {
      diagnose(attr->getLocation(), diag::borrowed_on_objc_protocol_requirement,
               D->getDescriptiveKind())
        .fixItRemove(attr->getRange());
      D->getAttrs().removeAttribute(attr);
      return;
    }
  }

  void visitTransparentAttr(TransparentAttr *attr);
  void visitMutationAttr(DeclAttribute *attr);
  void visitMutatingAttr(MutatingAttr *attr) { visitMutationAttr(attr); }
  void visitNonMutatingAttr(NonMutatingAttr *attr) { visitMutationAttr(attr); }
  void visitConsumingAttr(ConsumingAttr *attr) { visitMutationAttr(attr); }
  void visitDynamicAttr(DynamicAttr *attr);

  void visitIndirectAttr(IndirectAttr *attr) {
    if (auto caseDecl = dyn_cast<EnumElementDecl>(D)) {
      // An indirect case should have a payload.
      if (!caseDecl->hasAssociatedValues())
        diagnose(attr->getLocation(), diag::indirect_case_without_payload,
                 caseDecl->getBaseIdentifier());
      // If the enum is already indirect, its cases don't need to be.
      else if (caseDecl->getParentEnum()->getAttrs()
                 .hasAttribute<IndirectAttr>())
        diagnose(attr->getLocation(), diag::indirect_case_in_indirect_enum);
    }
  }

  void visitWarnUnqualifiedAccessAttr(WarnUnqualifiedAccessAttr *attr) {
    if (!D->getDeclContext()->isTypeContext()) {
      diagnoseAndRemoveAttr(attr, diag::attr_methods_only, attr);
    }
  }

  void visitFinalAttr(FinalAttr *attr);
  void visitCompileTimeConstAttr(CompileTimeConstAttr *attr) {}
  void visitIBActionAttr(IBActionAttr *attr);
  void visitIBSegueActionAttr(IBSegueActionAttr *attr);
  void visitLazyAttr(LazyAttr *attr);
  void visitIBDesignableAttr(IBDesignableAttr *attr);
  void visitIBInspectableAttr(IBInspectableAttr *attr);
  void visitGKInspectableAttr(GKInspectableAttr *attr);
  void visitIBOutletAttr(IBOutletAttr *attr);
  void visitLLDBDebuggerFunctionAttr(LLDBDebuggerFunctionAttr *attr);
  void visitNSManagedAttr(NSManagedAttr *attr);
  void visitOverrideAttr(OverrideAttr *attr);
  void visitNonOverrideAttr(NonOverrideAttr *attr);
  void visitAccessControlAttr(AccessControlAttr *attr);
  void visitSetterAccessAttr(SetterAccessAttr *attr);
  void visitSPIAccessControlAttr(SPIAccessControlAttr *attr);
  bool visitAbstractAccessControlAttr(AbstractAccessControlAttr *attr);

  void visitObjCAttr(ObjCAttr *attr);
  void visitNonObjCAttr(NonObjCAttr *attr);
  void visitObjCMembersAttr(ObjCMembersAttr *attr);

  void visitOptionalAttr(OptionalAttr *attr);

  void visitAvailableAttr(AvailableAttr *attr);

  void visitCDeclAttr(CDeclAttr *attr);

  void visitDynamicCallableAttr(DynamicCallableAttr *attr);

  void visitDynamicMemberLookupAttr(DynamicMemberLookupAttr *attr);

  void visitNSCopyingAttr(NSCopyingAttr *attr);
  void visitRequiredAttr(RequiredAttr *attr);
  void visitRethrowsAttr(RethrowsAttr *attr);

  void checkApplicationMainAttribute(DeclAttribute *attr,
                                     Identifier Id_ApplicationDelegate,
                                     Identifier Id_Kit,
                                     Identifier Id_ApplicationMain);

  void visitNSApplicationMainAttr(NSApplicationMainAttr *attr);
  void visitUIApplicationMainAttr(UIApplicationMainAttr *attr);
  void visitMainTypeAttr(MainTypeAttr *attr);

  void visitUnsafeNoObjCTaggedPointerAttr(UnsafeNoObjCTaggedPointerAttr *attr);
  void visitSwiftNativeObjCRuntimeBaseAttr(
                                         SwiftNativeObjCRuntimeBaseAttr *attr);

  void checkOperatorAttribute(DeclAttribute *attr);

  void visitInfixAttr(InfixAttr *attr) { checkOperatorAttribute(attr); }
  void visitPostfixAttr(PostfixAttr *attr) { checkOperatorAttribute(attr); }
  void visitPrefixAttr(PrefixAttr *attr) { checkOperatorAttribute(attr); }

  void visitSpecializeAttr(SpecializeAttr *attr);

  void visitFixedLayoutAttr(FixedLayoutAttr *attr);
  void visitUsableFromInlineAttr(UsableFromInlineAttr *attr);
  void visitInlinableAttr(InlinableAttr *attr);
  void visitOptimizeAttr(OptimizeAttr *attr);

  void visitDiscardableResultAttr(DiscardableResultAttr *attr);
  void visitDynamicReplacementAttr(DynamicReplacementAttr *attr);
  void visitTypeEraserAttr(TypeEraserAttr *attr);
  void visitImplementsAttr(ImplementsAttr *attr);
  void visitTypeSequenceAttr(TypeSequenceAttr *attr);

  void visitFrozenAttr(FrozenAttr *attr);

  void visitCustomAttr(CustomAttr *attr);
  void visitPropertyWrapperAttr(PropertyWrapperAttr *attr);
  void visitResultBuilderAttr(ResultBuilderAttr *attr);

  void visitImplementationOnlyAttr(ImplementationOnlyAttr *attr);
  void visitNonEphemeralAttr(NonEphemeralAttr *attr);
  void checkOriginalDefinedInAttrs(Decl *D, ArrayRef<OriginallyDefinedInAttr*> Attrs);

  void visitDifferentiableAttr(DifferentiableAttr *attr);
  void visitDerivativeAttr(DerivativeAttr *attr);
  void visitTransposeAttr(TransposeAttr *attr);

  void visitActorAttr(ActorAttr *attr);
  void visitDistributedActorAttr(DistributedActorAttr *attr);
  void visitGlobalActorAttr(GlobalActorAttr *attr);
  void visitAsyncAttr(AsyncAttr *attr);
  void visitMarkerAttr(MarkerAttr *attr);

  void visitReasyncAttr(ReasyncAttr *attr);
  void visitNonisolatedAttr(NonisolatedAttr *attr);

  void visitNoImplicitCopyAttr(NoImplicitCopyAttr *attr);

  void visitUnavailableFromAsyncAttr(UnavailableFromAsyncAttr *attr);
};

} // end anonymous namespace

void AttributeChecker::visitNoImplicitCopyAttr(NoImplicitCopyAttr *attr) {
  // Only allow for this attribute to be used when experimental move only is
  // enabled.
  if (!D->getASTContext().LangOpts.EnableExperimentalMoveOnly) {
    auto error =
        diag::experimental_moveonly_feature_can_only_be_used_when_enabled;
    diagnoseAndRemoveAttr(attr, error);
    return;
  }

  auto *dc = D->getDeclContext();

  // If we have a param decl that is marked as no implicit copy, change our
  // default specifier to be owned.
  if (auto *paramDecl = dyn_cast<ParamDecl>(D)) {
    // We only handle non-lvalue arguments today.
    if (paramDecl->getSpecifier() == ParamDecl::Specifier::InOut) {
      auto error = diag::noimplicitcopy_attr_valid_only_on_local_let_params;
      diagnoseAndRemoveAttr(attr, error);
      return;
    }
    return;
  }

  auto *vd = dyn_cast<VarDecl>(D);
  if (!vd) {
    auto error = diag::noimplicitcopy_attr_valid_only_on_local_let_params;
    diagnoseAndRemoveAttr(attr, error);
    return;
  }

  // If we have a 'var' instead of a 'let', bail. We only support on local
  // lets.
  if (!vd->isLet()) {
    auto error = diag::noimplicitcopy_attr_valid_only_on_local_let_params;
    diagnoseAndRemoveAttr(attr, error);
    return;
  }

  // We only support local lets.
  if (!dc->isLocalContext()) {
    auto error = diag::noimplicitcopy_attr_valid_only_on_local_let_params;
    diagnoseAndRemoveAttr(attr, error);
    return;
  }

  // We do not support static vars either yet.
  if (dc->isTypeContext() && vd->isStatic()) {
    auto error = diag::noimplicitcopy_attr_valid_only_on_local_let_params;
    diagnoseAndRemoveAttr(attr, error);
    return;
  }
}

void AttributeChecker::visitTransparentAttr(TransparentAttr *attr) {
  DeclContext *dc = D->getDeclContext();
  // Protocol declarations cannot be transparent.
  if (isa<ProtocolDecl>(dc))
    diagnoseAndRemoveAttr(attr, diag::transparent_in_protocols_not_supported);
  // Class declarations cannot be transparent.
  if (isa<ClassDecl>(dc)) {
    
    // @transparent is always ok on implicitly generated accessors: they can
    // be dispatched (even in classes) when the references are within the
    // class themself.
    if (!(isa<AccessorDecl>(D) && D->isImplicit()))
      diagnoseAndRemoveAttr(attr, diag::transparent_in_classes_not_supported);
  }

  if (auto *VD = dyn_cast<VarDecl>(D)) {
    // Stored properties and variables can't be transparent.
    if (VD->hasStorage())
      diagnoseAndRemoveAttr(attr, diag::attribute_invalid_on_stored_property,
                            attr);
  }
}

void AttributeChecker::visitMutationAttr(DeclAttribute *attr) {
  FuncDecl *FD = cast<FuncDecl>(D);

  SelfAccessKind attrModifier;
  switch (attr->getKind()) {
  case DeclAttrKind::DAK_Consuming:
    attrModifier = SelfAccessKind::Consuming;
    break;
  case DeclAttrKind::DAK_Mutating:
    attrModifier = SelfAccessKind::Mutating;
    break;
  case DeclAttrKind::DAK_NonMutating:
    attrModifier = SelfAccessKind::NonMutating;
    break;
  default:
    llvm_unreachable("unhandled attribute kind");
  }

  auto DC = FD->getDeclContext();
  // mutation attributes may only appear in type context.
  if (auto contextTy = DC->getDeclaredInterfaceType()) {
    // 'mutating' and 'nonmutating' are not valid on types
    // with reference semantics.
    if (contextTy->hasReferenceSemantics()) {
      if (attrModifier != SelfAccessKind::Consuming) {
        diagnoseAndRemoveAttr(attr, diag::mutating_invalid_classes,
                              attrModifier, FD->getDescriptiveKind(),
                              DC->getSelfProtocolDecl() != nullptr);
      }
    }
  } else {
    diagnoseAndRemoveAttr(attr, diag::mutating_invalid_global_scope,
                          attrModifier);
  }

  // Verify we don't have more than one of mutating, nonmutating,
  // and __consuming.
  if ((FD->getAttrs().hasAttribute<MutatingAttr>() +
          FD->getAttrs().hasAttribute<NonMutatingAttr>() +
          FD->getAttrs().hasAttribute<ConsumingAttr>()) > 1) {
    if (auto *NMA = FD->getAttrs().getAttribute<NonMutatingAttr>()) {
      if (attrModifier != SelfAccessKind::NonMutating) {
        diagnoseAndRemoveAttr(NMA, diag::functions_mutating_and_not,
                              SelfAccessKind::NonMutating, attrModifier);
      }
    }

    if (auto *MUA = FD->getAttrs().getAttribute<MutatingAttr>()) {
      if (attrModifier != SelfAccessKind::Mutating) {
        diagnoseAndRemoveAttr(MUA, diag::functions_mutating_and_not,
                                SelfAccessKind::Mutating, attrModifier);
      }
    }

    if (auto *CSA = FD->getAttrs().getAttribute<ConsumingAttr>()) {
      if (attrModifier != SelfAccessKind::Consuming) {
        diagnoseAndRemoveAttr(CSA, diag::functions_mutating_and_not,
                              SelfAccessKind::Consuming, attrModifier);
      }
    }
  }

  // Verify that we don't have a static function.
  if (FD->isStatic())
    diagnoseAndRemoveAttr(attr, diag::static_functions_not_mutating);
}

void AttributeChecker::visitDynamicAttr(DynamicAttr *attr) {
  // Members cannot be both dynamic and @_transparent.
  if (D->getAttrs().hasAttribute<TransparentAttr>())
    diagnoseAndRemoveAttr(attr, diag::dynamic_with_transparent);
}

static bool
validateIBActionSignature(ASTContext &ctx, DeclAttribute *attr,
                          const FuncDecl *FD, unsigned minParameters,
                          unsigned maxParameters, bool hasVoidResult = true) {
  bool valid = true;

  auto arity = FD->getParameters()->size();
  auto resultType = FD->getResultInterfaceType();

  if (arity < minParameters || arity > maxParameters) {
    auto diagID = diag::invalid_ibaction_argument_count;
    if (minParameters == maxParameters)
      diagID = diag::invalid_ibaction_argument_count_exact;
    else if (minParameters == 0)
      diagID = diag::invalid_ibaction_argument_count_max;
    ctx.Diags.diagnose(FD, diagID, attr->getAttrName(), minParameters,
                       maxParameters);
    valid = false;
  }

  if (resultType->isVoid() != hasVoidResult) {
    ctx.Diags.diagnose(FD, diag::invalid_ibaction_result, attr->getAttrName(),
                       hasVoidResult);
    valid = false;
  }

  // We don't need to check here that parameter or return types are
  // ObjC-representable; IsObjCRequest will validate that.

  if (!valid)
    attr->setInvalid();
  return valid;
}

static bool isiOS(ASTContext &ctx) {
  return ctx.LangOpts.Target.isiOS();
}

static bool iswatchOS(ASTContext &ctx) {
  return ctx.LangOpts.Target.isWatchOS();
}

static bool isRelaxedIBAction(ASTContext &ctx) {
  return isiOS(ctx) || iswatchOS(ctx);
}

void AttributeChecker::visitIBActionAttr(IBActionAttr *attr) {
  // Only instance methods can be IBActions.
  const FuncDecl *FD = cast<FuncDecl>(D);
  if (!FD->isPotentialIBActionTarget()) {
    diagnoseAndRemoveAttr(attr, diag::invalid_ibaction_decl,
                          attr->getAttrName());
    return;
  }

  if (isRelaxedIBAction(Ctx))
    // iOS, tvOS, and watchOS allow 0-2 parameters to an @IBAction method.
    validateIBActionSignature(Ctx, attr, FD, /*minParams=*/0, /*maxParams=*/2);
  else
    // macOS allows 1 parameter to an @IBAction method.
    validateIBActionSignature(Ctx, attr, FD, /*minParams=*/1, /*maxParams=*/1);
}

void AttributeChecker::visitIBSegueActionAttr(IBSegueActionAttr *attr) {
  // Only instance methods can be IBActions.
  const FuncDecl *FD = cast<FuncDecl>(D);
  if (!FD->isPotentialIBActionTarget())
    diagnoseAndRemoveAttr(attr, diag::invalid_ibaction_decl,
                          attr->getAttrName());

  if (!validateIBActionSignature(Ctx, attr, FD,
                                 /*minParams=*/1, /*maxParams=*/3,
                                 /*hasVoidResult=*/false))
    return;

  // If the IBSegueAction method's selector belongs to one of the ObjC method
  // families (like -newDocumentSegue: or -copyScreen), it would return the
  // object at +1, but the caller would expect it to be +0 and would therefore
  // leak it.
  //
  // To prevent that, diagnose if the selector belongs to one of the method
  // families and suggest that the user change the Swift name or Obj-C selector.
  auto currentSelector = FD->getObjCSelector();

  SmallString<32> prefix("make");

  switch (currentSelector.getSelectorFamily()) {
  case ObjCSelectorFamily::None:
    // No error--exit early.
    return;

  case ObjCSelectorFamily::Alloc:
  case ObjCSelectorFamily::Init:
  case ObjCSelectorFamily::New:
    // Fix-it will replace the "alloc"/"init"/"new" in the selector with "make".
    break;

  case ObjCSelectorFamily::Copy:
    // Fix-it will replace the "copy" in the selector with "makeCopy".
    prefix += "Copy";
    break;

  case ObjCSelectorFamily::MutableCopy:
    // Fix-it will replace the "mutable" in the selector with "makeMutable".
    prefix += "Mutable";
    break;
  }

  // Emit the actual error.
  diagnose(FD, diag::ibsegueaction_objc_method_family, attr->getAttrName(),
           currentSelector);

  // The rest of this is just fix-it generation.

  /// Replaces the first word of \c oldName with the prefix, where "word" is a
  /// sequence of lowercase characters.
  auto replacingPrefix = [&](Identifier oldName) -> Identifier {
    SmallString<32> scratch = prefix;
    scratch += oldName.str().drop_while(clang::isLowercase);
    return Ctx.getIdentifier(scratch);
  };

  // Suggest changing the Swift name of the method, unless there is already an
  // explicit selector.
  if (!FD->getAttrs().hasAttribute<ObjCAttr>() ||
      !FD->getAttrs().getAttribute<ObjCAttr>()->hasName()) {
    auto newSwiftBaseName = replacingPrefix(FD->getBaseIdentifier());
    auto argumentNames = FD->getName().getArgumentNames();
    DeclName newSwiftName(Ctx, newSwiftBaseName, argumentNames);

    auto diag = diagnose(FD, diag::fixit_rename_in_swift, newSwiftName);
    fixDeclarationName(diag, FD, newSwiftName);
  }

  // Suggest changing just the selector to one with a different first piece.
  auto oldPieces = currentSelector.getSelectorPieces();
  SmallVector<Identifier, 4> newPieces(oldPieces.begin(), oldPieces.end());
  newPieces[0] = replacingPrefix(newPieces[0]);
  ObjCSelector newSelector(Ctx, currentSelector.getNumArgs(), newPieces);

  auto diag = diagnose(FD, diag::fixit_rename_in_objc, newSelector);
  fixDeclarationObjCName(diag, FD, currentSelector, newSelector);
}

void AttributeChecker::visitIBDesignableAttr(IBDesignableAttr *attr) {
  if (auto *ED = dyn_cast<ExtensionDecl>(D)) {
    if (auto nominalDecl = ED->getExtendedNominal()) {
      if (!isa<ClassDecl>(nominalDecl))
        diagnoseAndRemoveAttr(attr, diag::invalid_ibdesignable_extension);
    }
  }
}

void AttributeChecker::visitIBInspectableAttr(IBInspectableAttr *attr) {
  // Only instance properties can be 'IBInspectable'.
  auto *VD = cast<VarDecl>(D);
  if (!VD->getDeclContext()->getSelfClassDecl() || VD->isStatic())
    diagnoseAndRemoveAttr(attr, diag::invalid_ibinspectable,
                                 attr->getAttrName());
}

void AttributeChecker::visitGKInspectableAttr(GKInspectableAttr *attr) {
  // Only instance properties can be 'GKInspectable'.
  auto *VD = cast<VarDecl>(D);
  if (!VD->getDeclContext()->getSelfClassDecl() || VD->isStatic())
    diagnoseAndRemoveAttr(attr, diag::invalid_ibinspectable,
                                 attr->getAttrName());
}

static Optional<Diag<bool,Type>>
isAcceptableOutletType(Type type, bool &isArray, ASTContext &ctx) {
  if (type->isObjCExistentialType() || type->isAny())
    return None; // @objc existential types are okay

  auto nominal = type->getAnyNominal();

  if (auto classDecl = dyn_cast_or_null<ClassDecl>(nominal)) {
    if (classDecl->isObjC())
      return None; // @objc class types are okay.
    return diag::iboutlet_nonobjc_class;
  }

  if (type->isString()) {
    // String is okay because it is bridged to NSString.
    // FIXME: BridgesTypes.def is almost sufficient for this.
    return None;
  }

  if (type->isArray()) {
    // Arrays of arrays are not allowed.
    if (isArray)
      return diag::iboutlet_nonobject_type;

    isArray = true;

    // Handle Array<T>. T must be an Objective-C class or protocol.
    auto boundTy = type->castTo<BoundGenericStructType>();
    auto boundArgs = boundTy->getGenericArgs();
    assert(boundArgs.size() == 1 && "invalid Array declaration");
    Type elementTy = boundArgs.front();
    return isAcceptableOutletType(elementTy, isArray, ctx);
  }

  if (type->isExistentialType())
    return diag::iboutlet_nonobjc_protocol;

  // No other types are permitted.
  return diag::iboutlet_nonobject_type;
}


void AttributeChecker::visitIBOutletAttr(IBOutletAttr *attr) {
  // Only instance properties can be 'IBOutlet'.
  auto *VD = cast<VarDecl>(D);
  if (!VD->getDeclContext()->getSelfClassDecl() || VD->isStatic())
    diagnoseAndRemoveAttr(attr, diag::invalid_iboutlet);

  if (!VD->isSettable(nullptr)) {
    // Allow non-mutable IBOutlet properties in module interfaces,
    // as they may have been private(set)
    SourceFile *Parent = VD->getDeclContext()->getParentSourceFile();
    if (!Parent || Parent->Kind != SourceFileKind::Interface)
      diagnoseAndRemoveAttr(attr, diag::iboutlet_only_mutable);
  }

  // Verify that the field type is valid as an outlet.
  auto type = VD->getType();

  if (VD->isInvalid())
    return;

  // Look through ownership types, and optionals.
  type = type->getReferenceStorageReferent();
  bool wasOptional = false;
  if (Type underlying = type->getOptionalObjectType()) {
    type = underlying;
    wasOptional = true;
  }

  bool isArray = false;
  if (auto isError = isAcceptableOutletType(type, isArray, Ctx))
    diagnoseAndRemoveAttr(attr, isError.getValue(),
                                 /*array=*/isArray, type);

  // If the type wasn't optional, an array, or unowned, complain.
  if (!wasOptional && !isArray) {
    diagnose(attr->getLocation(), diag::iboutlet_non_optional, type);
    auto typeRange = VD->getTypeSourceRangeForDiagnostics();
    { // Only one diagnostic can be active at a time.
      auto diag = diagnose(typeRange.Start, diag::note_make_optional,
                           OptionalType::get(type));
      if (type->hasSimpleTypeRepr()) {
        diag.fixItInsertAfter(typeRange.End, "?");
      } else {
        diag.fixItInsert(typeRange.Start, "(")
          .fixItInsertAfter(typeRange.End, ")?");
      }
    }
    { // Only one diagnostic can be active at a time.
      auto diag = diagnose(typeRange.Start,
                           diag::note_make_implicitly_unwrapped_optional);
      if (type->hasSimpleTypeRepr()) {
        diag.fixItInsertAfter(typeRange.End, "!");
      } else {
        diag.fixItInsert(typeRange.Start, "(")
          .fixItInsertAfter(typeRange.End, ")!");
      }
    }
  }
}

void AttributeChecker::visitNSManagedAttr(NSManagedAttr *attr) {
  // @NSManaged only applies to instance methods and properties within a class.
  if (cast<ValueDecl>(D)->isStatic() ||
      !D->getDeclContext()->getSelfClassDecl()) {
    diagnoseAndRemoveAttr(attr, diag::attr_NSManaged_not_instance_member);
  }

  if (auto *method = dyn_cast<FuncDecl>(D)) {
    // Separate out the checks for methods.
    if (method->hasBody())
      diagnoseAndRemoveAttr(attr, diag::attr_NSManaged_method_body);

    return;
  }

  // Everything below deals with restrictions on @NSManaged properties.
  auto *VD = cast<VarDecl>(D);

  // @NSManaged properties cannot be @NSCopying
  if (auto *NSCopy = VD->getAttrs().getAttribute<NSCopyingAttr>())
    diagnoseAndRemoveAttr(NSCopy, diag::attr_NSManaged_NSCopying);

}

void AttributeChecker::
visitLLDBDebuggerFunctionAttr(LLDBDebuggerFunctionAttr *attr) {
  // This is only legal when debugger support is on.
  if (!D->getASTContext().LangOpts.DebuggerSupport)
    diagnoseAndRemoveAttr(attr, diag::attr_for_debugger_support_only);
}

void AttributeChecker::visitOverrideAttr(OverrideAttr *attr) {
  if (!isa<ClassDecl>(D->getDeclContext()) &&
      !isa<ProtocolDecl>(D->getDeclContext()) &&
      !isa<ExtensionDecl>(D->getDeclContext()))
    diagnoseAndRemoveAttr(attr, diag::override_nonclass_decl);
}

void AttributeChecker::visitNonOverrideAttr(NonOverrideAttr *attr) {
  if (auto overrideAttr = D->getAttrs().getAttribute<OverrideAttr>())
    diagnoseAndRemoveAttr(overrideAttr, diag::nonoverride_and_override_attr);

  if (!isa<ClassDecl>(D->getDeclContext()) &&
      !isa<ProtocolDecl>(D->getDeclContext()) &&
      !isa<ExtensionDecl>(D->getDeclContext())) {
    diagnoseAndRemoveAttr(attr, diag::nonoverride_wrong_decl_context);
  }
}

void AttributeChecker::visitLazyAttr(LazyAttr *attr) {
  // lazy may only be used on properties.
  auto *VD = cast<VarDecl>(D);

  auto attrs = VD->getAttrs();
  // 'lazy' is not allowed to have reference attributes
  if (auto *refAttr = attrs.getAttribute<ReferenceOwnershipAttr>())
    diagnoseAndRemoveAttr(attr, diag::lazy_not_strong, refAttr->get());

  auto varDC = VD->getDeclContext();

  // 'lazy' is not allowed on a global variable or on a static property (which
  // are already lazily initialized).
  if (VD->isStatic() || varDC->isModuleScopeContext())
    diagnoseAndRemoveAttr(attr, diag::lazy_on_already_lazy_global);
}

bool AttributeChecker::visitAbstractAccessControlAttr(
    AbstractAccessControlAttr *attr) {
  // Access control attr may only be used on value decls and extensions.
  if (!isa<ValueDecl>(D) && !isa<ExtensionDecl>(D)) {
    diagnoseAndRemoveAttr(attr, diag::invalid_decl_modifier, attr);
    return true;
  }

  if (auto extension = dyn_cast<ExtensionDecl>(D)) {
    if (!extension->getInherited().empty()) {
      diagnoseAndRemoveAttr(attr, diag::extension_access_with_conformances,
                            attr);
      return true;
    }
  }

  // And not on certain value decls.
  if (isa<DestructorDecl>(D) || isa<EnumElementDecl>(D)) {
    diagnoseAndRemoveAttr(attr, diag::invalid_decl_modifier, attr);
    return true;
  }

  // Or within protocols.
  if (isa<ProtocolDecl>(D->getDeclContext())) {
    diagnoseAndRemoveAttr(attr, diag::access_control_in_protocol, attr);
    diagnose(attr->getLocation(), diag::access_control_in_protocol_detail);
    return true;
  }

  return false;
}

void AttributeChecker::visitAccessControlAttr(AccessControlAttr *attr) {
  visitAbstractAccessControlAttr(attr);

  if (auto extension = dyn_cast<ExtensionDecl>(D)) {
    if (attr->getAccess() == AccessLevel::Open) {
      auto diag =
          diagnose(attr->getLocation(), diag::access_control_extension_open);
      diag.fixItRemove(attr->getRange());
      for (auto Member : extension->getMembers()) {
        if (auto *VD = dyn_cast<ValueDecl>(Member)) {
          if (VD->getAttrs().hasAttribute<AccessControlAttr>())
            continue;

          StringRef accessLevel = VD->isObjC() ? "open " : "public ";

          if (auto *FD = dyn_cast<FuncDecl>(VD))
            diag.fixItInsert(FD->getFuncLoc(), accessLevel);

          if (auto *VAD = dyn_cast<VarDecl>(VD))
            diag.fixItInsert(VAD->getParentPatternBinding()->getLoc(),
                             accessLevel);
        }
      }

      attr->setInvalid();
      return;
    }

    NominalTypeDecl *nominal = extension->getExtendedNominal();

    // Extension is ill-formed; suppress the attribute.
    if (!nominal) {
      attr->setInvalid();
      return;
    }

    AccessLevel typeAccess = nominal->getFormalAccess();
    if (attr->getAccess() > typeAccess) {
      diagnose(attr->getLocation(), diag::access_control_extension_more,
               typeAccess, nominal->getDescriptiveKind(), attr->getAccess())
        .fixItRemove(attr->getRange());
      attr->setInvalid();
      return;
    }

  } else if (auto extension = dyn_cast<ExtensionDecl>(D->getDeclContext())) {
    AccessLevel maxAccess = extension->getMaxAccessLevel();
    if (std::min(attr->getAccess(), AccessLevel::Public) > maxAccess) {
      // FIXME: It would be nice to say what part of the requirements actually
      // end up being problematic.
      auto diag = diagnose(attr->getLocation(),
                           diag::access_control_ext_requirement_member_more,
                           attr->getAccess(),
                           D->getDescriptiveKind(),
                           maxAccess);
      swift::fixItAccess(diag, cast<ValueDecl>(D), maxAccess);
      return;
    }

    if (auto extAttr =
        extension->getAttrs().getAttribute<AccessControlAttr>()) {
      AccessLevel defaultAccess = extension->getDefaultAccessLevel();
      if (attr->getAccess() > defaultAccess) {
        auto diag = diagnose(attr->getLocation(),
                             diag::access_control_ext_member_more,
                             attr->getAccess(),
                             extAttr->getAccess());
        // Don't try to fix this one; it's just a warning, and fixing it can
        // lead to diagnostic fights between this and "declaration must be at
        // least this accessible" checking for overrides and protocol
        // requirements.
      } else if (attr->getAccess() == defaultAccess) {
        diagnose(attr->getLocation(),
                 diag::access_control_ext_member_redundant,
                 attr->getAccess(),
                 D->getDescriptiveKind(),
                 extAttr->getAccess())
          .fixItRemove(attr->getRange());
      }
    }
  }

  if (attr->getAccess() == AccessLevel::Open) {
    auto classDecl = dyn_cast<ClassDecl>(D);
    if (!(classDecl && !classDecl->isActor()) &&
        !D->isPotentiallyOverridable() &&
        !attr->isInvalid()) {
      diagnose(attr->getLocation(), diag::access_control_open_bad_decl)
        .fixItReplace(attr->getRange(), "public");
      attr->setInvalid();
    }
  }
}

void AttributeChecker::visitSetterAccessAttr(
    SetterAccessAttr *attr) {
  auto storage = dyn_cast<AbstractStorageDecl>(D);
  if (!storage)
    diagnoseAndRemoveAttr(attr, diag::access_control_setter, attr->getAccess());

  if (visitAbstractAccessControlAttr(attr))
    return;

  if (!storage->isSettable(storage->getDeclContext())) {
    // This must stay in sync with diag::access_control_setter_read_only.
    enum {
      SK_Constant = 0,
      SK_Variable,
      SK_Property,
      SK_Subscript
    } storageKind;
    if (isa<SubscriptDecl>(storage))
      storageKind = SK_Subscript;
    else if (storage->getDeclContext()->isTypeContext())
      storageKind = SK_Property;
    else if (cast<VarDecl>(storage)->isLet())
      storageKind = SK_Constant;
    else
      storageKind = SK_Variable;
    diagnoseAndRemoveAttr(attr, diag::access_control_setter_read_only,
                          attr->getAccess(), storageKind);
  }

  auto getterAccess = cast<ValueDecl>(D)->getFormalAccess();
  if (attr->getAccess() > getterAccess) {
    // This must stay in sync with diag::access_control_setter_more.
    enum {
      SK_Variable = 0,
      SK_Property,
      SK_Subscript
    } storageKind;
    if (isa<SubscriptDecl>(D))
      storageKind = SK_Subscript;
    else if (D->getDeclContext()->isTypeContext())
      storageKind = SK_Property;
    else
      storageKind = SK_Variable;
    diagnose(attr->getLocation(), diag::access_control_setter_more,
             getterAccess, storageKind, attr->getAccess());
    attr->setInvalid();
    return;

  } else if (attr->getAccess() == getterAccess) {
    diagnose(attr->getLocation(),
             diag::access_control_setter_redundant,
             attr->getAccess(),
             D->getDescriptiveKind(),
             getterAccess)
      .fixItRemove(attr->getRange());
    return;
  }
}

void AttributeChecker::visitSPIAccessControlAttr(SPIAccessControlAttr *attr) {
  if (auto VD = dyn_cast<ValueDecl>(D)) {
    // VD must be public or open to use an @_spi attribute.
    auto declAccess = VD->getFormalAccess();
    auto DC = VD->getDeclContext()->getAsDecl();
    if (declAccess < AccessLevel::Public &&
        !VD->getAttrs().hasAttribute<UsableFromInlineAttr>() &&
        !(DC && DC->isSPI())) {
      diagnoseAndRemoveAttr(attr,
                            diag::spi_attribute_on_non_public,
                            declAccess,
                            D->getDescriptiveKind());
    }

    // Forbid stored properties marked SPI in frozen types.
    if (auto property = dyn_cast<VarDecl>(VD)) {
      if (auto NTD = dyn_cast<NominalTypeDecl>(D->getDeclContext())) {
        if (property->isLayoutExposedToClients() && !NTD->isSPI()) {
          diagnoseAndRemoveAttr(attr,
                                diag::spi_attribute_on_frozen_stored_properties,
                                VD->getName());
        }
      }
    }
  }

  if (auto ID = dyn_cast<ImportDecl>(D)) {
    auto importedModule = ID->getModule();
    if (importedModule) {
      auto path = importedModule->getModuleFilename();
      if (llvm::sys::path::extension(path) == ".swiftinterface" &&
          !path.endswith(".private.swiftinterface")) {
        // If the module was built from the public swiftinterface, it can't
        // have any SPI.
        diagnose(attr->getLocation(),
                 diag::spi_attribute_on_import_of_public_module,
                 importedModule->getName(), path);
      }
    }
  }
}

static bool checkObjCDeclContext(Decl *D) {
  DeclContext *DC = D->getDeclContext();
  if (DC->getSelfClassDecl())
    return true;
  if (auto *PD = dyn_cast<ProtocolDecl>(DC))
    if (PD->isObjC())
      return true;
  return false;
}

static void diagnoseObjCAttrWithoutFoundation(ObjCAttr *attr, Decl *decl,
                                              ObjCReason reason,
                                              DiagnosticBehavior behavior) {
  auto *SF = decl->getDeclContext()->getParentSourceFile();
  assert(SF);

  // We only care about explicitly written @objc attributes.
  if (attr->isImplicit())
    return;

  // @objc enums do not require -enable-objc-interop or Foundation be have been
  // imported.
  if (isa<EnumDecl>(decl))
    return;

  auto &ctx = SF->getASTContext();

  if (!ctx.LangOpts.EnableObjCInterop) {
    diagnoseAndRemoveAttr(decl, attr, diag::objc_interop_disabled)
      .limitBehavior(behavior);
    return;
  }

  // Don't diagnose in a SIL file.
  if (SF->Kind == SourceFileKind::SIL)
    return;

  // Don't diagnose for -disable-objc-attr-requires-foundation-module.
  if (!ctx.LangOpts.EnableObjCAttrRequiresFoundation)
    return;

  // If we have the Foundation module, @objc is okay.
  auto *foundation = ctx.getLoadedModule(ctx.Id_Foundation);
  if (foundation && ctx.getImportCache().isImportedBy(foundation, SF))
    return;

  ctx.Diags.diagnose(attr->getLocation(),
                     diag::attr_used_without_required_module, attr,
                     ctx.Id_Foundation)
    .highlight(attr->getRangeWithAt())
    .limitBehavior(behavior);
  reason.describe(decl);
}

void AttributeChecker::visitObjCAttr(ObjCAttr *attr) {
  auto reason = objCReasonForObjCAttr(attr);
  auto behavior = behaviorLimitForObjCReason(reason, Ctx);

  // Only certain decls can be ObjC.
  Optional<Diag<>> error;
  if (isa<ClassDecl>(D) ||
      isa<ProtocolDecl>(D)) {
    /* ok */
  } else if (auto Ext = dyn_cast<ExtensionDecl>(D)) {
    if (!Ext->getSelfClassDecl())
      error = diag::objc_extension_not_class;
  } else if (auto ED = dyn_cast<EnumDecl>(D)) {
    if (ED->isGenericContext())
      error = diag::objc_enum_generic;
  } else if (auto EED = dyn_cast<EnumElementDecl>(D)) {
    auto ED = EED->getParentEnum();
    if (!ED->getAttrs().hasAttribute<ObjCAttr>())
      error = diag::objc_enum_case_req_objc_enum;
    else if (attr->hasName() && EED->getParentCase()->getElements().size() > 1)
      error = diag::objc_enum_case_multi;
  } else if (auto *func = dyn_cast<FuncDecl>(D)) {
    if (!checkObjCDeclContext(D))
      error = diag::invalid_objc_decl_context;
    else if (auto accessor = dyn_cast<AccessorDecl>(func))
      if (!accessor->isGetterOrSetter())
        error = diag::objc_observing_accessor;
  } else if (isa<ConstructorDecl>(D) ||
             isa<DestructorDecl>(D) ||
             isa<SubscriptDecl>(D) ||
             isa<VarDecl>(D)) {
    if (!checkObjCDeclContext(D))
      error = diag::invalid_objc_decl_context;
    /* ok */
  } else {
    error = diag::invalid_objc_decl;
  }

  if (error) {
    diagnoseAndRemoveAttr(attr, *error).limitBehavior(behavior);
    reason.describe(D);
    return;
  }

  auto correctNameUsingNewAttr = [&](ObjCAttr *newAttr) {
    if (attr->isInvalid()) newAttr->setInvalid();
    newAttr->setImplicit(attr->isImplicit());
    newAttr->setNameImplicit(attr->isNameImplicit());
    newAttr->setAddedByAccessNote(attr->getAddedByAccessNote());
    D->getAttrs().add(newAttr);

    D->getAttrs().removeAttribute(attr);
    attr->setInvalid();
  };

  // If there is a name, check whether the kind of name is
  // appropriate.
  if (auto objcName = attr->getName()) {
    if (isa<ClassDecl>(D) || isa<ProtocolDecl>(D) || isa<VarDecl>(D)
        || isa<EnumDecl>(D) || isa<EnumElementDecl>(D)
        || isa<ExtensionDecl>(D)) {
      // Types and properties can only have nullary
      // names. Complain and recover by chopping off everything
      // after the first name.
      if (objcName->getNumArgs() > 0) {
        SourceLoc firstNameLoc, afterFirstNameLoc;
        if (!attr->getNameLocs().empty()) {
          firstNameLoc = attr->getNameLocs().front();
          afterFirstNameLoc =
            Lexer::getLocForEndOfToken(Ctx.SourceMgr, firstNameLoc);
        }
        else {
          firstNameLoc = D->getLoc();
        }
        softenIfAccessNote(D, attr,
          diagnose(firstNameLoc, diag::objc_name_req_nullary,
                   D->getDescriptiveKind())
            .fixItRemoveChars(afterFirstNameLoc, attr->getRParenLoc())
            .limitBehavior(behavior));

        correctNameUsingNewAttr(
            ObjCAttr::createNullary(Ctx, attr->AtLoc, attr->getLocation(),
                                    attr->getLParenLoc(), firstNameLoc,
                                    objcName->getSelectorPieces()[0],
                                    attr->getRParenLoc()));
      }
    } else if (isa<SubscriptDecl>(D) || isa<DestructorDecl>(D)) {
      SourceLoc diagLoc = attr->getLParenLoc();
      if (diagLoc.isInvalid())
        diagLoc = D->getLoc();
      softenIfAccessNote(D, attr,
        diagnose(diagLoc,
                 isa<SubscriptDecl>(D)
                   ? diag::objc_name_subscript
                   : diag::objc_name_deinit)
            .limitBehavior(behavior));

      correctNameUsingNewAttr(
          ObjCAttr::createUnnamed(Ctx, attr->AtLoc, attr->getLocation()));
    } else {
      auto func = cast<AbstractFunctionDecl>(D);

      // Trigger lazy loading of any imported members with the same selector.
      // This ensures we correctly diagnose selector conflicts.
      if (auto *CD = D->getDeclContext()->getSelfClassDecl()) {
        (void) CD->lookupDirect(*objcName, !func->isStatic());
      }

      // We have a function. Make sure that the number of parameters
      // matches the "number of colons" in the name.
      auto params = func->getParameters();
      unsigned numParameters = params->size();
      if (auto CD = dyn_cast<ConstructorDecl>(func))
        if (CD->isObjCZeroParameterWithLongSelector())
          numParameters = 0;  // Something like "init(foo: ())"

      // An async method, even if it is also 'throws', has
      // one additional completion handler parameter in ObjC.
      if (func->hasAsync())
        ++numParameters;
      else if (func->hasThrows()) // A throwing method has an error parameter.
        ++numParameters;

      unsigned numArgumentNames = objcName->getNumArgs();
      if (numArgumentNames != numParameters) {
        SourceLoc firstNameLoc = func->getLoc();
        if (!attr->getNameLocs().empty())
          firstNameLoc = attr->getNameLocs().front();
        softenIfAccessNote(D, attr,
          diagnose(firstNameLoc,
                   diag::objc_name_func_mismatch,
                   isa<FuncDecl>(func),
                   numArgumentNames,
                   numArgumentNames != 1,
                   numParameters,
                   numParameters != 1,
                   func->hasThrows())
              .limitBehavior(behavior));
        
        correctNameUsingNewAttr(
            ObjCAttr::createUnnamed(Ctx, attr->AtLoc, attr->Range.Start));
      }
    }
  } else if (isa<EnumElementDecl>(D)) {
    // Enum elements require names.
    diagnoseAndRemoveAttr(attr, diag::objc_enum_case_req_name)
        .limitBehavior(behavior);
    reason.describe(D);
  }

  // Diagnose an @objc attribute used without importing Foundation.
  diagnoseObjCAttrWithoutFoundation(attr, D, reason, behavior);
}

void AttributeChecker::visitNonObjCAttr(NonObjCAttr *attr) {
  // Only extensions of classes; methods, properties, subscripts
  // and constructors can be NonObjC.
  // The last three are handled automatically by generic attribute
  // validation -- for the first one, we have to check FuncDecls
  // ourselves.
  auto func = dyn_cast<FuncDecl>(D);
  if (func &&
      (isa<DestructorDecl>(func) ||
       !checkObjCDeclContext(func) ||
       (isa<AccessorDecl>(func) &&
        !cast<AccessorDecl>(func)->isGetterOrSetter()))) {
    diagnoseAndRemoveAttr(attr, diag::invalid_nonobjc_decl);
  }

  if (auto ext = dyn_cast<ExtensionDecl>(D)) {
    if (!ext->getSelfClassDecl())
      diagnoseAndRemoveAttr(attr, diag::invalid_nonobjc_extension);
  }
}

void AttributeChecker::visitObjCMembersAttr(ObjCMembersAttr *attr) {
  if (!isa<ClassDecl>(D))
    diagnoseAndRemoveAttr(attr, diag::objcmembers_attribute_nonclass);
}

void AttributeChecker::visitOptionalAttr(OptionalAttr *attr) {
  if (!isa<ProtocolDecl>(D->getDeclContext())) {
    diagnoseAndRemoveAttr(attr, diag::optional_attribute_non_protocol);
  } else if (!cast<ProtocolDecl>(D->getDeclContext())->isObjC()) {
    diagnoseAndRemoveAttr(attr, diag::optional_attribute_non_objc_protocol);
  } else if (isa<ConstructorDecl>(D)) {
    diagnoseAndRemoveAttr(attr, diag::optional_attribute_initializer);
  } else {
    auto objcAttr = D->getAttrs().getAttribute<ObjCAttr>();
    if (!objcAttr || objcAttr->isImplicit()) {
      auto diag = diagnose(attr->getLocation(),
                           diag::optional_attribute_missing_explicit_objc);
      if (auto VD = dyn_cast<ValueDecl>(D))
        diag.fixItInsert(VD->getAttributeInsertionLoc(false), "@objc ");
    }
  }
}

void TypeChecker::checkDeclAttributes(Decl *D) {
  if (auto VD = dyn_cast<ValueDecl>(D))
    TypeChecker::applyAccessNote(VD);

  AttributeChecker Checker(D);
  // We need to check all OriginallyDefinedInAttr relative to each other, so
  // collect them and check in batch later.
  llvm::SmallVector<OriginallyDefinedInAttr*, 4> ODIAttrs;
  for (auto attr : D->getAttrs()) {
    if (!attr->isValid()) continue;

    // If Attr.def says that the attribute cannot appear on this kind of
    // declaration, diagnose it and disable it.
    if (attr->canAppearOnDecl(D)) {
      if (auto *ODI = dyn_cast<OriginallyDefinedInAttr>(attr)) {
        ODIAttrs.push_back(ODI);
      } else {
        // Otherwise, check it.
        Checker.visit(attr);
      }
      continue;
    }

    // Otherwise, this attribute cannot be applied to this declaration.  If the
    // attribute is only valid on one kind of declaration (which is pretty
    // common) give a specific helpful error.
    auto PossibleDeclKinds = attr->getOptions() & DeclAttribute::OnAnyDecl;
    StringRef OnlyKind;
    switch (PossibleDeclKinds) {
    case DeclAttribute::OnAccessor:    OnlyKind = "accessor"; break;
    case DeclAttribute::OnClass:       OnlyKind = "class"; break;
    case DeclAttribute::OnConstructor: OnlyKind = "init"; break;
    case DeclAttribute::OnDestructor:  OnlyKind = "deinit"; break;
    case DeclAttribute::OnEnum:        OnlyKind = "enum"; break;
    case DeclAttribute::OnEnumCase:    OnlyKind = "case"; break;
    case DeclAttribute::OnFunc | DeclAttribute::OnAccessor: // FIXME
    case DeclAttribute::OnFunc:        OnlyKind = "func"; break;
    case DeclAttribute::OnImport:      OnlyKind = "import"; break;
    case DeclAttribute::OnModule:      OnlyKind = "module"; break;
    case DeclAttribute::OnParam:       OnlyKind = "parameter"; break;
    case DeclAttribute::OnProtocol:    OnlyKind = "protocol"; break;
    case DeclAttribute::OnStruct:      OnlyKind = "struct"; break;
    case DeclAttribute::OnSubscript:   OnlyKind = "subscript"; break;
    case DeclAttribute::OnTypeAlias:   OnlyKind = "typealias"; break;
    case DeclAttribute::OnVar:         OnlyKind = "var"; break;
    default: break;
    }

    if (!OnlyKind.empty())
      Checker.diagnoseAndRemoveAttr(attr, diag::attr_only_one_decl_kind,
                                    attr, OnlyKind);
    else if (attr->isDeclModifier())
      Checker.diagnoseAndRemoveAttr(attr, diag::invalid_decl_modifier, attr);
    else
      Checker.diagnoseAndRemoveAttr(attr, diag::invalid_decl_attribute, attr);
  }
  Checker.checkOriginalDefinedInAttrs(D, ODIAttrs);
}

/// Returns true if the given method is an valid implementation of a
/// @dynamicCallable attribute requirement. The method is given to be defined
/// as one of the following: `dynamicallyCall(withArguments:)` or
/// `dynamicallyCall(withKeywordArguments:)`.
bool swift::isValidDynamicCallableMethod(FuncDecl *decl, ModuleDecl *module,
                                         bool hasKeywordArguments) {
  auto &ctx = module->getASTContext();
  // There are two cases to check.
  // 1. `dynamicallyCall(withArguments:)`.
  //    In this case, the method is valid if the argument has type `A` where
  //    `A` conforms to `ExpressibleByArrayLiteral`.
  //    `A.ArrayLiteralElement` and the return type can be arbitrary.
  // 2. `dynamicallyCall(withKeywordArguments:)`
  //    In this case, the method is valid if the argument has type `D` where
  //    `D` conforms to `ExpressibleByDictionaryLiteral` and `D.Key` conforms to
  //    `ExpressibleByStringLiteral`.
  //    `D.Value` and the return type can be arbitrary.

  auto paramList = decl->getParameters();
  if (paramList->size() != 1 || paramList->get(0)->isVariadic()) return false;
  auto argType = paramList->get(0)->getType();

  // If non-keyword (positional) arguments, check that argument type conforms to
  // `ExpressibleByArrayLiteral`.
  if (!hasKeywordArguments) {
    auto arrayLitProto =
      ctx.getProtocol(KnownProtocolKind::ExpressibleByArrayLiteral);
    return (bool)TypeChecker::conformsToProtocol(argType, arrayLitProto, module);
  }
  // If keyword arguments, check that argument type conforms to
  // `ExpressibleByDictionaryLiteral` and that the `Key` associated type
  // conforms to `ExpressibleByStringLiteral`.
  auto stringLitProtocol =
    ctx.getProtocol(KnownProtocolKind::ExpressibleByStringLiteral);
  auto dictLitProto =
    ctx.getProtocol(KnownProtocolKind::ExpressibleByDictionaryLiteral);
  auto dictConf = TypeChecker::conformsToProtocol(argType, dictLitProto, module);
  if (dictConf.isInvalid())
    return false;
  auto keyType = dictConf.getTypeWitnessByName(argType, ctx.Id_Key);
  return (bool)TypeChecker::conformsToProtocol(keyType, stringLitProtocol, module);
}

/// Returns true if the given nominal type has a valid implementation of a
/// @dynamicCallable attribute requirement with the given argument name.
static bool hasValidDynamicCallableMethod(NominalTypeDecl *decl,
                                          Identifier argumentName,
                                          bool hasKeywordArgs) {
  auto &ctx = decl->getASTContext();
  auto declType = decl->getDeclaredType();
  DeclNameRef methodName({ ctx, ctx.Id_dynamicallyCall, { argumentName } });
  auto candidates = TypeChecker::lookupMember(decl, declType, methodName);
  if (candidates.empty()) return false;

  // Filter valid candidates.
  auto *module = decl->getParentModule();
  candidates.filter([&](LookupResultEntry entry, bool isOuter) {
    auto candidate = cast<FuncDecl>(entry.getValueDecl());
    return isValidDynamicCallableMethod(candidate, module, hasKeywordArgs);
  });

  // If there are no valid candidates, return false.
  if (candidates.size() == 0) return false;
  return true;
}

void AttributeChecker::
visitDynamicCallableAttr(DynamicCallableAttr *attr) {
  // This attribute is only allowed on nominal types.
  auto decl = cast<NominalTypeDecl>(D);
  auto type = decl->getDeclaredType();

  bool hasValidMethod = false;
  hasValidMethod |=
    hasValidDynamicCallableMethod(decl, Ctx.Id_withArguments,
                                  /*hasKeywordArgs*/ false);
  hasValidMethod |=
    hasValidDynamicCallableMethod(decl, Ctx.Id_withKeywordArguments,
                                  /*hasKeywordArgs*/ true);
  if (!hasValidMethod) {
    diagnose(attr->getLocation(), diag::invalid_dynamic_callable_type, type);
    attr->setInvalid();
  }
}

static bool hasSingleNonVariadicParam(SubscriptDecl *decl,
                                      Identifier expectedLabel,
                                      bool ignoreLabel = false) {
  auto *indices = decl->getIndices();
  if (decl->isInvalid() || indices->size() != 1)
    return false;

  auto *index = indices->get(0);
  if (index->isVariadic() || !index->hasInterfaceType())
    return false;

  if (ignoreLabel) {
    return true;
  }

  return index->getArgumentName() == expectedLabel;
}

/// Returns true if the given subscript method is an valid implementation of
/// the `subscript(dynamicMember:)` requirement for @dynamicMemberLookup.
/// The method is given to be defined as `subscript(dynamicMember:)`.
bool swift::isValidDynamicMemberLookupSubscript(SubscriptDecl *decl,
                                                ModuleDecl *module,
                                                bool ignoreLabel) {
  // It could be
  // - `subscript(dynamicMember: {Writable}KeyPath<...>)`; or
  // - `subscript(dynamicMember: String*)`
  return isValidKeyPathDynamicMemberLookup(decl, ignoreLabel) ||
         isValidStringDynamicMemberLookup(decl, module, ignoreLabel);
}

bool swift::isValidStringDynamicMemberLookup(SubscriptDecl *decl,
                                             ModuleDecl *module,
                                             bool ignoreLabel) {
  auto &ctx = decl->getASTContext();
  // There are two requirements:
  // - The subscript method has exactly one, non-variadic parameter.
  // - The parameter type conforms to `ExpressibleByStringLiteral`.
  if (!hasSingleNonVariadicParam(decl, ctx.Id_dynamicMember,
                                 ignoreLabel))
    return false;

  const auto *param = decl->getIndices()->get(0);
  auto paramType = param->getType();

  // If this is `subscript(dynamicMember: String*)`
  return TypeChecker::conformsToKnownProtocol(
      paramType, KnownProtocolKind::ExpressibleByStringLiteral, module);
}

bool swift::isValidKeyPathDynamicMemberLookup(SubscriptDecl *decl,
                                              bool ignoreLabel) {
  auto &ctx = decl->getASTContext();
  if (!hasSingleNonVariadicParam(decl, ctx.Id_dynamicMember,
                                 ignoreLabel))
    return false;

  auto paramTy = decl->getIndices()->get(0)->getInterfaceType();
  return paramTy->isKeyPath() ||
         paramTy->isWritableKeyPath() ||
         paramTy->isReferenceWritableKeyPath();
}

/// The @dynamicMemberLookup attribute is only allowed on types that have at
/// least one subscript member declared like this:
///
/// subscript<KeywordType: ExpressibleByStringLiteral, LookupValue>
///   (dynamicMember name: KeywordType) -> LookupValue { get }
///
/// ... but doesn't care about the mutating'ness of the getter/setter.
/// We just manually check the requirements here.
void AttributeChecker::
visitDynamicMemberLookupAttr(DynamicMemberLookupAttr *attr) {
  // This attribute is only allowed on nominal types.
  auto decl = cast<NominalTypeDecl>(D);
  auto type = decl->getDeclaredType();
  auto &ctx = decl->getASTContext();

  auto *module = decl->getParentModule();

  auto emitInvalidTypeDiagnostic = [&](const SourceLoc loc) {
    diagnose(loc, diag::invalid_dynamic_member_lookup_type, type);
    attr->setInvalid();
  };

  // Look up `subscript(dynamicMember:)` candidates.
  DeclNameRef subscriptName(
      { ctx, DeclBaseName::createSubscript(), { ctx.Id_dynamicMember } });
  auto candidates = TypeChecker::lookupMember(decl, type, subscriptName);

  if (!candidates.empty()) {
    // If no candidates are valid, then reject one.
    auto oneCandidate = candidates.front().getValueDecl();
    candidates.filter([&](LookupResultEntry entry, bool isOuter) -> bool {
      auto cand = cast<SubscriptDecl>(entry.getValueDecl());
      return isValidDynamicMemberLookupSubscript(cand, module);
    });

    if (candidates.empty()) {
      emitInvalidTypeDiagnostic(oneCandidate->getLoc());
    }

    return;
  }

  // If we couldn't find any candidates, it's likely because:
  //
  // 1. We don't have a subscript with `dynamicMember` label.
  // 2. We have a subscript with `dynamicMember` label, but no argument label.
  //
  // Let's do another lookup using just the base name.
  auto newCandidates =
      TypeChecker::lookupMember(decl, type, DeclNameRef::createSubscript());

  // Validate the candidates while ignoring the label.
  newCandidates.filter([&](const LookupResultEntry entry, bool isOuter) {
    auto cand = cast<SubscriptDecl>(entry.getValueDecl());
    return isValidDynamicMemberLookupSubscript(cand, module,
                                               /*ignoreLabel*/ true);
  });

  // If there were no potentially valid candidates, then throw an error.
  if (newCandidates.empty()) {
    emitInvalidTypeDiagnostic(attr->getLocation());
    return;
  }

  // For each candidate, emit a diagnostic. If we don't have an explicit
  // argument label, then emit a fix-it to suggest the user to add one.
  for (auto cand : newCandidates) {
    auto SD = cast<SubscriptDecl>(cand.getValueDecl());
    auto index = SD->getIndices()->get(0);
    diagnose(SD, diag::invalid_dynamic_member_lookup_type, type);

    // If we have something like `subscript(foo:)` then we want to insert
    // `dynamicMember` before `foo`.
    if (index->getParameterNameLoc().isValid() &&
        index->getArgumentNameLoc().isInvalid()) {
      diagnose(SD, diag::invalid_dynamic_member_subscript)
          .highlight(index->getSourceRange())
          .fixItInsert(index->getParameterNameLoc(), "dynamicMember ");
    }
  }

  attr->setInvalid();
  return;
}

/// Get the innermost enclosing declaration for a declaration.
static Decl *getEnclosingDeclForDecl(Decl *D) {
  // If the declaration is an accessor, treat its storage declaration
  // as the enclosing declaration.
  if (auto *accessor = dyn_cast<AccessorDecl>(D)) {
    return accessor->getStorage();
  }

  return D->getDeclContext()->getInnermostDeclarationDeclContext();
}

void AttributeChecker::visitAvailableAttr(AvailableAttr *attr) {
  if (Ctx.LangOpts.DisableAvailabilityChecking)
    return;

  if (auto *PD = dyn_cast<ProtocolDecl>(D->getDeclContext())) {
    if (auto *VD = dyn_cast<ValueDecl>(D)) {
      if (VD->isProtocolRequirement()) {
        if (attr->isActivePlatform(Ctx) ||
            attr->isLanguageVersionSpecific() ||
            attr->isPackageDescriptionVersionSpecific()) {
          auto versionAvailability = attr->getVersionAvailability(Ctx);
          if (attr->isUnconditionallyUnavailable() ||
              versionAvailability == AvailableVersionComparison::Obsoleted ||
              versionAvailability == AvailableVersionComparison::Unavailable) {
              if (!PD->isObjC()) {
                diagnoseAndRemoveAttr(attr, diag::unavailable_method_non_objc_protocol);
                return;
              }
            }
          }
        }
    }
  }

  if (!attr->hasPlatform() || !attr->isActivePlatform(Ctx) ||
      !attr->Introduced.hasValue()) {
    return;
  }

  // Make sure there isn't a more specific attribute we should be using instead.
  // findMostSpecificActivePlatform() is O(N), so only do this if we're checking
  // an iOS attribute while building for macCatalyst.
  if (attr->Platform == PlatformKind::iOS &&
      isPlatformActive(PlatformKind::macCatalyst, Ctx.LangOpts)) {
    if (attr != D->getAttrs().findMostSpecificActivePlatform(Ctx)) {
      return;
    }
  }

  SourceLoc attrLoc = attr->getLocation();

  // Find the innermost enclosing declaration with an availability
  // range annotation and ensure that this attribute's available version range
  // is fully contained within that declaration's range. If there is no such
  // enclosing declaration, then there is nothing to check.
  Optional<AvailabilityContext> EnclosingAnnotatedRange;
  Decl *EnclosingDecl = getEnclosingDeclForDecl(D);

  while (EnclosingDecl) {
    EnclosingAnnotatedRange =
        AvailabilityInference::annotatedAvailableRange(EnclosingDecl, Ctx);

    if (EnclosingAnnotatedRange.hasValue())
      break;

    EnclosingDecl = getEnclosingDeclForDecl(EnclosingDecl);
  }

  AvailabilityContext AttrRange{
      VersionRange::allGTE(attr->Introduced.getValue())};

  if (EnclosingDecl) {
    if (!AttrRange.isContainedIn(EnclosingAnnotatedRange.getValue())) {
      diagnose(attr->getLocation(), diag::availability_decl_more_than_enclosing);
      diagnose(EnclosingDecl->getLoc(),
               diag::availability_decl_more_than_enclosing_enclosing_here);
    }
  }

  Optional<Diag<>> MaybeNotAllowed =
      TypeChecker::diagnosticIfDeclCannotBePotentiallyUnavailable(D);
  if (MaybeNotAllowed.hasValue()) {
    AvailabilityContext DeploymentRange
        = AvailabilityContext::forDeploymentTarget(Ctx);
    if (EnclosingAnnotatedRange.hasValue())
      DeploymentRange.intersectWith(*EnclosingAnnotatedRange);

    if (!DeploymentRange.isContainedIn(AttrRange))
      diagnose(attrLoc, MaybeNotAllowed.getValue());
  }
}

void AttributeChecker::visitCDeclAttr(CDeclAttr *attr) {
  // Only top-level func decls are currently supported.
  if (D->getDeclContext()->isTypeContext())
    diagnose(attr->getLocation(), diag::cdecl_not_at_top_level);

  // The name must not be empty.
  if (attr->Name.empty())
    diagnose(attr->getLocation(), diag::cdecl_empty_name);
}

void AttributeChecker::visitUnsafeNoObjCTaggedPointerAttr(
                                          UnsafeNoObjCTaggedPointerAttr *attr) {
  // Only class protocols can have the attribute.
  auto proto = dyn_cast<ProtocolDecl>(D);
  if (!proto) {
    diagnose(attr->getLocation(),
             diag::no_objc_tagged_pointer_not_class_protocol);
    attr->setInvalid();
  }
  
  if (!proto->requiresClass()
      && !proto->getAttrs().hasAttribute<ObjCAttr>()) {
    diagnose(attr->getLocation(),
             diag::no_objc_tagged_pointer_not_class_protocol);
    attr->setInvalid();    
  }
}

void AttributeChecker::visitSwiftNativeObjCRuntimeBaseAttr(
                                         SwiftNativeObjCRuntimeBaseAttr *attr) {
  // Only root classes can have the attribute.
  auto theClass = dyn_cast<ClassDecl>(D);
  if (!theClass) {
    diagnose(attr->getLocation(),
             diag::swift_native_objc_runtime_base_not_on_root_class);
    attr->setInvalid();
    return;
  }

  if (theClass->hasSuperclass()) {
    diagnose(attr->getLocation(),
             diag::swift_native_objc_runtime_base_not_on_root_class);
    attr->setInvalid();
    return;
  }
}

void AttributeChecker::visitFinalAttr(FinalAttr *attr) {
  // Reject combining 'final' with 'open'.
  if (auto accessAttr = D->getAttrs().getAttribute<AccessControlAttr>()) {
    if (accessAttr->getAccess() == AccessLevel::Open) {
      diagnose(attr->getLocation(), diag::open_decl_cannot_be_final,
               D->getDescriptiveKind());
      return;
    }
  }

  if (isa<ClassDecl>(D))
    return;

  // 'final' only makes sense in the context of a class declaration.
  // Reject it on global functions, protocols, structs, enums, etc.
  if (!D->getDeclContext()->getSelfClassDecl()) {
    diagnose(attr->getLocation(), diag::member_cannot_be_final)
      .fixItRemove(attr->getRange());

    // Remove the attribute so child declarations are not flagged as final
    // and duplicate the error message.
    D->getAttrs().removeAttribute(attr);
    return;
  }

  // We currently only support final on var/let, func and subscript
  // declarations.
  if (!isa<VarDecl>(D) && !isa<FuncDecl>(D) && !isa<SubscriptDecl>(D)) {
    diagnose(attr->getLocation(), diag::final_not_allowed_here)
      .fixItRemove(attr->getRange());
    return;
  }

  if (auto *accessor = dyn_cast<AccessorDecl>(D)) {
    if (!attr->isImplicit()) {
      unsigned Kind = 2;
      if (auto *VD = dyn_cast<VarDecl>(accessor->getStorage()))
        Kind = VD->isLet() ? 1 : 0;
      diagnose(attr->getLocation(), diag::final_not_on_accessors, Kind)
        .fixItRemove(attr->getRange());
      return;
    }
  }
}

/// Return true if this is a builtin operator that cannot be defined in user
/// code.
static bool isBuiltinOperator(StringRef name, DeclAttribute *attr) {
  return ((isa<PrefixAttr>(attr)  && name == "&") ||   // lvalue to inout
          (isa<PostfixAttr>(attr) && name == "!") ||   // optional unwrapping
          // FIXME: Not actually a builtin operator, but should probably
          // be allowed and accounted for in Sema?
          (isa<PrefixAttr>(attr)  && name == "?") ||
          (isa<PostfixAttr>(attr) && name == "?") ||   // optional chaining
          (isa<InfixAttr>(attr)   && name == "?") ||   // ternary operator
          (isa<PostfixAttr>(attr) && name == ">") ||   // generic argument list
          (isa<PrefixAttr>(attr)  && name == "<") ||   // generic argument list
                                     name == "="  ||   // Assignment
          // FIXME: Should probably be allowed in expression position?
                                     name == "->");
}

void AttributeChecker::checkOperatorAttribute(DeclAttribute *attr) {
  // Check out the operator attributes.  They may be attached to an operator
  // declaration or a function.
  if (auto *OD = dyn_cast<OperatorDecl>(D)) {
    // Reject attempts to define builtin operators.
    if (isBuiltinOperator(OD->getName().str(), attr)) {
      diagnose(D->getStartLoc(), diag::redefining_builtin_operator,
               attr->getAttrName(), OD->getName().str());
      attr->setInvalid();
      return;
    }

    // Otherwise, the attribute is always ok on an operator.
    return;
  }

  // Operators implementations may only be defined as functions.
  auto *FD = dyn_cast<FuncDecl>(D);
  if (!FD) {
    diagnose(D->getLoc(), diag::operator_not_func);
    attr->setInvalid();
    return;
  }

  // Only functions with an operator identifier can be declared with as an
  // operator.
  if (!FD->isOperator()) {
    diagnose(D->getStartLoc(), diag::attribute_requires_operator_identifier,
             attr->getAttrName());
    attr->setInvalid();
    return;
  }

  // Reject attempts to define builtin operators.
  if (isBuiltinOperator(FD->getBaseIdentifier().str(), attr)) {
    diagnose(D->getStartLoc(), diag::redefining_builtin_operator,
             attr->getAttrName(), FD->getBaseIdentifier().str());
    attr->setInvalid();
    return;
  }

  // Otherwise, must be unary.
  if (!FD->isUnaryOperator()) {
    diagnose(attr->getLocation(), diag::attribute_requires_single_argument,
             attr->getAttrName());
    attr->setInvalid();
    return;
  }
}

void AttributeChecker::visitNSCopyingAttr(NSCopyingAttr *attr) {
  // The @NSCopying attribute is only allowed on stored properties.
  auto *VD = cast<VarDecl>(D);

  // It may only be used on class members.
  auto classDecl = D->getDeclContext()->getSelfClassDecl();
  if (!classDecl) {
    diagnose(attr->getLocation(), diag::nscopying_only_on_class_properties);
    attr->setInvalid();
    return;
  }

  if (!VD->isSettable(VD->getDeclContext())) {
    diagnose(attr->getLocation(), diag::nscopying_only_mutable);
    attr->setInvalid();
    return;
  }

  if (!VD->hasStorage()) {
    diagnose(attr->getLocation(), diag::nscopying_only_stored_property);
    attr->setInvalid();
    return;
  }

  if (VD->hasInterfaceType()) {
    if (TypeChecker::checkConformanceToNSCopying(VD).isInvalid()) {
      attr->setInvalid();
      return;
    }
  }

  assert(VD->getOverriddenDecl() == nullptr &&
         "Can't have value with storage that is an override");

  // Check the type.  It must be an [unchecked]optional, weak, a normal
  // class, AnyObject, or classbound protocol.
  // It must conform to the NSCopying protocol.

}

void AttributeChecker::checkApplicationMainAttribute(DeclAttribute *attr,
                                             Identifier Id_ApplicationDelegate,
                                             Identifier Id_Kit,
                                             Identifier Id_ApplicationMain) {
  // %select indexes for ApplicationMain diagnostics.
  enum : unsigned {
    UIApplicationMainClass,
    NSApplicationMainClass,
  };

  unsigned applicationMainKind;
  if (isa<UIApplicationMainAttr>(attr))
    applicationMainKind = UIApplicationMainClass;
  else if (isa<NSApplicationMainAttr>(attr))
    applicationMainKind = NSApplicationMainClass;
  else
    llvm_unreachable("not an ApplicationMain attr");

  auto *CD = dyn_cast<ClassDecl>(D);

  // The applicant not being a class should have been diagnosed by the early
  // checker.
  if (!CD) return;

  // The class cannot be generic.
  if (CD->isGenericContext()) {
    diagnose(attr->getLocation(),
             diag::attr_generic_ApplicationMain_not_supported,
             applicationMainKind);
    attr->setInvalid();
    return;
  }

  // @XXApplicationMain classes must conform to the XXApplicationDelegate
  // protocol.
  auto *SF = cast<SourceFile>(CD->getModuleScopeContext());
  auto &C = SF->getASTContext();

  auto KitModule = C.getLoadedModule(Id_Kit);
  ProtocolDecl *ApplicationDelegateProto = nullptr;
  if (KitModule) {
    SmallVector<ValueDecl *, 1> decls;
    namelookup::lookupInModule(KitModule, Id_ApplicationDelegate,
                               decls, NLKind::QualifiedLookup,
                               namelookup::ResolutionKind::TypesOnly,
                               SF, NL_QualifiedDefault);
    if (decls.size() == 1)
      ApplicationDelegateProto = dyn_cast<ProtocolDecl>(decls[0]);
  }

  if (!ApplicationDelegateProto ||
      !TypeChecker::conformsToProtocol(CD->getDeclaredInterfaceType(),
                                       ApplicationDelegateProto,
                                       CD->getParentModule())) {
    diagnose(attr->getLocation(),
             diag::attr_ApplicationMain_not_ApplicationDelegate,
             applicationMainKind);
    attr->setInvalid();
  }

  if (attr->isInvalid())
    return;

  // Register the class as the main class in the module. If there are multiples
  // they will be diagnosed.
  if (SF->registerMainDecl(CD, attr->getLocation()))
    attr->setInvalid();
}

void AttributeChecker::visitNSApplicationMainAttr(NSApplicationMainAttr *attr) {
  auto &C = D->getASTContext();
  checkApplicationMainAttribute(attr,
                                C.getIdentifier("NSApplicationDelegate"),
                                C.getIdentifier("AppKit"),
                                C.getIdentifier("NSApplicationMain"));
}
void AttributeChecker::visitUIApplicationMainAttr(UIApplicationMainAttr *attr) {
  auto &C = D->getASTContext();
  checkApplicationMainAttribute(attr,
                                C.getIdentifier("UIApplicationDelegate"),
                                C.getIdentifier("UIKit"),
                                C.getIdentifier("UIApplicationMain"));
}

namespace {
struct MainTypeAttrParams {
  FuncDecl *mainFunction;
  MainTypeAttr *attr;
};

}
static std::pair<BraceStmt *, bool>
synthesizeMainBody(AbstractFunctionDecl *fn, void *arg) {
  ASTContext &context = fn->getASTContext();
  MainTypeAttrParams *params = (MainTypeAttrParams *) arg;

  FuncDecl *mainFunction = params->mainFunction;
  auto location = params->attr->getLocation();
  NominalTypeDecl *nominal = fn->getDeclContext()->getSelfNominalTypeDecl();

  auto *typeExpr = TypeExpr::createImplicit(nominal->getDeclaredType(), context);

  SubstitutionMap substitutionMap;
  if (auto *environment = mainFunction->getGenericEnvironment()) {
    substitutionMap = SubstitutionMap::get(
      environment->getGenericSignature(),
      [&](SubstitutableType *type) { return nominal->getDeclaredType(); },
      LookUpConformanceInModule(nominal->getModuleContext()));
  } else {
    substitutionMap = SubstitutionMap();
  }

  auto funcDeclRef = ConcreteDeclRef(mainFunction, substitutionMap);

  auto *memberRefExpr = new (context) MemberRefExpr(
      typeExpr, SourceLoc(), funcDeclRef, DeclNameLoc(location),
      /*Implicit*/ true);
  memberRefExpr->setImplicit(true);

  auto *callExpr = CallExpr::createImplicitEmpty(context, memberRefExpr);
  callExpr->setImplicit(true);
  callExpr->setType(context.TheEmptyTupleType);

  Expr *returnedExpr;

  if (mainFunction->hasAsync()) {
    // Ensure that the concurrency module is loaded
    auto *concurrencyModule = context.getLoadedModule(context.Id_Concurrency);
    if (!concurrencyModule) {
      context.Diags.diagnose(mainFunction->getAsyncLoc(),
                             diag::async_main_no_concurrency);
      auto result = new (context) ErrorExpr(mainFunction->getSourceRange());
      ASTNode stmts[] = {result};
      auto body = BraceStmt::create(context, SourceLoc(), stmts, SourceLoc(),
                                    /*Implicit*/ true);
      return std::make_pair(body, /*typechecked*/true);
    }

    // $main() async { await main() }
    Expr *awaitExpr =
        new (context) AwaitExpr(callExpr->getLoc(), callExpr,
                                context.TheEmptyTupleType, /*implicit*/ true);
    if (mainFunction->hasThrows()) {
      // $main() async throws { try await main() }
      awaitExpr =
          new (context) TryExpr(callExpr->getLoc(), awaitExpr,
                                context.TheEmptyTupleType, /*implicit*/ true);
    }
    returnedExpr = awaitExpr;
  } else if (mainFunction->hasThrows()) {
    auto *tryExpr = new (context) TryExpr(
        callExpr->getLoc(), callExpr, context.TheEmptyTupleType, /*implicit=*/true);
    returnedExpr = tryExpr;
  } else {
    returnedExpr = callExpr;
  }

  auto *returnStmt =
      new (context) ReturnStmt(SourceLoc(), returnedExpr, /*Implicit=*/true);

  SmallVector<ASTNode, 1> stmts;
  stmts.push_back(returnStmt);
  auto *body = BraceStmt::create(context, SourceLoc(), stmts,
                                SourceLoc(), /*Implicit*/true);

  return std::make_pair(body, /*typechecked=*/false);
}

FuncDecl *
SynthesizeMainFunctionRequest::evaluate(Evaluator &evaluator,
                                        Decl *D) const {
  auto &context = D->getASTContext();

  MainTypeAttr *attr = D->getAttrs().getAttribute<MainTypeAttr>();
  if (attr == nullptr)
    return nullptr;

  auto *extension = dyn_cast<ExtensionDecl>(D);

  IterableDeclContext *iterableDeclContext;
  DeclContext *declContext;
  NominalTypeDecl *nominal;
  SourceRange braces;

  if (extension) {
    nominal = extension->getExtendedNominal();
    iterableDeclContext = extension;
    declContext = extension;
    braces = extension->getBraces();
  } else {
    nominal = dyn_cast<NominalTypeDecl>(D);
    iterableDeclContext = nominal;
    declContext = nominal;
    braces = nominal->getBraces();
  }

  assert(nominal && "Should have already recognized that the MainType decl "
                    "isn't applicable to decls other than NominalTypeDecls");
  assert(iterableDeclContext);
  assert(declContext);

  // The type cannot be generic.
  if (nominal->isGenericContext()) {
    context.Diags.diagnose(attr->getLocation(),
                           diag::attr_generic_ApplicationMain_not_supported, 2);
    attr->setInvalid();
    return nullptr;
  }

  // Create a function
  //
  //     func $main() {
  //         return MainType.main()
  //     }
  //
  // to be called as the entry point.  The advantage of setting up such a
  // function is that we get full type-checking for mainType.main() as part of
  // usual type-checking.  The alternative would be to directly call
  // mainType.main() from the entry point, and that would require fully
  // type-checking the call to mainType.main().

  auto resolution = resolveValueMember(
      *declContext, nominal->getInterfaceType(), context.Id_main);

  FuncDecl *mainFunction = nullptr;

  if (resolution.hasBestOverload()) {
    auto best = resolution.getBestOverload();
    if (auto function = dyn_cast<FuncDecl>(best)) {
      if (function->isMainTypeMainMethod()) {
        mainFunction = function;
      }
    }
  }

  if (mainFunction == nullptr) {
    SmallVector<FuncDecl *, 4> viableCandidates;

    for (auto *candidate : resolution.getMemberDecls(Viable)) {
      if (auto func = dyn_cast<FuncDecl>(candidate)) {
        if (func->isMainTypeMainMethod()) {
          viableCandidates.push_back(func);
        }
      }
    }

    if (viableCandidates.size() != 1) {
      context.Diags.diagnose(attr->getLocation(),
                             diag::attr_MainType_without_main,
                             nominal->getBaseName());
      attr->setInvalid();
      return nullptr;
    }
    mainFunction = viableCandidates[0];
  }

  auto where = ExportContext::forDeclSignature(D);
  diagnoseDeclAvailability(mainFunction, attr->getRange(), nullptr, where, None);

  auto *const func = FuncDecl::createImplicit(
      context, StaticSpellingKind::KeywordStatic,
      DeclName(context, DeclBaseName(context.Id_MainEntryPoint),
               ParameterList::createEmpty(context)),
      /*NameLoc=*/SourceLoc(),
      /*Async=*/mainFunction->hasAsync(),
      /*Throws=*/mainFunction->hasThrows(),
      /*GenericParams=*/nullptr, ParameterList::createEmpty(context),
      /*FnRetType=*/TupleType::getEmpty(context), declContext);
  func->setSynthesized(true);
  // It's never useful to provide a dynamic replacement of this function--it is
  // just a pass-through to MainType.main.
  func->setIsDynamic(false);

  auto *params = context.Allocate<MainTypeAttrParams>();
  params->mainFunction = mainFunction;
  params->attr = attr;
  func->setBodySynthesizer(synthesizeMainBody, params);

  iterableDeclContext->addMember(func);

  return func;
}

void AttributeChecker::visitMainTypeAttr(MainTypeAttr *attr) {
  auto &context = D->getASTContext();

  SourceFile *file = D->getDeclContext()->getParentSourceFile();
  assert(file);

  auto *func = evaluateOrDefault(context.evaluator,
                                 SynthesizeMainFunctionRequest{D},
                                 nullptr);

  if (!func)
    return;

  // Register the func as the main decl in the module. If there are multiples
  // they will be diagnosed.
  if (file->registerMainDecl(func, attr->getLocation()))
    attr->setInvalid();
}

/// Determine whether the given context is an extension to an Objective-C class
/// where the class is defined in the Objective-C module and the extension is
/// defined within its module.
static bool isObjCClassExtensionInOverlay(DeclContext *dc) {
  // Check whether we have an extension.
  auto ext = dyn_cast<ExtensionDecl>(dc);
  if (!ext)
    return false;

  // Find the extended class.
  auto classDecl = ext->getSelfClassDecl();
  if (!classDecl)
    return false;

  auto clangLoader = dc->getASTContext().getClangModuleLoader();
  if (!clangLoader) return false;
  return clangLoader->isInOverlayModuleForImportedModule(ext, classDecl);
}

void AttributeChecker::visitRequiredAttr(RequiredAttr *attr) {
  // The required attribute only applies to constructors.
  auto ctor = cast<ConstructorDecl>(D);
  auto parentTy = ctor->getDeclContext()->getDeclaredInterfaceType();
  if (!parentTy) {
    // Constructor outside of nominal type context; we've already complained
    // elsewhere.
    attr->setInvalid();
    return;
  }
  // Only classes can have required constructors.
  if (parentTy->getClassOrBoundGenericClass() &&
      !parentTy->getClassOrBoundGenericClass()->isActor()) {
    // The constructor must be declared within the class itself.
    // FIXME: Allow an SDK overlay to add a required initializer to a class
    // defined in Objective-C
    if (!isa<ClassDecl>(ctor->getDeclContext()) &&
        !isObjCClassExtensionInOverlay(ctor->getDeclContext())) {
      diagnose(ctor, diag::required_initializer_in_extension, parentTy)
        .highlight(attr->getLocation());
      attr->setInvalid();
      return;
    }
  } else {
    if (!parentTy->hasError()) {
      diagnose(ctor, diag::required_initializer_nonclass, parentTy)
        .highlight(attr->getLocation());
    }
    attr->setInvalid();
    return;
  }
}

void AttributeChecker::visitRethrowsAttr(RethrowsAttr *attr) {
  // Make sure the function takes a 'throws' function argument or a
  // conformance to a '@rethrows' protocol.
  auto fn = dyn_cast<AbstractFunctionDecl>(D);
  if (fn->getPolymorphicEffectKind(EffectKind::Throws)
        != PolymorphicEffectKind::Invalid) {
    return;
  }

  diagnose(attr->getLocation(), diag::rethrows_without_throwing_parameter);
  attr->setInvalid();
}

/// Ensure that the requirements provided by the @_specialize attribute
/// can be supported by the SIL EagerSpecializer pass.
static void checkSpecializeAttrRequirements(SpecializeAttr *attr,
                                            GenericSignature originalSig,
                                            GenericSignature specializedSig,
                                            ASTContext &ctx) {
  bool hadError = false;

  auto specializedReqs = specializedSig.requirementsNotSatisfiedBy(originalSig);
  for (auto specializedReq : specializedReqs) {
    if (!specializedReq.getFirstType()->is<GenericTypeParamType>()) {
      ctx.Diags.diagnose(attr->getLocation(),
                         diag::specialize_attr_only_generic_param_req);
      hadError = true;
      continue;
    }

    switch (specializedReq.getKind()) {
    case RequirementKind::Conformance:
    case RequirementKind::Superclass:
      ctx.Diags.diagnose(attr->getLocation(),
                         diag::specialize_attr_unsupported_kind_of_req);
      hadError = true;
      break;

    case RequirementKind::SameType:
      if (specializedReq.getSecondType()->isTypeParameter()) {
        ctx.Diags.diagnose(attr->getLocation(),
                           diag::specialize_attr_non_concrete_same_type_req);
        hadError = true;
      }
      break;

    case RequirementKind::Layout:
      break;
    }
  }

  if (hadError)
    return;

  if (!attr->isFullSpecialization())
    return;

  if (specializedSig->areAllParamsConcrete())
    return;

  SmallVector<GenericTypeParamType *, 2> unspecializedParams;

  for (auto *paramTy : specializedSig.getGenericParams()) {
    auto canTy = paramTy->getCanonicalType();
    if (specializedSig->isCanonicalTypeInContext(canTy) &&
        (!specializedSig->getLayoutConstraint(canTy) ||
         originalSig->getLayoutConstraint(canTy))) {
      unspecializedParams.push_back(paramTy);
    }
  }

  unsigned expectedCount = specializedSig.getGenericParams().size();
  unsigned gotCount = expectedCount - unspecializedParams.size();

  if (expectedCount == gotCount)
    return;

  ctx.Diags.diagnose(
      attr->getLocation(), diag::specialize_attr_type_parameter_count_mismatch,
      gotCount, expectedCount);

  for (auto paramTy : unspecializedParams) {
    ctx.Diags.diagnose(attr->getLocation(),
                       diag::specialize_attr_missing_constraint,
                       paramTy->getName());
  }
}

/// Type check that a set of requirements provided by @_specialize.
/// Store the set of requirements in the attribute.
void AttributeChecker::visitSpecializeAttr(SpecializeAttr *attr) {
  DeclContext *DC = D->getDeclContext();
  auto *FD = cast<AbstractFunctionDecl>(D);
  auto genericSig = FD->getGenericSignature();
  auto *trailingWhereClause = attr->getTrailingWhereClause();

  if (!trailingWhereClause) {
    // Report a missing "where" clause.
    diagnose(attr->getLocation(), diag::specialize_missing_where_clause);
    return;
  }

  if (trailingWhereClause->getRequirements().empty()) {
    // Report an empty "where" clause.
    diagnose(attr->getLocation(), diag::specialize_empty_where_clause);
    return;
  }

  if (!genericSig) {
    // Only generic functions are permitted to have trailing where clauses.
    diagnose(attr->getLocation(),
             diag::specialize_attr_nongeneric_trailing_where, FD->getName())
        .highlight(trailingWhereClause->getSourceRange());
    return;
  }

  InferredGenericSignatureRequest request{
      DC->getParentModule(),
      genericSig.getPointer(),
      /*genericParams=*/nullptr,
      WhereClauseOwner(FD, attr),
      /*addedRequirements=*/{},
      /*inferenceSources=*/{},
      /*allowConcreteGenericParams=*/true};

  auto specializedSig = evaluateOrDefault(Ctx.evaluator, request,
                                          GenericSignatureWithError())
      .getPointer();

  // Check the validity of provided requirements.
  checkSpecializeAttrRequirements(attr, genericSig, specializedSig, Ctx);

  attr->setSpecializedSignature(specializedSig);

  // Check the target function if there is one.
  attr->getTargetFunctionDecl(FD);
}

void AttributeChecker::visitFixedLayoutAttr(FixedLayoutAttr *attr) {
  if (isa<StructDecl>(D)) {
    diagnose(attr->getLocation(), diag::fixed_layout_struct)
      .fixItReplace(attr->getRange(), "@frozen");
  }

  auto *VD = cast<ValueDecl>(D);

  if (VD->getFormalAccess() < AccessLevel::Public &&
      !VD->getAttrs().hasAttribute<UsableFromInlineAttr>()) {
    diagnoseAndRemoveAttr(attr, diag::fixed_layout_attr_on_internal_type,
                          VD->getName(), VD->getFormalAccess());
  }
}

void AttributeChecker::visitUsableFromInlineAttr(UsableFromInlineAttr *attr) {
  auto *VD = cast<ValueDecl>(D);

  // FIXME: Once protocols can contain nominal types, do we want to allow
  // these nominal types to have access control (and also @usableFromInline)?
  if (isa<ProtocolDecl>(VD->getDeclContext())) {
    diagnoseAndRemoveAttr(attr, diag::usable_from_inline_attr_in_protocol);
    return;
  }

  // @usableFromInline can only be applied to internal declarations.
  if (VD->getFormalAccess() != AccessLevel::Internal) {
    diagnoseAndRemoveAttr(attr,
                          diag::usable_from_inline_attr_with_explicit_access,
                          VD->getName(), VD->getFormalAccess());
    return;
  }

  // On internal declarations, @inlinable implies @usableFromInline.
  if (VD->getAttrs().hasAttribute<InlinableAttr>()) {
    if (Ctx.isSwiftVersionAtLeast(4,2))
      diagnoseAndRemoveAttr(attr, diag::inlinable_implies_usable_from_inline);
    return;
  }
}

void AttributeChecker::visitInlinableAttr(InlinableAttr *attr) {
  // @inlinable cannot be applied to stored properties.
  //
  // If the type is fixed-layout, the accessors are inlinable anyway;
  // if the type is resilient, the accessors cannot be inlinable
  // because clients cannot directly access storage.
  if (auto *VD = dyn_cast<VarDecl>(D)) {
    if (VD->hasStorage() || VD->getAttrs().hasAttribute<LazyAttr>()) {
      diagnoseAndRemoveAttr(attr,
                            diag::attribute_invalid_on_stored_property,
                            attr);
      return;
    }
  }

  auto *VD = cast<ValueDecl>(D);

  // Calls to dynamically-dispatched declarations are never devirtualized,
  // so marking them as @inlinable does not make sense.
  if (VD->isDynamic()) {
    diagnoseAndRemoveAttr(attr, diag::inlinable_dynamic_not_supported);
    return;
  }

  // @inlinable can only be applied to public or internal declarations.
  auto access = VD->getFormalAccess();
  if (access < AccessLevel::Internal) {
    diagnoseAndRemoveAttr(attr, diag::inlinable_decl_not_public,
                          VD->getBaseName(),
                          access);
    return;
  }

  // @inlinable cannot be applied to deinitializers in resilient classes.
  if (auto *DD = dyn_cast<DestructorDecl>(D)) {
    if (auto *CD = dyn_cast<ClassDecl>(DD->getDeclContext())) {
      if (CD->isResilient()) {
        diagnoseAndRemoveAttr(attr, diag::inlinable_resilient_deinit);
        return;
      }
    }
  }
}

void AttributeChecker::visitOptimizeAttr(OptimizeAttr *attr) {
  if (auto *VD = dyn_cast<VarDecl>(D)) {
    if (VD->hasStorage()) {
      diagnoseAndRemoveAttr(attr,
                            diag::attribute_invalid_on_stored_property,
                            attr);
      return;
    }
  }
}

void AttributeChecker::visitDiscardableResultAttr(DiscardableResultAttr *attr) {
  if (auto *FD = dyn_cast<FuncDecl>(D)) {
    if (auto result = FD->getResultInterfaceType()) {
      auto resultIsVoid = result->isVoid();
      if (resultIsVoid || result->isUninhabited()) {
        diagnoseAndRemoveAttr(attr,
                              diag::discardable_result_on_void_never_function,
                              resultIsVoid);
      }
    }
  }
}

/// Lookup the replaced decl in the replacments scope.
static void lookupReplacedDecl(DeclNameRef replacedDeclName,
                               const DeclAttribute  *attr,
                               const ValueDecl *replacement,
                               SmallVectorImpl<ValueDecl *> &results) {
  auto *declCtxt = replacement->getDeclContext();

  // Look at the accessors' storage's context.
  if (auto *accessor = dyn_cast<AccessorDecl>(replacement)) {
    auto *storage = accessor->getStorage();
    declCtxt = storage->getDeclContext();
  }

  auto *moduleScopeCtxt = declCtxt->getModuleScopeContext();
  if (isa<FileUnit>(declCtxt)) {
    auto &ctx = declCtxt->getASTContext();
    auto descriptor = UnqualifiedLookupDescriptor(
        replacedDeclName, moduleScopeCtxt, attr->getLocation());
    auto lookup = evaluateOrDefault(ctx.evaluator,
                                    UnqualifiedLookupRequest{descriptor}, {});
    for (auto entry : lookup) {
      results.push_back(entry.getValueDecl());
    }
    return;
  }

  assert(declCtxt->isTypeContext());
  auto typeCtx = dyn_cast<NominalTypeDecl>(declCtxt->getAsDecl());
  if (!typeCtx)
    typeCtx = cast<ExtensionDecl>(declCtxt->getAsDecl())->getExtendedNominal();

  auto options = NL_QualifiedDefault;
  if (declCtxt->isInSpecializeExtensionContext())
    options |= NL_IncludeUsableFromInline;

  if (typeCtx)
    moduleScopeCtxt->lookupQualified({typeCtx}, replacedDeclName, options,
                                     results);
}

/// Remove any argument labels from the interface type of the given value that
/// are extraneous from the type system's point of view, producing the
/// type to compare against for the purposes of dynamic replacement.
static Type getDynamicComparisonType(ValueDecl *value) {
  unsigned numArgumentLabels = 0;

  if (isa<AbstractFunctionDecl>(value)) {
    ++numArgumentLabels;

    if (value->getDeclContext()->isTypeContext())
      ++numArgumentLabels;
  } else if (isa<SubscriptDecl>(value)) {
    ++numArgumentLabels;
  }

  auto interfaceType = value->getInterfaceType();
  return interfaceType->removeArgumentLabels(numArgumentLabels);
}

static FuncDecl *findSimilarAccessor(DeclNameRef replacedVarName,
                                     const AccessorDecl *replacement,
                                     DeclAttribute *attr, ASTContext &ctx,
                                     bool forDynamicReplacement) {

  // Retrieve the replaced abstract storage decl.
  SmallVector<ValueDecl *, 4> results;
  lookupReplacedDecl(replacedVarName, attr, replacement, results);

  // Filter out any accessors that won't work.
  if (!results.empty()) {
    auto replacementStorage = replacement->getStorage();
    Type replacementStorageType = getDynamicComparisonType(replacementStorage);
    results.erase(std::remove_if(results.begin(), results.end(),
        [&](ValueDecl *result) {
          // Protocol requirements are not replaceable.
          if (isa<ProtocolDecl>(result->getDeclContext()))
            return true;
          // Check for static/instance mismatch.
          if (result->isStatic() != replacementStorage->isStatic())
            return true;

          // Check for type mismatch.
          auto resultType = getDynamicComparisonType(result);
          if (!resultType->isEqual(replacementStorageType) &&
              !resultType->matches(
                  replacementStorageType,
                  TypeMatchFlags::AllowCompatibleOpaqueTypeArchetypes)) {
            return true;
          }

          return false;
        }),
        results.end());
  }

  auto &Diags = ctx.Diags;
  if (results.empty()) {
    Diags.diagnose(attr->getLocation(),
                   diag::dynamic_replacement_accessor_not_found,
                   replacedVarName);
    attr->setInvalid();
    return nullptr;
  }

  if (results.size() > 1) {
    Diags.diagnose(attr->getLocation(),
                   diag::dynamic_replacement_accessor_ambiguous,
                   replacedVarName);
    for (auto result : results) {
      Diags.diagnose(result,
                     diag::dynamic_replacement_accessor_ambiguous_candidate,
                     result->getModuleContext()->getName());
    }
    attr->setInvalid();
    return nullptr;
  }

  assert(!isa<FuncDecl>(results[0]));

  auto *origStorage = cast<AbstractStorageDecl>(results[0]);
  if (forDynamicReplacement && !origStorage->isDynamic()) {
    Diags.diagnose(attr->getLocation(),
                   diag::dynamic_replacement_accessor_not_dynamic,
                   origStorage->getName());
    attr->setInvalid();
    return nullptr;
  }

  // Find the accessor in the replaced storage decl.
  auto *origAccessor = origStorage->getOpaqueAccessor(
      replacement->getAccessorKind());
  if (!origAccessor)
    return nullptr;

  if (origAccessor->isImplicit() &&
      !(origStorage->getReadImpl() == ReadImplKind::Stored &&
        origStorage->getWriteImpl() == WriteImplKind::Stored)) {
    Diags.diagnose(attr->getLocation(),
                   diag::dynamic_replacement_accessor_not_explicit,
                   (unsigned)origAccessor->getAccessorKind(),
                   origStorage->getName());
    attr->setInvalid();
    return nullptr;
  }

  return origAccessor;
}

static FuncDecl *findReplacedAccessor(DeclNameRef replacedVarName,
                                      const AccessorDecl *replacement,
                                      DeclAttribute *attr,
                                      ASTContext &ctx) {
  return findSimilarAccessor(replacedVarName, replacement, attr, ctx,
                             /*forDynamicReplacement*/ true);
}

static FuncDecl *findTargetAccessor(DeclNameRef replacedVarName,
                                      const AccessorDecl *replacement,
                                      DeclAttribute *attr,
                                      ASTContext &ctx) {
  return findSimilarAccessor(replacedVarName, replacement, attr, ctx,
                             /*forDynamicReplacement*/ false);
}

static AbstractFunctionDecl *
findSimilarFunction(DeclNameRef replacedFunctionName,
                    const AbstractFunctionDecl *base, DeclAttribute *attr,
                    DiagnosticEngine *Diags, bool forDynamicReplacement) {

  // Note: we might pass a constant attribute when typechecker is nullptr.
  // Any modification to attr must be guarded by a null check on TC.
  //
  SmallVector<ValueDecl *, 4> results;
  lookupReplacedDecl(replacedFunctionName, attr, base, results);

  for (auto *result : results) {
    // Protocol requirements are not replaceable.
    if (isa<ProtocolDecl>(result->getDeclContext()))
      continue;
    // Check for static/instance mismatch.
    if (result->isStatic() != base->isStatic())
      continue;

    auto resultTy = result->getInterfaceType();
    auto replaceTy = base->getInterfaceType();
    TypeMatchOptions matchMode = TypeMatchFlags::AllowABICompatible;
    matchMode |= TypeMatchFlags::AllowCompatibleOpaqueTypeArchetypes;
    if (resultTy->matches(replaceTy, matchMode)) {
      if (forDynamicReplacement && !result->isDynamic()) {
        if (Diags) {
          Diags->diagnose(attr->getLocation(),
                          diag::dynamic_replacement_function_not_dynamic,
                          result->getName());
          attr->setInvalid();
        }
        return nullptr;
      }
      return cast<AbstractFunctionDecl>(result);
    }
  }

  if (!Diags)
    return nullptr;

  if (results.empty()) {
    Diags->diagnose(attr->getLocation(),
                    forDynamicReplacement
                        ? diag::dynamic_replacement_function_not_found
                        : diag::specialize_target_function_not_found,
                    replacedFunctionName);
  } else {
    Diags->diagnose(attr->getLocation(),
                    forDynamicReplacement
                        ? diag::dynamic_replacement_function_of_type_not_found
                        : diag::specialize_target_function_of_type_not_found,
                    replacedFunctionName,
                    base->getInterfaceType()->getCanonicalType());

    for (auto *result : results) {
      Diags->diagnose(SourceLoc(),
                      forDynamicReplacement
                          ? diag::dynamic_replacement_found_function_of_type
                          : diag::specialize_found_function_of_type,
                      result->getName(),
                      result->getInterfaceType()->getCanonicalType());
    }
  }
  attr->setInvalid();
  return nullptr;
}

static AbstractFunctionDecl *
findReplacedFunction(DeclNameRef replacedFunctionName,
                     const AbstractFunctionDecl *replacement,
                     DynamicReplacementAttr *attr, DiagnosticEngine *Diags) {
  return findSimilarFunction(replacedFunctionName, replacement, attr, Diags,
                             true /*forDynamicReplacement*/);
}

static AbstractFunctionDecl *
findTargetFunction(DeclNameRef targetFunctionName,
                   const AbstractFunctionDecl *base,
                   SpecializeAttr * attr, DiagnosticEngine *diags) {
  return findSimilarFunction(targetFunctionName, base, attr, diags,
                             false /*forDynamicReplacement*/);
}

static AbstractStorageDecl *
findReplacedStorageDecl(DeclNameRef replacedFunctionName,
                        const AbstractStorageDecl *replacement,
                        const DynamicReplacementAttr *attr) {

  SmallVector<ValueDecl *, 4> results;
  lookupReplacedDecl(replacedFunctionName, attr, replacement, results);

  for (auto *result : results) {
    // Check for static/instance mismatch.
    if (result->isStatic() != replacement->isStatic())
      continue;
    auto resultTy = result->getInterfaceType();
    auto replaceTy = replacement->getInterfaceType();
    TypeMatchOptions matchMode = TypeMatchFlags::AllowABICompatible;
    matchMode |= TypeMatchFlags::AllowCompatibleOpaqueTypeArchetypes;
    if (resultTy->matches(replaceTy, matchMode)) {
      if (!result->isDynamic()) {
        return nullptr;
      }
      return cast<AbstractStorageDecl>(result);
    }
  }
  return nullptr;
}

void AttributeChecker::visitDynamicReplacementAttr(DynamicReplacementAttr *attr) {
  assert(isa<AbstractFunctionDecl>(D) || isa<AbstractStorageDecl>(D));
  auto *replacement = cast<ValueDecl>(D);

  if (!isa<ExtensionDecl>(replacement->getDeclContext()) &&
      !replacement->getDeclContext()->isModuleScopeContext()) {
    diagnose(attr->getLocation(), diag::dynamic_replacement_not_in_extension,
             replacement->getBaseName());
    attr->setInvalid();
    return;
  }

  if (replacement->shouldUseNativeDynamicDispatch()) {
    diagnose(attr->getLocation(), diag::dynamic_replacement_must_not_be_dynamic,
             replacement->getBaseName());
    attr->setInvalid();
    return;
  }

  auto *original = replacement->getDynamicallyReplacedDecl();
  if (!original) {
    attr->setInvalid();
    return;
  }

  if (original->isObjC() && !replacement->isObjC()) {
    diagnose(attr->getLocation(),
             diag::dynamic_replacement_replacement_not_objc_dynamic,
             replacement->getName());
    attr->setInvalid();
  }
  if (!original->isObjC() && replacement->isObjC()) {
    diagnose(attr->getLocation(),
             diag::dynamic_replacement_replaced_not_objc_dynamic,
             original->getName());
    attr->setInvalid();
  }

  if (auto *CD = dyn_cast<ConstructorDecl>(replacement)) {
    auto *attr = CD->getAttrs().getAttribute<DynamicReplacementAttr>();
    auto replacedIsConvenienceInit =
        cast<ConstructorDecl>(original)->isConvenienceInit();
    if (replacedIsConvenienceInit &&!CD->isConvenienceInit()) {
      diagnose(attr->getLocation(),
               diag::dynamic_replacement_replaced_constructor_is_convenience,
               attr->getReplacedFunctionName());
    } else if (!replacedIsConvenienceInit && CD->isConvenienceInit()) {
      diagnose(
          attr->getLocation(),
          diag::dynamic_replacement_replaced_constructor_is_not_convenience,
          attr->getReplacedFunctionName());
    }
  }
}

Type
ResolveTypeEraserTypeRequest::evaluate(Evaluator &evaluator,
                                       ProtocolDecl *PD,
                                       TypeEraserAttr *attr) const {
  if (auto *typeEraserRepr = attr->getParsedTypeEraserTypeRepr()) {
    return TypeResolution::resolveContextualType(
        typeEraserRepr, PD, None,
        // Unbound generics and placeholders
        // are not allowed within this
        // attribute.
        /*unboundTyOpener*/ nullptr,
        /*placeholderHandler*/ nullptr);
  } else {
    auto *LazyResolver = attr->Resolver;
    assert(LazyResolver && "type eraser was neither parsed nor deserialized?");
    auto ty = LazyResolver->loadTypeEraserType(attr, attr->ResolverContextData);
    attr->Resolver = nullptr;
    if (!ty) {
      return ErrorType::get(PD->getASTContext());
    }
    return ty;
  }
}

bool
TypeEraserHasViableInitRequest::evaluate(Evaluator &evaluator,
                                         TypeEraserAttr *attr,
                                         ProtocolDecl *protocol) const {
  DeclContext *dc = protocol->getDeclContext();
  ModuleDecl *module = dc->getParentModule();
  auto &ctx = module->getASTContext();
  auto &diags = ctx.Diags;
  Type protocolType = protocol->getDeclaredInterfaceType();

  // Get the NominalTypeDecl for the type eraser.
  Type typeEraser = attr->getResolvedType(protocol);
  if (typeEraser->hasError())
    return false;

  // The type eraser must be a concrete nominal type
  auto nominalTypeDecl = typeEraser->getAnyNominal();
  if (auto typeAliasDecl = dyn_cast_or_null<TypeAliasDecl>(nominalTypeDecl))
    nominalTypeDecl = typeAliasDecl->getUnderlyingType()->getAnyNominal();

  if (!nominalTypeDecl || isa<ProtocolDecl>(nominalTypeDecl)) {
    diags.diagnose(attr->getLoc(), diag::non_nominal_type_eraser);
    return false;
  }

  // The nominal type must be accessible wherever the protocol is accessible
  if (nominalTypeDecl->getFormalAccess() < protocol->getFormalAccess()) {
    diags.diagnose(attr->getLoc(), diag::type_eraser_not_accessible,
                   nominalTypeDecl->getFormalAccess(), nominalTypeDecl->getName(),
                   protocolType, protocol->getFormalAccess());
    diags.diagnose(nominalTypeDecl->getLoc(), diag::type_eraser_declared_here);
    return false;
  }

  // The type eraser must conform to the annotated protocol
  if (!TypeChecker::conformsToProtocol(typeEraser, protocol, module)) {
    diags.diagnose(attr->getLoc(), diag::type_eraser_does_not_conform,
                   typeEraser, protocolType);
    diags.diagnose(nominalTypeDecl->getLoc(), diag::type_eraser_declared_here);
    return false;
  }

  // The type eraser must have an init of the form init<T: Protocol>(erasing: T)
  auto lookupResult = TypeChecker::lookupMember(dc, typeEraser,
                                                DeclNameRef::createConstructor());

  // Keep track of unviable init candidates for diagnostics
  enum class UnviableReason {
    Failable,
    UnsatisfiedRequirements,
    Inaccessible,
  };
  SmallVector<std::tuple<ConstructorDecl *, UnviableReason, Type>, 2> unviable;

  bool foundMatch = llvm::any_of(lookupResult, [&](const LookupResultEntry &entry) {
    auto *init = cast<ConstructorDecl>(entry.getValueDecl());
    if (!init->isGeneric() || init->getGenericParams()->size() != 1)
      return false;

    auto genericSignature = init->getGenericSignature();
    auto genericParamType = genericSignature.getInnermostGenericParams().front();

    // Fow now, only allow one parameter.
    auto params = init->getParameters();
    if (params->size() != 1)
      return false;

    // The parameter must have the form `erasing: T` where T conforms to the protocol.
    ParamDecl *param = *init->getParameters()->begin();
    if (param->getArgumentName() != ctx.Id_erasing ||
        !param->getInterfaceType()->isEqual(genericParamType) ||
        !genericSignature->requiresProtocol(genericParamType, protocol))
      return false;

    // Allow other constraints as long as the init can be called with any
    // type conforming to the annotated protocol. We will check this by
    // substituting the protocol's Self type for the generic arg and check that
    // the requirements in the generic signature are satisfied.
    auto *module = nominalTypeDecl->getParentModule();
    auto baseMap =
        typeEraser->getContextSubstitutionMap(module,
                                              nominalTypeDecl);
    QuerySubstitutionMap getSubstitution{baseMap};

    // Use invalid 'SourceLoc's to suppress diagnostics.
    auto result = TypeChecker::checkGenericArguments(
          module, genericSignature.getRequirements(),
          [&](SubstitutableType *type) -> Type {
            if (type->isEqual(genericParamType))
              return protocol->getSelfTypeInContext();

            return getSubstitution(type);
          });

    if (result != RequirementCheckResult::Success) {
      unviable.push_back(
          std::make_tuple(init, UnviableReason::UnsatisfiedRequirements,
                          genericParamType));
      return false;
    }

    if (init->isFailable()) {
      unviable.push_back(
          std::make_tuple(init, UnviableReason::Failable, genericParamType));
      return false;
    }

    if (init->getFormalAccess() < protocol->getFormalAccess()) {
      unviable.push_back(
          std::make_tuple(init, UnviableReason::Inaccessible, genericParamType));
      return false;
    }

    return true;
  });

  if (!foundMatch) {
    if (unviable.empty()) {
      diags.diagnose(attr->getLocation(), diag::type_eraser_missing_init,
                     typeEraser, protocol->getName().str());
      diags.diagnose(nominalTypeDecl->getLoc(), diag::type_eraser_declared_here);
      return false;
    }

    diags.diagnose(attr->getLocation(), diag::type_eraser_unviable_init,
                   typeEraser, protocol->getName().str());
    for (auto &candidate: unviable) {
      auto init = std::get<0>(candidate);
      auto reason = std::get<1>(candidate);
      auto genericParamType = std::get<2>(candidate);

      switch (reason) {
      case UnviableReason::Failable:
        diags.diagnose(init->getLoc(), diag::type_eraser_failable_init);
        break;
      case UnviableReason::UnsatisfiedRequirements:
        diags.diagnose(init->getLoc(),
                       diag::type_eraser_init_unsatisfied_requirements,
                       genericParamType, protocol->getName().str());
        break;
      case UnviableReason::Inaccessible:
        diags.diagnose(init->getLoc(), diag::type_eraser_init_not_accessible,
                       init->getFormalAccess(), protocolType,
                       protocol->getFormalAccess());
        break;
      }
    }
    return false;
  }

  return true;
}

void AttributeChecker::visitTypeEraserAttr(TypeEraserAttr *attr) {
  assert(isa<ProtocolDecl>(D));
  // Invoke the request.
  (void)attr->hasViableTypeEraserInit(cast<ProtocolDecl>(D));
}

void AttributeChecker::visitImplementsAttr(ImplementsAttr *attr) {
  DeclContext *DC = D->getDeclContext();

  Type T = attr->getProtocolType();
  if (!T && attr->getProtocolTypeRepr()) {
    T = TypeResolution::resolveContextualType(attr->getProtocolTypeRepr(), DC,
                                              None, /*unboundTyOpener*/ nullptr,
                                              /*placeholderHandler*/ nullptr);
  }

  // Definite error-types were already diagnosed in resolveType.
  if (T->hasError())
    return;
  attr->setProtocolType(T);

  // Check that we got a ProtocolType.
  if (auto PT = T->getAs<ProtocolType>()) {
    ProtocolDecl *PD = PT->getDecl();

    // Check that the ProtocolType has the specified member.
    LookupResult R =
        TypeChecker::lookupMember(PD->getDeclContext(), PT,
                                  DeclNameRef(attr->getMemberName()));
    if (!R) {
      diagnose(attr->getLocation(),
               diag::implements_attr_protocol_lacks_member,
               PD->getName(), attr->getMemberName())
        .highlight(attr->getMemberNameLoc().getSourceRange());
    }

    // Check that the decl we're decorating is a member of a type that actually
    // conforms to the specified protocol.
    NominalTypeDecl *NTD = DC->getSelfNominalTypeDecl();
    SmallVector<ProtocolConformance *, 2> conformances;
    if (!NTD->lookupConformance(PD, conformances)) {
      diagnose(attr->getLocation(),
               diag::implements_attr_protocol_not_conformed_to,
               NTD->getName(), PD->getName())
        .highlight(attr->getProtocolTypeRepr()->getSourceRange());
    }

  } else {
    diagnose(attr->getLocation(), diag::implements_attr_non_protocol_type)
      .highlight(attr->getProtocolTypeRepr()->getSourceRange());
  }
}

void AttributeChecker::visitFrozenAttr(FrozenAttr *attr) {
  if (auto *ED = dyn_cast<EnumDecl>(D)) {
    if (!ED->getModuleContext()->isResilient()) {
      attr->setInvalid();
      return;
    }

    if (ED->getFormalAccess() < AccessLevel::Public &&
        !ED->getAttrs().hasAttribute<UsableFromInlineAttr>()) {
      diagnoseAndRemoveAttr(attr, diag::enum_frozen_nonpublic, attr);
      return;
    }
  }

  auto *VD = cast<ValueDecl>(D);

  if (VD->getFormalAccess() < AccessLevel::Public &&
      !VD->getAttrs().hasAttribute<UsableFromInlineAttr>()) {
    diagnoseAndRemoveAttr(attr, diag::frozen_attr_on_internal_type,
                          VD->getName(), VD->getFormalAccess());
  }
}

void AttributeChecker::visitCustomAttr(CustomAttr *attr) {
  auto dc = D->getDeclContext();

  // Figure out which nominal declaration this custom attribute refers to.
  auto nominal = evaluateOrDefault(
    Ctx.evaluator, CustomAttrNominalRequest{attr, dc}, nullptr);

  // Diagnose errors.
  if (!nominal) {
    auto typeRepr = attr->getTypeRepr();

    auto type = TypeResolution::forInterface(dc, TypeResolverContext::CustomAttr,
                                             // Unbound generics and placeholders
                                             // are not allowed within this
                                             // attribute.
                                             /*unboundTyOpener*/ nullptr,
                                             /*placeholderHandler*/ nullptr)
        .resolveType(typeRepr);

    if (type->is<ErrorType>()) {
      // Type resolution has failed, and we should have diagnosed something already.
      assert(Ctx.hadError());
    } else {
      // Otherwise, something odd happened.
      std::string typeName;
      llvm::raw_string_ostream out(typeName);
      typeRepr->print(out);

      Ctx.Diags.diagnose(attr->getLocation(), diag::unknown_attribute, typeName);
    }

    attr->setInvalid();
    return;
  }

  // If the nominal type is a property wrapper type, we can be delegating
  // through a property.
  if (nominal->getAttrs().hasAttribute<PropertyWrapperAttr>()) {
    // property wrappers can only be applied to variables
    if (!isa<VarDecl>(D)) {
      diagnose(attr->getLocation(),
               diag::property_wrapper_attribute_not_on_property,
               nominal->getName());
      attr->setInvalid();
      return;
    }

    if (isa<ParamDecl>(D)) {
      // Check for unsupported declarations.
      auto *context = D->getDeclContext()->getAsDecl();
      if (isa_and_nonnull<SubscriptDecl>(context)) {
        diagnose(attr->getLocation(),
                 diag::property_wrapper_param_not_supported,
                 context->getDescriptiveKind());
        attr->setInvalid();
        return;
      }
    }

    return;
  }

  // If the nominal type is a result builder type, verify that D is a
  // function, storage with an explicit getter, or parameter of function type.
  if (nominal->getAttrs().hasAttribute<ResultBuilderAttr>()) {
    ValueDecl *decl;
    if (auto param = dyn_cast<ParamDecl>(D)) {
      decl = param;
    } else if (auto func = dyn_cast<FuncDecl>(D)) {
      decl = func;
    } else if (auto storage = dyn_cast<AbstractStorageDecl>(D)) {
      decl = storage;

      // Check whether this is a storage declaration that is not permitted
      // to have a result builder attached.
      auto shouldDiagnose = [&]() -> bool {
        // An uninitialized stored property in a struct can have a function
        // builder attached.
        if (auto var = dyn_cast<VarDecl>(decl)) {
          if (var->isInstanceMember() &&
              isa<StructDecl>(var->getDeclContext()) &&
              !var->getParentInitializer()) {
            return false;
          }
        }

        auto getter = storage->getParsedAccessor(AccessorKind::Get);
        if (!getter)
          return true;

        // Module interfaces don't print bodies for all getters, so allow getters
        // that don't have a body if we're compiling a module interface.
        // Within a protocol definition, there will never be a body.
        SourceFile *parent = storage->getDeclContext()->getParentSourceFile();
        bool isInInterface = parent && parent->Kind == SourceFileKind::Interface;
        if (!isInInterface && !getter->hasBody() &&
            !isa<ProtocolDecl>(storage->getDeclContext()))
          return true;

        return false;
      };

      if (shouldDiagnose()) {
        diagnose(attr->getLocation(),
                 diag::result_builder_attribute_on_storage_without_getter,
                 nominal->getName(),
                 isa<SubscriptDecl>(storage) ? 0
                   : storage->getDeclContext()->isTypeContext() ? 1
                   : cast<VarDecl>(storage)->isLet() ? 2 : 3);
        attr->setInvalid();
        return;
      }
    } else {
      diagnose(attr->getLocation(),
               diag::result_builder_attribute_not_allowed_here,
               nominal->getName());
      attr->setInvalid();
      return;
    }

    // Diagnose and ignore arguments.
    if (attr->hasArgs()) {
      diagnose(attr->getLocation(), diag::result_builder_arguments)
        .highlight(attr->getArgs()->getSourceRange());
    }

    // Complain if this isn't the primary result-builder attribute.
    auto attached = decl->getAttachedResultBuilder();
    if (attached != attr) {
      diagnose(attr->getLocation(), diag::result_builder_multiple,
               isa<ParamDecl>(decl));
      diagnose(attached->getLocation(), diag::previous_result_builder_here);
      attr->setInvalid();
      return;
    } else {
      // Force any diagnostics associated with computing the result-builder
      // type.
      (void) decl->getResultBuilderType();
    }

    return;
  }

  // If the nominal type is a global actor, let the global actor attribute
  // retrieval request perform checking for us.
  if (nominal->isGlobalActor()) {
    (void)D->getGlobalActorAttr();
    if (auto value = dyn_cast<ValueDecl>(D))
      (void)getActorIsolation(value);
    return;
  }

  diagnose(attr->getLocation(), diag::nominal_type_not_attribute,
           nominal->getDescriptiveKind(), nominal->getName());
  nominal->diagnose(diag::decl_declared_here, nominal->getName());
  attr->setInvalid();
}


void AttributeChecker::visitPropertyWrapperAttr(PropertyWrapperAttr *attr) {
  auto nominal = dyn_cast<NominalTypeDecl>(D);
  if (!nominal)
    return;

  // Force checking of the property wrapper type.
  (void)nominal->getPropertyWrapperTypeInfo();
}

void AttributeChecker::visitResultBuilderAttr(ResultBuilderAttr *attr) {
  auto *nominal = dyn_cast<NominalTypeDecl>(D);
  SmallVector<ValueDecl *, 4> potentialMatches;
  bool supportsBuildBlock = TypeChecker::typeSupportsBuilderOp(
      nominal->getDeclaredType(), nominal, D->getASTContext().Id_buildBlock,
      /*argLabels=*/{}, &potentialMatches);

  if (!supportsBuildBlock) {
    {
      auto diag = diagnose(
          nominal->getLoc(), diag::result_builder_static_buildblock);

      // If there were no close matches, propose adding a stub.
      SourceLoc buildInsertionLoc;
      std::string stubIndent;
      Type componentType;
      std::tie(buildInsertionLoc, stubIndent, componentType) =
          determineResultBuilderBuildFixItInfo(nominal);
      if (buildInsertionLoc.isValid() && potentialMatches.empty()) {
        std::string fixItString;
        {
          llvm::raw_string_ostream out(fixItString);
          printResultBuilderBuildFunction(
              nominal, componentType,
              ResultBuilderBuildFunction::BuildBlock,
              stubIndent, out);
        }

        diag.fixItInsert(buildInsertionLoc, fixItString);
      }
    }

    // For any close matches, attempt to explain to the user why they aren't
    // valid.
    for (auto *member : potentialMatches) {
      if (member->isStatic() && isa<FuncDecl>(member))
        continue;

      if (isa<FuncDecl>(member) &&
          member->getDeclContext()->getSelfNominalTypeDecl() == nominal)
        diagnose(member->getLoc(), diag::result_builder_non_static_buildblock)
          .fixItInsert(member->getAttributeInsertionLoc(true), "static ");
      else if (isa<EnumElementDecl>(member))
        diagnose(member->getLoc(), diag::result_builder_buildblock_enum_case);
      else
        diagnose(member->getLoc(),
                 diag::result_builder_buildblock_not_static_method);
    }
  }
}

void
AttributeChecker::visitImplementationOnlyAttr(ImplementationOnlyAttr *attr) {
  if (isa<ImportDecl>(D)) {
    // These are handled elsewhere.
    return;
  }

  auto *VD = cast<ValueDecl>(D);
  auto *overridden = VD->getOverriddenDecl();
  if (!overridden) {
    diagnoseAndRemoveAttr(attr, diag::implementation_only_decl_non_override);
    return;
  }

  // Check if VD has the exact same type as what it overrides.
  // Note: This is specifically not using `swift::getMemberTypeForComparison`
  // because that erases more information than we want, like `throws`-ness.
  auto baseInterfaceTy = overridden->getInterfaceType();
  auto derivedInterfaceTy = VD->getInterfaceType();

  auto selfInterfaceTy = VD->getDeclContext()->getDeclaredInterfaceType();

  auto overrideInterfaceTy =
      selfInterfaceTy->adjustSuperclassMemberDeclType(overridden, VD,
                                                      baseInterfaceTy);

  if (isa<AbstractFunctionDecl>(VD)) {
    // Drop the 'Self' parameter.
    // FIXME: The real effect here, though, is dropping the generic signature.
    // This should be okay because it should already be checked as part of
    // making an override, but that isn't actually the case as of this writing,
    // and it's kind of suspect anyway.
    derivedInterfaceTy =
        derivedInterfaceTy->castTo<AnyFunctionType>()->getResult();
    overrideInterfaceTy =
        overrideInterfaceTy->castTo<AnyFunctionType>()->getResult();
  } else if (isa<SubscriptDecl>(VD)) {
    // For subscripts, we don't have a 'Self' type, but turn it
    // into a monomorphic function type.
    // FIXME: does this actually make sense, though?
    auto derivedInterfaceFuncTy = derivedInterfaceTy->castTo<AnyFunctionType>();
    // FIXME: Verify ExtInfo state is correct, not working by accident.
    FunctionType::ExtInfo derivedInterfaceInfo;
    derivedInterfaceTy = FunctionType::get(derivedInterfaceFuncTy->getParams(),
                                           derivedInterfaceFuncTy->getResult(),
                                           derivedInterfaceInfo);
    auto overrideInterfaceFuncTy =
        overrideInterfaceTy->castTo<AnyFunctionType>();
    // FIXME: Verify ExtInfo state is correct, not working by accident.
    FunctionType::ExtInfo overrideInterfaceInfo;
    overrideInterfaceTy = FunctionType::get(
        overrideInterfaceFuncTy->getParams(),
        overrideInterfaceFuncTy->getResult(), overrideInterfaceInfo);
  }

  if (!derivedInterfaceTy->isEqual(overrideInterfaceTy)) {
    diagnose(VD, diag::implementation_only_override_changed_type,
             overrideInterfaceTy);
    diagnose(overridden, diag::overridden_here);
    return;
  }

  // FIXME: When compiling without library evolution enabled, this should also
  // check whether VD or any of its accessors need a new vtable entry, even if
  // it won't necessarily be able to say why.
}

void AttributeChecker::visitTypeSequenceAttr(TypeSequenceAttr *attr) {
  if (!isa<GenericTypeParamDecl>(D)) {
    attr->setInvalid();
    diagnoseAndRemoveAttr(attr, diag::type_sequence_on_non_generic_param);
  }
}

void AttributeChecker::visitNonEphemeralAttr(NonEphemeralAttr *attr) {
  auto *param = cast<ParamDecl>(D);
  auto type = param->getInterfaceType()->lookThroughSingleOptionalType();

  // Can only be applied to Unsafe[...]Pointer types
  if (type->getAnyPointerElementType())
    return;

  // ... or the protocol Self type.
  auto *outerDC = param->getDeclContext()->getParent();
  if (outerDC->getSelfProtocolDecl() &&
      type->isEqual(outerDC->getProtocolSelfType())) {
    return;
  }

  diagnose(attr->getLocation(), diag::non_ephemeral_non_pointer_type);
  attr->setInvalid();
}

void AttributeChecker::checkOriginalDefinedInAttrs(Decl *D,
    ArrayRef<OriginallyDefinedInAttr*> Attrs) {
  if (Attrs.empty())
    return;
  auto &Ctx = D->getASTContext();
  std::map<PlatformKind, SourceLoc> seenPlatforms;

  // Attrs are in the reverse order of the source order. We need to visit them
  // in source order to diagnose the later attribute.
  for (auto *Attr: Attrs) {
    if (!Attr->isActivePlatform(Ctx))
      continue;
    auto AtLoc = Attr->AtLoc;
    auto Platform = Attr->Platform;
    if (!seenPlatforms.insert({Platform, AtLoc}).second) {
      // We've seen the platform before, emit error to the previous one which
      // comes later in the source order.
      diagnose(seenPlatforms[Platform],
               diag::originally_defined_in_dupe_platform,
               platformString(Platform));
      return;
    }
    static StringRef AttrName = "_originallyDefinedIn";
    if (!D->getDeclContext()->isModuleScopeContext()) {
      diagnose(AtLoc, diag::originally_definedin_topleve_decl, AttrName);
      return;
    }
    auto IntroVer = D->getIntroducedOSVersion(Platform);
    if (!IntroVer.hasValue()) {
      diagnose(AtLoc, diag::originally_definedin_need_available,
               AttrName);
      return;
    }
    if (IntroVer.getValue() > Attr->MovedVersion) {
      diagnose(AtLoc,
               diag::originally_definedin_must_not_before_available_version,
               AttrName);
      return;
    }
  }
}

Type TypeChecker::checkReferenceOwnershipAttr(VarDecl *var, Type type,
                                              ReferenceOwnershipAttr *attr) {
  auto &Diags = var->getASTContext().Diags;
  auto *dc = var->getDeclContext();

  // Don't check ownership attribute if the type is invalid.
  if (attr->isInvalid() || type->is<ErrorType>())
    return type;

  auto ownershipKind = attr->get();

  // A weak variable must have type R? or R! for some ownership-capable type R.
  auto underlyingType = type->getOptionalObjectType();
  auto isOptional = bool(underlyingType);

  switch (optionalityOf(ownershipKind)) {
  case ReferenceOwnershipOptionality::Disallowed:
    if (isOptional) {
      var->diagnose(diag::invalid_ownership_with_optional, ownershipKind)
          .fixItReplace(attr->getRange(), "weak");
      attr->setInvalid();
    }
    break;
  case ReferenceOwnershipOptionality::Allowed:
    break;
  case ReferenceOwnershipOptionality::Required:
    if (var->isLet()) {
      var->diagnose(diag::invalid_ownership_is_let, ownershipKind);
      attr->setInvalid();
    }

    if (!isOptional) {
      attr->setInvalid();

      // @IBOutlet has its own diagnostic when the property type is
      // non-optional.
      if (var->getAttrs().hasAttribute<IBOutletAttr>())
        break;

      auto diag = var->diagnose(diag::invalid_ownership_not_optional,
                                ownershipKind, OptionalType::get(type));
      auto typeRange = var->getTypeSourceRangeForDiagnostics();
      if (type->hasSimpleTypeRepr()) {
        diag.fixItInsertAfter(typeRange.End, "?");
      } else {
        diag.fixItInsert(typeRange.Start, "(")
          .fixItInsertAfter(typeRange.End, ")?");
      }
    }
    break;
  }

  if (!underlyingType)
    underlyingType = type;

  auto sig = var->getDeclContext()->getGenericSignatureOfContext();
  if (!underlyingType->allowsOwnership(sig.getPointer())) {
    auto D = diag::invalid_ownership_type;

    if (underlyingType->isExistentialType() ||
        underlyingType->isTypeParameter()) {
      // Suggest the possibility of adding a class bound.
      D = diag::invalid_ownership_protocol_type;
    }

    var->diagnose(D, ownershipKind, underlyingType);
    attr->setInvalid();
  }

  ClassDecl *underlyingClass = underlyingType->getClassOrBoundGenericClass();
  if (underlyingClass && underlyingClass->isIncompatibleWithWeakReferences()) {
    Diags
        .diagnose(attr->getLocation(),
                  diag::invalid_ownership_incompatible_class, underlyingType,
                  ownershipKind)
        .fixItRemove(attr->getRange());
    attr->setInvalid();
  }

  auto PDC = dyn_cast<ProtocolDecl>(dc);
  if (PDC && !PDC->isObjC()) {
    // Ownership does not make sense in protocols, except for "weak" on
    // properties of Objective-C protocols.
    auto D = diag::ownership_invalid_in_protocols;
    Diags.diagnose(attr->getLocation(), D, ownershipKind)
        .warnUntilSwiftVersion(5)
        .fixItRemove(attr->getRange());
    attr->setInvalid();
  }

  if (attr->isInvalid())
    return type;

  // Change the type to the appropriate reference storage type.
  return ReferenceStorageType::get(type, ownershipKind, var->getASTContext());
}

Optional<Diag<>>
TypeChecker::diagnosticIfDeclCannotBePotentiallyUnavailable(const Decl *D) {
  auto *DC = D->getDeclContext();

  if (auto *VD = dyn_cast<VarDecl>(D)) {
    if (!VD->hasStorage())
      return None;

    // Do not permit potential availability of script-mode global variables;
    // their initializer expression is not lazily evaluated, so this would
    // not be safe.
    if (VD->isTopLevelGlobal())
      return diag::availability_global_script_no_potential;

    // Globals and statics are lazily initialized, so they are safe
    // for potential unavailability.
    if (!VD->isStatic() && !DC->isModuleScopeContext())
      return diag::availability_stored_property_no_potential;

  } else if (auto *EED = dyn_cast<EnumElementDecl>(D)) {
    // An enum element with an associated value cannot be potentially
    // unavailable.
    if (EED->hasAssociatedValues()) {
      auto &ctx = DC->getASTContext();
      auto *SF = DC->getParentSourceFile();

      if (SF->Kind == SourceFileKind::Interface ||
          ctx.LangOpts.WarnOnPotentiallyUnavailableEnumCase) {
        return diag::availability_enum_element_no_potential_warn;
      } else {
        return diag::availability_enum_element_no_potential;
      }
    }
  }

  return None;
}

static bool shouldBlockImplicitDynamic(Decl *D) {
  if (D->getAttrs().hasAttribute<NonObjCAttr>() ||
      D->getAttrs().hasAttribute<SILGenNameAttr>() ||
      D->getAttrs().hasAttribute<TransparentAttr>() ||
      D->getAttrs().hasAttribute<InlinableAttr>())
    return true;
  return false;
}
void TypeChecker::addImplicitDynamicAttribute(Decl *D) {
  if (!D->getModuleContext()->isImplicitDynamicEnabled())
    return;

  // Add the attribute if the decl kind allows it and it is not an accessor
  // decl. Accessor decls should always infer the var/subscript's attribute.
  if (!DeclAttribute::canAttributeAppearOnDecl(DAK_Dynamic, D) ||
      isa<AccessorDecl>(D))
    return;

  // Don't add dynamic if decl is inlinable or tranparent.
  if (shouldBlockImplicitDynamic(D))
   return;

  if (auto *FD = dyn_cast<FuncDecl>(D)) {
    // Don't add dynamic to defer bodies.
    if (FD->isDeferBody())
      return;
    // Don't add dynamic to functions with a cdecl.
    if (FD->getAttrs().hasAttribute<CDeclAttr>())
      return;
    // Don't add dynamic to local function definitions.
    if (!FD->getDeclContext()->isTypeContext() &&
        FD->getDeclContext()->isLocalContext())
      return;
  }

  // Don't add dynamic if accessor is inlinable or transparent.
  if (auto *asd = dyn_cast<AbstractStorageDecl>(D)) {
    bool blocked = false;
    asd->visitParsedAccessors([&](AccessorDecl *accessor) {
      blocked |= shouldBlockImplicitDynamic(accessor);
    });
    if (blocked)
      return;
  }

  if (auto *VD = dyn_cast<VarDecl>(D)) {
    // Don't turn stored into computed properties. This could conflict with
    // exclusivity checking.
    // If there is a didSet or willSet function we allow dynamic replacement.
    if (VD->hasStorage() &&
        !VD->getParsedAccessor(AccessorKind::DidSet) &&
        !VD->getParsedAccessor(AccessorKind::WillSet))
      return;
    // Don't add dynamic to local variables.
    if (VD->getDeclContext()->isLocalContext())
      return;
    // Don't add to implicit variables.
    if (VD->isImplicit())
      return;
  }

  if (!D->getAttrs().hasAttribute<DynamicAttr>() &&
      !D->getAttrs().hasAttribute<DynamicReplacementAttr>()) {
    auto attr = new (D->getASTContext()) DynamicAttr(/*implicit=*/true);
    D->getAttrs().add(attr);
  }
}

ValueDecl *
DynamicallyReplacedDeclRequest::evaluate(Evaluator &evaluator,
                                         ValueDecl *VD) const {
  // Dynamic replacements must be explicit.
  if (VD->isImplicit())
    return nullptr;

  auto *attr = VD->getAttrs().getAttribute<DynamicReplacementAttr>();
  if (!attr) {
    // It's likely that the accessor isn't annotated but its storage is.
    if (auto *AD = dyn_cast<AccessorDecl>(VD)) {
      // Try to grab the attribute from the storage.
      attr = AD->getStorage()->getAttrs().getAttribute<DynamicReplacementAttr>();
    }

    if (!attr) {
      // Otherwise, it's not dynamically replacing anything.
      return nullptr;
    }
  }

  // If the attribute is invalid, bail.
  if (attr->isInvalid())
    return nullptr;

  // If we can lazily resolve the function, do so now.
  if (auto *LazyResolver = attr->Resolver) {
    auto decl = LazyResolver->loadDynamicallyReplacedFunctionDecl(
        attr, attr->ResolverContextData);
    attr->Resolver = nullptr;
    return decl;
  }

  auto &Ctx = VD->getASTContext();
  if (auto *AD = dyn_cast<AccessorDecl>(VD)) {
    return findReplacedAccessor(attr->getReplacedFunctionName(), AD, attr, Ctx);
  }

  if (auto *AFD = dyn_cast<AbstractFunctionDecl>(VD)) {
    return findReplacedFunction(attr->getReplacedFunctionName(), AFD,
                                attr, &Ctx.Diags);
  }

  if (auto *SD = dyn_cast<AbstractStorageDecl>(VD)) {
    return findReplacedStorageDecl(attr->getReplacedFunctionName(), SD, attr);
  }

  return nullptr;
}

ValueDecl *
SpecializeAttrTargetDeclRequest::evaluate(Evaluator &evaluator,
                                          const ValueDecl *vd,
                                          SpecializeAttr *attr) const {
  if (auto *lazyResolver = attr->resolver) {
    auto *decl =
        lazyResolver->loadTargetFunctionDecl(attr, attr->resolverContextData);
    attr->resolver = nullptr;
    return decl;
  }

  auto &ctx = vd->getASTContext();

  auto targetFunctionName = attr->getTargetFunctionName();
  if (!targetFunctionName)
    return nullptr;

  if (auto *ad = dyn_cast<AccessorDecl>(vd)) {
    return findTargetAccessor(targetFunctionName, ad, attr, ctx);
  }

  if (auto *afd = dyn_cast<AbstractFunctionDecl>(vd)) {
    return findTargetFunction(targetFunctionName, afd, attr, &ctx.Diags);
  }

  return nullptr;

}
/// Returns true if the given type conforms to `Differentiable` in the given
/// context. If `tangentVectorEqualsSelf` is true, also check whether the given
/// type satisfies `TangentVector == Self`.
static bool conformsToDifferentiable(Type type, ModuleDecl *module,
                                     bool tangentVectorEqualsSelf = false) {
  auto &ctx = module->getASTContext();
  auto *differentiableProto =
      ctx.getProtocol(KnownProtocolKind::Differentiable);
  auto conf = TypeChecker::conformsToProtocol(type, differentiableProto, module);
  if (conf.isInvalid())
    return false;
  if (!tangentVectorEqualsSelf)
    return true;
  auto tanType = conf.getTypeWitnessByName(type, ctx.Id_TangentVector);
  return type->isEqual(tanType);
}

IndexSubset *TypeChecker::inferDifferentiabilityParameters(
    AbstractFunctionDecl *AFD, GenericEnvironment *derivativeGenEnv) {
  auto *module = AFD->getParentModule();
  auto &ctx = module->getASTContext();
  auto *functionType = AFD->getInterfaceType()->castTo<AnyFunctionType>();
  auto numUncurriedParams = functionType->getNumParams();
  if (auto *resultFnType =
          functionType->getResult()->getAs<AnyFunctionType>()) {
    numUncurriedParams += resultFnType->getNumParams();
  }
  llvm::SmallBitVector parameterBits(numUncurriedParams);
  SmallVector<Type, 4> allParamTypes;

  // Returns true if the i-th parameter type is differentiable.
  auto isDifferentiableParam = [&](unsigned i) -> bool {
    if (i >= allParamTypes.size())
      return false;
    auto paramType = allParamTypes[i];
    if (derivativeGenEnv)
      paramType = derivativeGenEnv->mapTypeIntoContext(paramType);
    else
      paramType = AFD->mapTypeIntoContext(paramType);
    // Return false for existential types.
    if (paramType->isExistentialType())
      return false;
    // Return true if the type conforms to `Differentiable`.
    return conformsToDifferentiable(paramType, module);
  };

  // Get all parameter types.
  // NOTE: To be robust, result function type parameters should be added only if
  // `functionType` comes from a static/instance method, and not a free function
  // returning a function type. In practice, this code path should not be
  // reachable for free functions returning a function type.
  if (auto resultFnType = functionType->getResult()->getAs<AnyFunctionType>())
    for (auto &param : resultFnType->getParams())
      allParamTypes.push_back(param.getPlainType());
  for (auto &param : functionType->getParams())
    allParamTypes.push_back(param.getPlainType());

  // Set differentiability parameters.
  for (unsigned i : range(parameterBits.size()))
    if (isDifferentiableParam(i))
      parameterBits.set(i);

  return IndexSubset::get(ctx, parameterBits);
}

/// Computes the differentiability parameter indices from the given parsed
/// differentiability parameters for the given original or derivative
/// `AbstractFunctionDecl` and derivative generic environment. On error, emits
/// diagnostics and returns `nullptr`.
/// - If parsed parameters are empty, infer parameter indices.
/// - Otherwise, build parameter indices from parsed parameters.
/// The attribute name/location are used in diagnostics.
static IndexSubset *computeDifferentiabilityParameters(
    ArrayRef<ParsedAutoDiffParameter> parsedDiffParams,
    AbstractFunctionDecl *function, GenericEnvironment *derivativeGenEnv,
    StringRef attrName, SourceLoc attrLoc) {
  auto *module = function->getParentModule();
  auto &ctx = module->getASTContext();
  auto &diags = ctx.Diags;

  // Get function type and parameters.
  auto *functionType = function->getInterfaceType()->castTo<AnyFunctionType>();
  auto &params = *function->getParameters();
  auto numParams = function->getParameters()->size();
  auto isInstanceMethod = function->isInstanceMember();

  // Diagnose if function has no parameters.
  if (params.size() == 0) {
    // If function is not an instance method, diagnose immediately.
    if (!isInstanceMethod) {
      diags
          .diagnose(attrLoc, diag::diff_function_no_parameters,
                    function->getName())
          .highlight(function->getSignatureSourceRange());
      return nullptr;
    }
    // If function is an instance method, diagnose only if `self` does not
    // conform to `Differentiable`.
    else {
      auto selfType = function->getImplicitSelfDecl()->getInterfaceType();
      if (derivativeGenEnv)
        selfType = derivativeGenEnv->mapTypeIntoContext(selfType);
      else
        selfType = function->mapTypeIntoContext(selfType);
      if (!conformsToDifferentiable(selfType, module)) {
        diags
            .diagnose(attrLoc, diag::diff_function_no_parameters,
                      function->getName())
            .highlight(function->getSignatureSourceRange());
        return nullptr;
      }
    }
  }

  // If parsed differentiability parameters are empty, infer parameter indices
  // from the function type.
  if (parsedDiffParams.empty())
    return TypeChecker::inferDifferentiabilityParameters(function,
                                                         derivativeGenEnv);

  // Otherwise, build parameter indices from parsed differentiability
  // parameters.
  auto numUncurriedParams = functionType->getNumParams();
  if (auto *resultFnType =
          functionType->getResult()->getAs<AnyFunctionType>()) {
    numUncurriedParams += resultFnType->getNumParams();
  }
  llvm::SmallBitVector parameterBits(numUncurriedParams);
  int lastIndex = -1;
  for (unsigned i : indices(parsedDiffParams)) {
    auto paramLoc = parsedDiffParams[i].getLoc();
    switch (parsedDiffParams[i].getKind()) {
    case ParsedAutoDiffParameter::Kind::Named: {
      auto nameIter = llvm::find_if(params.getArray(), [&](ParamDecl *param) {
        return param->getName() == parsedDiffParams[i].getName();
      });
      // Parameter name must exist.
      if (nameIter == params.end()) {
        diags.diagnose(paramLoc, diag::diff_params_clause_param_name_unknown,
                       parsedDiffParams[i].getName());
        return nullptr;
      }
      // Parameter names must be specified in the original order.
      unsigned index = std::distance(params.begin(), nameIter);
      if ((int)index <= lastIndex) {
        diags.diagnose(paramLoc,
                       diag::diff_params_clause_params_not_original_order);
        return nullptr;
      }
      parameterBits.set(index);
      lastIndex = index;
      break;
    }
    case ParsedAutoDiffParameter::Kind::Self: {
      // 'self' is only applicable to instance methods.
      if (!isInstanceMethod) {
        diags.diagnose(paramLoc,
                       diag::diff_params_clause_self_instance_method_only);
        return nullptr;
      }
      // 'self' can only be the first in the list.
      if (i > 0) {
        diags.diagnose(paramLoc, diag::diff_params_clause_self_must_be_first);
        return nullptr;
      }
      parameterBits.set(parameterBits.size() - 1);
      break;
    }
    case ParsedAutoDiffParameter::Kind::Ordered: {
      auto index = parsedDiffParams[i].getIndex();
      if (index >= numParams) {
        diags.diagnose(paramLoc,
                       diag::diff_params_clause_param_index_out_of_range);
        return nullptr;
      }
      // Parameter names must be specified in the original order.
      if ((int)index <= lastIndex) {
        diags.diagnose(paramLoc,
                       diag::diff_params_clause_params_not_original_order);
        return nullptr;
      }
      parameterBits.set(index);
      lastIndex = index;
      break;
    }
    }
  }
  return IndexSubset::get(ctx, parameterBits);
}

/// Returns the `DescriptiveDeclKind` corresponding to the given `AccessorKind`.
/// Used for diagnostics.
static DescriptiveDeclKind getAccessorDescriptiveDeclKind(AccessorKind kind) {
  switch (kind) {
  case AccessorKind::Get:
    return DescriptiveDeclKind::Getter;
  case AccessorKind::Set:
    return DescriptiveDeclKind::Setter;
  case AccessorKind::Read:
    return DescriptiveDeclKind::ReadAccessor;
  case AccessorKind::Modify:
    return DescriptiveDeclKind::ModifyAccessor;
  case AccessorKind::WillSet:
    return DescriptiveDeclKind::WillSet;
  case AccessorKind::DidSet:
    return DescriptiveDeclKind::DidSet;
  case AccessorKind::Address:
    return DescriptiveDeclKind::Addressor;
  case AccessorKind::MutableAddress:
    return DescriptiveDeclKind::MutableAddressor;
  }
}

/// An abstract function declaration lookup error.
enum class AbstractFunctionDeclLookupErrorKind {
  /// No lookup candidates could be found.
  NoCandidatesFound,
  /// There are multiple valid lookup candidates.
  CandidatesAmbiguous,
  /// Lookup candidate does not have the expected type.
  CandidateTypeMismatch,
  /// Lookup candidate is in the wrong type context.
  CandidateWrongTypeContext,
  /// Lookup candidate does not have the requested accessor.
  CandidateMissingAccessor,
  /// Lookup candidate is a protocol requirement.
  CandidateProtocolRequirement,
  /// Lookup candidate could be resolved to an `AbstractFunctionDecl`.
  CandidateNotFunctionDeclaration
};

/// Returns the original function (in the context of a derivative or transpose
/// function) declaration corresponding to the given base type (optional),
/// function name, lookup context, and the expected original function type.
///
/// If the base type of the function is specified, member lookup is performed.
/// Otherwise, unqualified lookup is performed.
///
/// If the expected original function type has a generic signature, any
/// candidate with a less constrained type signature than the expected original
/// function type will be treated as a viable candidate.
///
/// If the function declaration cannot be resolved, emits a diagnostic and
/// returns nullptr.
///
/// Used for resolving the referenced declaration in `@derivative` and
/// `@transpose` attributes.
static AbstractFunctionDecl *findAutoDiffOriginalFunctionDecl(
    DeclAttribute *attr, Type baseType, DeclNameRefWithLoc funcNameWithLoc,
    DeclContext *lookupContext, NameLookupOptions lookupOptions,
    const llvm::function_ref<Optional<AbstractFunctionDeclLookupErrorKind>(
        AbstractFunctionDecl *)> &isValidCandidate,
    AnyFunctionType *expectedOriginalFnType) {
  assert(lookupContext);
  auto &ctx = lookupContext->getASTContext();
  auto &diags = ctx.Diags;

  auto funcName = funcNameWithLoc.Name;
  auto funcNameLoc = funcNameWithLoc.Loc;
  auto maybeAccessorKind = funcNameWithLoc.AccessorKind;

  // Perform lookup.
  LookupResult results;
  // If `baseType` is not null but `lookupContext` is a type context, set
  // `baseType` to the `self` type of `lookupContext` to perform member lookup.
  if (!baseType && lookupContext->isTypeContext())
    baseType = lookupContext->getSelfTypeInContext();
  if (baseType) {
    results = TypeChecker::lookupMember(lookupContext, baseType, funcName);
  } else {
    results = TypeChecker::lookupUnqualified(
        lookupContext, funcName, funcNameLoc.getBaseNameLoc(), lookupOptions);
  }

  // Error if no candidates were found.
  if (results.empty()) {
    diags.diagnose(funcNameLoc, diag::cannot_find_in_scope, funcName,
                   funcName.isOperator());
    return nullptr;
  }

  // Track invalid and valid candidates.
  using LookupErrorKind = AbstractFunctionDeclLookupErrorKind;
  SmallVector<std::pair<ValueDecl *, LookupErrorKind>, 2> invalidCandidates;
  SmallVector<AbstractFunctionDecl *, 2> validCandidates;

  // Filter lookup results.
  for (auto choice : results) {
    auto *decl = choice.getValueDecl();
    // Cast the candidate to an `AbstractFunctionDecl`.
    auto *candidate = dyn_cast<AbstractFunctionDecl>(decl);
    // If the candidate is an `AbstractStorageDecl`, use one of its accessors as
    // the candidate.
    if (auto *asd = dyn_cast<AbstractStorageDecl>(decl)) {
      // If accessor kind is specified, use corresponding accessor from the
      // candidate. Otherwise, use the getter by default.
      auto accessorKind = maybeAccessorKind.getValueOr(AccessorKind::Get);
      candidate = asd->getOpaqueAccessor(accessorKind);
      // Error if candidate is missing the requested accessor.
      if (!candidate) {
        invalidCandidates.push_back(
            {decl, LookupErrorKind::CandidateMissingAccessor});
        continue;
      }
    }
    // Error if the candidate is not an `AbstractStorageDecl` but an accessor is
    // requested.
    else if (maybeAccessorKind.hasValue()) {
      invalidCandidates.push_back(
          {decl, LookupErrorKind::CandidateMissingAccessor});
      continue;
    }
    // Error if candidate is not a `AbstractFunctionDecl`.
    if (!candidate) {
      invalidCandidates.push_back(
          {decl, LookupErrorKind::CandidateNotFunctionDeclaration});
      continue;
    }
    // Error if candidate is not valid.
    auto invalidCandidateKind = isValidCandidate(candidate);
    if (invalidCandidateKind.hasValue()) {
      invalidCandidates.push_back({candidate, *invalidCandidateKind});
      continue;
    }
    // Otherwise, record valid candidate.
    validCandidates.push_back(candidate);
  }
  // If there are no valid candidates, emit diagnostics for invalid candidates.
  if (validCandidates.empty()) {
    assert(!invalidCandidates.empty());
    diags.diagnose(funcNameLoc, diag::autodiff_attr_original_decl_none_valid,
                   funcName);
    for (auto invalidCandidatePair : invalidCandidates) {
      auto *invalidCandidate = invalidCandidatePair.first;
      auto invalidCandidateKind = invalidCandidatePair.second;
      auto declKind = invalidCandidate->getDescriptiveKind();
      switch (invalidCandidateKind) {
      case AbstractFunctionDeclLookupErrorKind::NoCandidatesFound:
        diags.diagnose(invalidCandidate, diag::cannot_find_in_scope, funcName,
                       funcName.isOperator());
        break;
      case AbstractFunctionDeclLookupErrorKind::CandidatesAmbiguous:
        diags.diagnose(invalidCandidate, diag::attr_ambiguous_reference_to_decl,
                       funcName, attr->getAttrName());
        break;
      case AbstractFunctionDeclLookupErrorKind::CandidateTypeMismatch: {
        // If the expected original function type has a generic signature, emit
        // "candidate does not have type equal to or less constrained than ..."
        // diagnostic.
        //
        // This is significant because derivative/transpose functions may have
        // more constrained generic signatures than their referenced original
        // declarations.
        if (auto genSig = expectedOriginalFnType->getOptGenericSignature()) {
          diags.diagnose(invalidCandidate,
                         diag::autodiff_attr_original_decl_type_mismatch,
                         declKind, expectedOriginalFnType,
                         /*hasGenericSignature*/ true);
          break;
        }
        // Otherwise, emit a "candidate does not have expected type ..." error.
        diags.diagnose(invalidCandidate,
                       diag::autodiff_attr_original_decl_type_mismatch,
                       declKind, expectedOriginalFnType,
                       /*hasGenericSignature*/ false);
        break;
      }
      case AbstractFunctionDeclLookupErrorKind::CandidateWrongTypeContext:
        diags.diagnose(invalidCandidate,
                       diag::autodiff_attr_original_decl_not_same_type_context,
                       declKind);
        break;
      case AbstractFunctionDeclLookupErrorKind::CandidateMissingAccessor: {
        auto accessorKind = maybeAccessorKind.getValueOr(AccessorKind::Get);
        auto accessorDeclKind = getAccessorDescriptiveDeclKind(accessorKind);
        diags.diagnose(invalidCandidate,
                       diag::autodiff_attr_original_decl_missing_accessor,
                       declKind, accessorDeclKind);
        break;
      }
      case AbstractFunctionDeclLookupErrorKind::CandidateProtocolRequirement:
        diags.diagnose(invalidCandidate,
                       diag::derivative_attr_protocol_requirement_unsupported);
        break;
      case AbstractFunctionDeclLookupErrorKind::CandidateNotFunctionDeclaration:
        diags.diagnose(invalidCandidate,
                       diag::autodiff_attr_original_decl_invalid_kind,
                       declKind);
        break;
      }
    }
    return nullptr;
  }
  // Error if there are multiple valid candidates.
  if (validCandidates.size() > 1) {
    diags.diagnose(funcNameLoc, diag::autodiff_attr_original_decl_ambiguous,
                   funcName);
    for (auto *validCandidate : validCandidates) {
      auto declKind = validCandidate->getDescriptiveKind();
      diags.diagnose(validCandidate,
                     diag::autodiff_attr_original_decl_ambiguous_candidate,
                     declKind);
    }
    return nullptr;
  }
  // Success if there is one unambiguous valid candidate.
  return validCandidates.front();
}

/// Checks that the `candidate` function type equals the `required` function
/// type, disregarding parameter labels and tuple result labels.
/// `checkGenericSignature` is used to check generic signatures, if specified.
/// Otherwise, generic signatures are checked for equality.
static bool checkFunctionSignature(
    CanAnyFunctionType required, CanType candidate) {
  // Check that candidate is actually a function.
  auto candidateFnTy = dyn_cast<AnyFunctionType>(candidate);
  if (!candidateFnTy)
    return false;

  // Erase dynamic self types.
  required = dyn_cast<AnyFunctionType>(required->getCanonicalType());
  candidateFnTy = dyn_cast<AnyFunctionType>(candidateFnTy->getCanonicalType());

  // Check that generic signatures match.
  auto requiredGenSig = required.getOptGenericSignature();
  auto candidateGenSig = candidateFnTy.getOptGenericSignature();
  // Check that the candidate signature's generic parameters are a subset of
  // those of the required signature.
  if (requiredGenSig && candidateGenSig &&
      candidateGenSig.getGenericParams().size()
          > requiredGenSig.getGenericParams().size())
    return false;
  // Check that the requirements are satisfied.
  if (!candidateGenSig.requirementsNotSatisfiedBy(requiredGenSig).empty())
    return false;

  // Check that parameter types match, disregarding labels.
  if (required->getNumParams() != candidateFnTy->getNumParams())
    return false;
  if (!std::equal(required->getParams().begin(), required->getParams().end(),
                  candidateFnTy->getParams().begin(),
                  [&](AnyFunctionType::Param x, AnyFunctionType::Param y) {
                    auto xInstanceTy = x.getOldType()->getMetatypeInstanceType();
                    auto yInstanceTy = y.getOldType()->getMetatypeInstanceType();
                    return xInstanceTy->isEqual(
                        requiredGenSig.getCanonicalTypeInContext(yInstanceTy));
                  }))
    return false;

  // If required result type is not a function type, check that result types
  // match exactly.
  auto requiredResultFnTy = dyn_cast<AnyFunctionType>(required.getResult());
  auto candidateResultTy =
      requiredGenSig.getCanonicalTypeInContext(candidateFnTy.getResult());
  if (!requiredResultFnTy) {
    auto requiredResultTupleTy = dyn_cast<TupleType>(required.getResult());
    auto candidateResultTupleTy = dyn_cast<TupleType>(candidateResultTy);
    if (!requiredResultTupleTy || !candidateResultTupleTy)
      return required.getResult()->isEqual(candidateResultTy);
    // If result types are tuple types, check that element types match,
    // ignoring labels.
    if (requiredResultTupleTy->getNumElements() !=
        candidateResultTupleTy->getNumElements())
      return false;
    return std::equal(requiredResultTupleTy.getElementTypes().begin(),
                      requiredResultTupleTy.getElementTypes().end(),
                      candidateResultTupleTy.getElementTypes().begin(),
                      [](CanType x, CanType y) { return x->isEqual(y); });
  }

  // Required result type is a function. Recurse.
  return checkFunctionSignature(requiredResultFnTy, candidateResultTy);
}

/// Returns an `AnyFunctionType` from the given parameters, result type, and
/// generic signature.
static AnyFunctionType *
makeFunctionType(ArrayRef<AnyFunctionType::Param> parameters, Type resultType,
                 GenericSignature genericSignature) {
  // FIXME: Verify ExtInfo state is correct, not working by accident.
  if (genericSignature) {
    GenericFunctionType::ExtInfo info;
    return GenericFunctionType::get(genericSignature, parameters, resultType,
                                    info);
  }
  FunctionType::ExtInfo info;
  return FunctionType::get(parameters, resultType, info);
}

/// Computes the original function type corresponding to the given derivative
/// function type. Used for `@derivative` attribute type-checking.
static AnyFunctionType *
getDerivativeOriginalFunctionType(AnyFunctionType *derivativeFnTy) {
  // Unwrap curry levels. At most, two parameter lists are necessary, for
  // curried method types with a `(Self)` parameter list.
  SmallVector<AnyFunctionType *, 2> curryLevels;
  auto *currentLevel = derivativeFnTy;
  for (unsigned i : range(2)) {
    (void)i;
    if (currentLevel == nullptr)
      break;
    curryLevels.push_back(currentLevel);
    currentLevel = currentLevel->getResult()->getAs<AnyFunctionType>();
  }

  auto derivativeResult = curryLevels.back()->getResult()->getAs<TupleType>();
  assert(derivativeResult && derivativeResult->getNumElements() == 2 &&
         "Expected derivative result to be a two-element tuple");
  auto originalResult = derivativeResult->getElement(0).getType();
  auto *originalType = makeFunctionType(
      curryLevels.back()->getParams(), originalResult,
      curryLevels.size() == 1 ? derivativeFnTy->getOptGenericSignature()
                              : nullptr);

  // Wrap the derivative function type in additional curry levels.
  auto curryLevelsWithoutLast =
      ArrayRef<AnyFunctionType *>(curryLevels).drop_back(1);
  for (auto pair : enumerate(llvm::reverse(curryLevelsWithoutLast))) {
    unsigned i = pair.index();
    AnyFunctionType *curryLevel = pair.value();
    originalType =
        makeFunctionType(curryLevel->getParams(), originalType,
                         i == curryLevelsWithoutLast.size() - 1
                             ? derivativeFnTy->getOptGenericSignature()
                             : nullptr);
  }
  return originalType;
}

/// Computes the original function type corresponding to the given transpose
/// function type. Used for `@transpose` attribute type-checking.
static AnyFunctionType *
getTransposeOriginalFunctionType(AnyFunctionType *transposeFnType,
                                 IndexSubset *linearParamIndices,
                                 bool wrtSelf) {
  unsigned transposeParamsIndex = 0;

  // Get the transpose function's parameters and result type.
  auto transposeParams = transposeFnType->getParams();
  auto transposeResult = transposeFnType->getResult();
  bool isCurried = transposeResult->is<AnyFunctionType>();
  if (isCurried) {
    auto methodType = transposeResult->castTo<AnyFunctionType>();
    transposeParams = methodType->getParams();
    transposeResult = methodType->getResult();
  }

  // Get the original function's result type.
  // The original result type is always equal to the type of the last
  // parameter of the transpose function type.
  auto originalResult = transposeParams.back().getPlainType();

  // Get transposed result types.
  // The transpose function result type may be a singular type or a tuple type.
  SmallVector<TupleTypeElt, 4> transposeResultTypes;
  if (auto transposeResultTupleType = transposeResult->getAs<TupleType>()) {
    transposeResultTypes.append(transposeResultTupleType->getElements().begin(),
                                transposeResultTupleType->getElements().end());
  } else {
    transposeResultTypes.push_back(transposeResult);
  }

  // Get the `Self` type, if the transpose function type is curried.
  // - If `self` is a linearity parameter, use the first transpose result type.
  // - Otherwise, use the first transpose parameter type.
  unsigned transposeResultTypesIndex = 0;
  Type selfType;
  if (isCurried && wrtSelf) {
    selfType = transposeResultTypes.front().getType();
    ++transposeResultTypesIndex;
  } else if (isCurried) {
    selfType = transposeFnType->getParams().front().getPlainType();
  }

  // Get the original function's parameters.
  SmallVector<AnyFunctionType::Param, 8> originalParams;
  // The number of original parameters is equal to the sum of:
  // - The number of original non-transposed parameters.
  //   - This is the number of transpose parameters minus one. All transpose
  //     parameters come from the original function, except the last parameter
  //     (the transposed original result).
  // - The number of original transposed parameters.
  //   - This is the number of linearity parameters.
  unsigned originalParameterCount =
      transposeParams.size() - 1 + linearParamIndices->getNumIndices();
  // Iterate over all original parameter indices.
  for (auto i : range(originalParameterCount)) {
    // Skip `self` parameter if `self` is a linearity parameter.
    // The `self` is handled specially later to form a curried function type.
    bool isSelfParameterAndWrtSelf =
        wrtSelf && i == linearParamIndices->getCapacity() - 1;
    if (isSelfParameterAndWrtSelf)
      continue;
    // If `i` is a linearity parameter index, the next original parameter is
    // the next transpose result.
    if (linearParamIndices->contains(i)) {
      auto resultType =
          transposeResultTypes[transposeResultTypesIndex++].getType();
      originalParams.push_back(AnyFunctionType::Param(resultType));
    }
    // Otherwise, the next original parameter is the next transpose parameter.
    else {
      originalParams.push_back(transposeParams[transposeParamsIndex++]);
    }
  }

  // Compute the original function type.
  AnyFunctionType *originalType;
  // If the transpose type is curried, the original function type is:
  // `(Self) -> (<original parameters>) -> <original result>`.
  if (isCurried) {
    assert(selfType && "`Self` type should be resolved");
    originalType = makeFunctionType(originalParams, originalResult, nullptr);
    originalType =
        makeFunctionType(AnyFunctionType::Param(selfType), originalType,
                         transposeFnType->getOptGenericSignature());
  }
  // Otherwise, the original function type is simply:
  // `(<original parameters>) -> <original result>`.
  else {
    originalType = makeFunctionType(originalParams, originalResult,
                                    transposeFnType->getOptGenericSignature());
  }
  return originalType;
}

/// Given a `@differentiable` attribute, attempts to resolve the original
/// `AbstractFunctionDecl` for which it is registered, using the declaration
/// on which it is actually declared. On error, emits diagnostic and returns
/// `nullptr`.
AbstractFunctionDecl *
resolveDifferentiableAttrOriginalFunction(DifferentiableAttr *attr) {
  auto *D = attr->getOriginalDeclaration();
  assert(D &&
         "Original declaration should be resolved by parsing/deserialization");
  auto *original = dyn_cast<AbstractFunctionDecl>(D);
  if (auto *asd = dyn_cast<AbstractStorageDecl>(D)) {
    // If `@differentiable` attribute is declared directly on a
    // `AbstractStorageDecl` (a stored/computed property or subscript),
    // forward the attribute to the storage's getter.
    // TODO(TF-129): Forward `@differentiable` attributes to setters after
    // differentiation supports inout parameters.
    // TODO(TF-1080): Forward `@differentiable` attributes to `read` and
    // `modify` accessors after differentiation supports `inout` parameters.
    if (!asd->getDeclContext()->isModuleScopeContext()) {
      original = asd->getSynthesizedAccessor(AccessorKind::Get);
    } else {
      original = nullptr;
    }
  }
  // Non-`get` accessors are not yet supported: `set`, `read`, and `modify`.
  // TODO(TF-1080): Enable `read` and `modify` when differentiation supports
  // coroutines.
  if (auto *accessor = dyn_cast_or_null<AccessorDecl>(original))
    if (!accessor->isGetter() && !accessor->isSetter())
      original = nullptr;
  // Diagnose if original `AbstractFunctionDecl` could not be resolved.
  if (!original) {
    diagnoseAndRemoveAttr(D, attr, diag::invalid_decl_attribute, attr);
    attr->setInvalid();
    return nullptr;
  }
  // If the original function has an error interface type, return.
  // A diagnostic should have already been emitted.
  if (original->getInterfaceType()->hasError())
    return nullptr;
  return original;
}

/// Given a `@differentiable` attribute, attempts to resolve the derivative
/// generic signature. The derivative generic signature is returned as
/// `derivativeGenSig`. On error, emits diagnostic, assigns `nullptr` to
/// `derivativeGenSig`, and returns true.
bool resolveDifferentiableAttrDerivativeGenericSignature(
    DifferentiableAttr *attr, AbstractFunctionDecl *original,
    GenericSignature &derivativeGenSig) {
  derivativeGenSig = nullptr;

  auto &ctx = original->getASTContext();
  auto &diags = ctx.Diags;

  bool isOriginalProtocolRequirement =
      isa<ProtocolDecl>(original->getDeclContext()) &&
      original->isProtocolRequirement();

  // Compute the derivative generic signature for the `@differentiable`
  // attribute:
  // - If the `@differentiable` attribute has a `where` clause, use it to
  //   compute the derivative generic signature.
  // - Otherwise, use the original function's generic signature by default.
  auto originalGenSig = original->getGenericSignature();
  derivativeGenSig = originalGenSig;

  // Handle the `where` clause, if it exists.
  // - Resolve attribute where clause requirements and store in the attribute
  //   for serialization.
  // - Compute generic signature for autodiff derivative functions based on
  //   the original function's generate signature and the attribute's where
  //   clause requirements.
  if (auto *whereClause = attr->getWhereClause()) {
    // `@differentiable` attributes on protocol requirements do not support
    // `where` clauses.
    if (isOriginalProtocolRequirement) {
      diags.diagnose(attr->getLocation(),
                     diag::differentiable_attr_protocol_req_where_clause);
      attr->setInvalid();
      return true;
    }
    if (whereClause->getRequirements().empty()) {
      // `where` clause must not be empty.
      diags.diagnose(attr->getLocation(),
                     diag::differentiable_attr_empty_where_clause);
      attr->setInvalid();
      return true;
    }

    if (!originalGenSig) {
      // `where` clauses are valid only when the original function is generic.
      diags
          .diagnose(
              attr->getLocation(),
              diag::differentiable_attr_where_clause_for_nongeneric_original,
              original->getName())
          .highlight(whereClause->getSourceRange());
      attr->setInvalid();
      return true;
    }

    InferredGenericSignatureRequest request{
        original->getParentModule(),
        originalGenSig.getPointer(),
        /*genericParams=*/nullptr,
        WhereClauseOwner(original, attr),
        /*addedRequirements=*/{},
        /*inferenceSources=*/{},
        /*allowConcreteParams=*/true};

    // Compute generic signature for derivative functions.
    derivativeGenSig = evaluateOrDefault(ctx.evaluator, request,
                                         GenericSignatureWithError())
        .getPointer();

    bool hadInvalidRequirements = false;
    for (auto req : derivativeGenSig.requirementsNotSatisfiedBy(originalGenSig)) {
      if (req.getKind() == RequirementKind::Layout) {
        // Layout requirements are not supported.
        diags
            .diagnose(attr->getLocation(),
                      diag::differentiable_attr_layout_req_unsupported);
        hadInvalidRequirements = true;
      }
    }

    if (hadInvalidRequirements) {
      attr->setInvalid();
      return true;
    }
  }

  attr->setDerivativeGenericSignature(derivativeGenSig);
  return false;
}

/// Given a `@differentiable` attribute, attempts to resolve and validate the
/// differentiability parameter indices. The parameter indices are returned as
/// `diffParamIndices`. On error, emits diagnostic, assigns `nullptr` to
/// `diffParamIndices`, and returns true.
bool resolveDifferentiableAttrDifferentiabilityParameters(
    DifferentiableAttr *attr, AbstractFunctionDecl *original,
    AnyFunctionType *originalFnRemappedTy, GenericEnvironment *derivativeGenEnv,
    IndexSubset *&diffParamIndices) {
  diffParamIndices = nullptr;
  auto &ctx = original->getASTContext();
  auto &diags = ctx.Diags;

  // Get the parsed differentiability parameter indices, which have not yet been
  // resolved. Parsed differentiability parameter indices are defined only for
  // parsed attributes.
  auto parsedDiffParams = attr->getParsedParameters();

  diffParamIndices = computeDifferentiabilityParameters(
      parsedDiffParams, original, derivativeGenEnv, attr->getAttrName(),
      attr->getLocation());
  if (!diffParamIndices) {
    attr->setInvalid();
    return true;
  }

  // Check if differentiability parameter indices are valid.
  // Do this by compute the expected differential type and checking whether
  // there is an error.
  auto expectedLinearMapTypeOrError =
      originalFnRemappedTy->getAutoDiffDerivativeFunctionLinearMapType(
          diffParamIndices, AutoDiffLinearMapKind::Differential,
          LookUpConformanceInModule(original->getModuleContext()),
          /*makeSelfParamFirst*/ true);

  // Helper for diagnosing derivative function type errors.
  auto errorHandler = [&](const DerivativeFunctionTypeError &error) {
    attr->setInvalid();
    switch (error.kind) {
    case DerivativeFunctionTypeError::Kind::NoSemanticResults:
      diags
          .diagnose(attr->getLocation(),
                    diag::autodiff_attr_original_void_result,
                    original->getName())
          .highlight(original->getSourceRange());
      return;
    case DerivativeFunctionTypeError::Kind::MultipleSemanticResults:
      diags
          .diagnose(attr->getLocation(),
                    diag::autodiff_attr_original_multiple_semantic_results)
          .highlight(original->getSourceRange());
      return;
    case DerivativeFunctionTypeError::Kind::NoDifferentiabilityParameters:
      diags.diagnose(attr->getLocation(),
                     diag::diff_params_clause_no_inferred_parameters);
      return;
    case DerivativeFunctionTypeError::Kind::
        NonDifferentiableDifferentiabilityParameter: {
      auto nonDiffParam = error.getNonDifferentiableTypeAndIndex();
      SourceLoc loc = parsedDiffParams.empty()
                          ? attr->getLocation()
                          : parsedDiffParams[nonDiffParam.second].getLoc();
      diags.diagnose(loc, diag::diff_params_clause_param_not_differentiable,
                     nonDiffParam.first);
      return;
    }
    case DerivativeFunctionTypeError::Kind::NonDifferentiableResult:
      auto nonDiffResult = error.getNonDifferentiableTypeAndIndex();
      diags.diagnose(attr->getLocation(),
                     diag::autodiff_attr_result_not_differentiable,
                     nonDiffResult.first);
      return;
    }
  };
  // Diagnose any derivative function type errors.
  if (!expectedLinearMapTypeOrError) {
    auto error = expectedLinearMapTypeOrError.takeError();
    handleAllErrors(std::move(error), errorHandler);
    return true;
  }

  return false;
}

/// Checks whether differentiable programming is enabled for the given
/// differentiation-related attribute. Returns true on error.
bool checkIfDifferentiableProgrammingEnabled(ASTContext &ctx,
                                             DeclAttribute *attr,
                                             DeclContext *DC) {
  auto &diags = ctx.Diags;
  auto *SF = DC->getParentSourceFile();
  assert(SF && "Source file not found");
  // The `Differentiable` protocol must be available.
  // If unavailable, the `_Differentiation` module should be imported.
  if (isDifferentiableProgrammingEnabled(*SF))
    return false;
  diags
      .diagnose(attr->getLocation(), diag::attr_used_without_required_module,
                attr, ctx.Id_Differentiation)
      .highlight(attr->getRangeWithAt());
  return true;
}

IndexSubset *DifferentiableAttributeTypeCheckRequest::evaluate(
    Evaluator &evaluator, DifferentiableAttr *attr) const {
  // Skip type-checking for implicit `@differentiable` attributes. We currently
  // assume that all implicit `@differentiable` attributes are valid.
  //
  // Motivation: some implicit attributes do not have a `where` clause, and this
  // function assumes that the `where` clauses exist. Propagating `where`
  // clauses and requirements consistently is a larger problem, to be revisited.
  if (attr->isImplicit())
    return nullptr;

  auto *D = attr->getOriginalDeclaration();
  auto &ctx = D->getASTContext();
  auto &diags = ctx.Diags;
  // `@differentiable` attribute requires experimental differentiable
  // programming to be enabled.
  if (checkIfDifferentiableProgrammingEnabled(ctx, attr, D->getDeclContext()))
    return nullptr;

  // Resolve the original `AbstractFunctionDecl`.
  auto *original = resolveDifferentiableAttrOriginalFunction(attr);
  if (!original)
    return nullptr;

  auto *originalFnTy = original->getInterfaceType()->castTo<AnyFunctionType>();

  // Diagnose if original function has opaque result types.
  if (auto *opaqueResultTypeDecl = original->getOpaqueResultTypeDecl()) {
    diags.diagnose(
        attr->getLocation(),
        diag::autodiff_attr_opaque_result_type_unsupported);
    attr->setInvalid();
    return nullptr;
  }

  // Diagnose if original function is an invalid class member.
  bool isOriginalClassMember = original->getDeclContext() &&
                               original->getDeclContext()->getSelfClassDecl();
  if (isOriginalClassMember) {
    auto *classDecl = original->getDeclContext()->getSelfClassDecl();
    assert(classDecl);
    // Class members returning dynamic `Self` are not supported.
    // Dynamic `Self` is supported only as a single top-level result for class
    // members. JVP/VJP functions returning `(Self, ...)` tuples would not
    // type-check.
    bool diagnoseDynamicSelfResult = original->hasDynamicSelfResult();
    if (diagnoseDynamicSelfResult) {
      // Diagnose class initializers in non-final classes.
      if (isa<ConstructorDecl>(original)) {
        if (!classDecl->isSemanticallyFinal()) {
          diags.diagnose(
              attr->getLocation(),
              diag::differentiable_attr_nonfinal_class_init_unsupported,
              classDecl->getDeclaredInterfaceType());
          attr->setInvalid();
          return nullptr;
        }
      }
      // Diagnose all other declarations returning dynamic `Self`.
      else {
        diags.diagnose(
            attr->getLocation(),
            diag::
                differentiable_attr_class_member_dynamic_self_result_unsupported);
        attr->setInvalid();
        return nullptr;
      }
    }
  }

  // Resolve the derivative generic signature.
  GenericSignature derivativeGenSig = nullptr;
  if (resolveDifferentiableAttrDerivativeGenericSignature(attr, original,
                                                          derivativeGenSig))
    return nullptr;
  auto *derivativeGenEnv = derivativeGenSig.getGenericEnvironment();

  // Compute the derivative function type.
  auto originalFnRemappedTy = originalFnTy;
  if (derivativeGenEnv)
    originalFnRemappedTy =
        derivativeGenEnv->mapTypeIntoContext(originalFnRemappedTy)
            ->castTo<AnyFunctionType>();

  // Resolve and validate the differentiability parameters.
  IndexSubset *resolvedDiffParamIndices = nullptr;
  if (resolveDifferentiableAttrDifferentiabilityParameters(
          attr, original, originalFnRemappedTy, derivativeGenEnv,
          resolvedDiffParamIndices))
    return nullptr;

  if (auto *asd = dyn_cast<AbstractStorageDecl>(D)) {
    // Remove `@differentiable` attribute from storage declaration to prevent
    // duplicate attribute registration during SILGen.
    D->getAttrs().removeAttribute(attr);
    // Transfer `@differentiable` attribute from storage declaration to
    // getter accessor.
    auto *getterDecl = asd->getOpaqueAccessor(AccessorKind::Get);
    auto *newAttr = DifferentiableAttr::create(
        getterDecl, /*implicit*/ true, attr->AtLoc, attr->getRange(),
        attr->getDifferentiabilityKind(), resolvedDiffParamIndices,
        attr->getDerivativeGenericSignature());
    auto insertion = ctx.DifferentiableAttrs.try_emplace(
        {getterDecl, resolvedDiffParamIndices}, newAttr);
    // Reject duplicate `@differentiable` attributes.
    if (!insertion.second) {
      diagnoseAndRemoveAttr(D, attr, diag::differentiable_attr_duplicate);
      diags.diagnose(insertion.first->getSecond()->getLocation(),
                     diag::differentiable_attr_duplicate_note);
      return nullptr;
    }
    getterDecl->getAttrs().add(newAttr);
    // Register derivative function configuration.
    auto *resultIndices = IndexSubset::get(ctx, 1, {0});
    getterDecl->addDerivativeFunctionConfiguration(
        {resolvedDiffParamIndices, resultIndices, derivativeGenSig});
    return resolvedDiffParamIndices;
  }
  // Reject duplicate `@differentiable` attributes.
  auto insertion =
      ctx.DifferentiableAttrs.try_emplace({D, resolvedDiffParamIndices}, attr);
  if (!insertion.second && insertion.first->getSecond() != attr) {
    diagnoseAndRemoveAttr(D, attr, diag::differentiable_attr_duplicate);
    diags.diagnose(insertion.first->getSecond()->getLocation(),
                   diag::differentiable_attr_duplicate_note);
    return nullptr;
  }
  // Register derivative function configuration.
  auto *resultIndices = IndexSubset::get(ctx, 1, {0});
  original->addDerivativeFunctionConfiguration(
      {resolvedDiffParamIndices, resultIndices, derivativeGenSig});
  return resolvedDiffParamIndices;
}

void AttributeChecker::visitDifferentiableAttr(DifferentiableAttr *attr) {
  // Call `getParameterIndices` to trigger
  // `DifferentiableAttributeTypeCheckRequest`.
  (void)attr->getParameterIndices();
}

/// Type-checks the given `@derivative` attribute `attr` on declaration `D`.
///
/// Effects are:
/// - Sets the original function and parameter indices on `attr`.
/// - Diagnoses errors.
/// - Stores the attribute in `ASTContext::DerivativeAttrs`.
///
/// \returns true on error, false on success.
static bool typeCheckDerivativeAttr(ASTContext &Ctx, Decl *D,
                                    DerivativeAttr *attr) {
  // Note: Implementation must be idempotent because it may be called multiple
  // times for the same attribute.
  auto &diags = Ctx.Diags;
  // `@derivative` attribute requires experimental differentiable programming
  // to be enabled.
  if (checkIfDifferentiableProgrammingEnabled(Ctx, attr, D->getDeclContext()))
    return true;
  auto *derivative = cast<FuncDecl>(D);
  auto originalName = attr->getOriginalFunctionName();

  auto *derivativeInterfaceType =
      derivative->getInterfaceType()->castTo<AnyFunctionType>();

  // Perform preliminary `@derivative` declaration checks.
  // The result type should be a two-element tuple.
  // Either a value and pullback:
  //     (value: R, pullback: (R.TangentVector) -> (T.TangentVector...)
  // Or a value and differential:
  //     (value: R, differential: (T.TangentVector...) -> (R.TangentVector)
  auto derivativeResultType = derivative->getResultInterfaceType();
  auto derivativeResultTupleType = derivativeResultType->getAs<TupleType>();
  if (!derivativeResultTupleType ||
      derivativeResultTupleType->getNumElements() != 2) {
    diags.diagnose(attr->getLocation(),
                   diag::derivative_attr_expected_result_tuple);
    return true;
  }
  auto valueResultElt = derivativeResultTupleType->getElement(0);
  auto funcResultElt = derivativeResultTupleType->getElement(1);
  // Get derivative kind and derivative function identifier.
  AutoDiffDerivativeFunctionKind kind;
  if (valueResultElt.getName().str() != "value") {
    diags.diagnose(attr->getLocation(),
                   diag::derivative_attr_invalid_result_tuple_value_label);
    return true;
  }
  if (funcResultElt.getName().str() == "differential") {
    kind = AutoDiffDerivativeFunctionKind::JVP;
  } else if (funcResultElt.getName().str() == "pullback") {
    kind = AutoDiffDerivativeFunctionKind::VJP;
  } else {
    diags.diagnose(attr->getLocation(),
                   diag::derivative_attr_invalid_result_tuple_func_label);
    return true;
  }
  attr->setDerivativeKind(kind);

  // Compute expected original function type and look up original function.
  auto *originalFnType =
      getDerivativeOriginalFunctionType(derivativeInterfaceType);

  // Returns true if the derivative function and original function candidate are
  // defined in compatible type contexts. If the derivative function and the
  // original function candidate have different parents, return false.
  auto hasValidTypeContext = [&](AbstractFunctionDecl *originalCandidate) {
    // Check if both functions are top-level.
    if (!derivative->getInnermostTypeContext() &&
        !originalCandidate->getInnermostTypeContext())
      return true;
    // Check if both functions are defined in the same type context.
    if (auto typeCtx1 = derivative->getInnermostTypeContext())
      if (auto typeCtx2 = originalCandidate->getInnermostTypeContext()) {
        return typeCtx1->getSelfNominalTypeDecl() ==
               typeCtx2->getSelfNominalTypeDecl();
      }
    return derivative->getParent() == originalCandidate->getParent();
  };

  auto isValidOriginalCandidate = [&](AbstractFunctionDecl *originalCandidate)
      -> Optional<AbstractFunctionDeclLookupErrorKind> {
    // Error if the original candidate is a protocol requirement. Derivative
    // registration does not yet support protocol requirements.
    // TODO(TF-982): Allow default derivative implementations for protocol
    // requirements.
    if (isa<ProtocolDecl>(originalCandidate->getDeclContext()))
      return AbstractFunctionDeclLookupErrorKind::CandidateProtocolRequirement;
    // Error if the original candidate is not defined in a type context
    // compatible with the derivative function.
    if (!hasValidTypeContext(originalCandidate))
      return AbstractFunctionDeclLookupErrorKind::CandidateWrongTypeContext;
    // Error if the original candidate does not have the expected type.
    if (!checkFunctionSignature(
            cast<AnyFunctionType>(originalFnType->getCanonicalType()),
            originalCandidate->getInterfaceType()->getCanonicalType()))
      return AbstractFunctionDeclLookupErrorKind::CandidateTypeMismatch;
    return None;
  };

  Type baseType;
  if (auto *baseTypeRepr = attr->getBaseTypeRepr()) {
    const auto options =
        TypeResolutionOptions(None) | TypeResolutionFlags::AllowModule;
    baseType = TypeResolution::resolveContextualType(
        baseTypeRepr, derivative->getDeclContext(), options,
        /*unboundTyOpener*/ nullptr,
        /*placeholderHandler*/ nullptr);
  }
  if (baseType && baseType->hasError())
    return true;
  auto lookupOptions = attr->getBaseTypeRepr()
                           ? defaultMemberLookupOptions
                           : defaultUnqualifiedLookupOptions;
  auto derivativeTypeCtx = derivative->getInnermostTypeContext();
  if (!derivativeTypeCtx)
    derivativeTypeCtx = derivative->getParent();
  assert(derivativeTypeCtx);

  // Diagnose unsupported original accessor kinds.
  // Currently, only getters and setters are supported.
  if (originalName.AccessorKind.hasValue()) {
    if (*originalName.AccessorKind != AccessorKind::Get &&
        *originalName.AccessorKind != AccessorKind::Set) {
      attr->setInvalid();
      diags.diagnose(
          originalName.Loc, diag::derivative_attr_unsupported_accessor_kind,
          getAccessorDescriptiveDeclKind(*originalName.AccessorKind));
      return true;
    }
  }

  // Look up original function.
  auto *originalAFD = findAutoDiffOriginalFunctionDecl(
      attr, baseType, originalName, derivativeTypeCtx, lookupOptions,
      isValidOriginalCandidate, originalFnType);
  if (!originalAFD) {
    attr->setInvalid();
    return true;
  }

  // Diagnose original stored properties. Stored properties cannot have custom
  // registered derivatives.
  if (auto *accessorDecl = dyn_cast<AccessorDecl>(originalAFD)) {
    // Diagnose original stored properties. Stored properties cannot have custom
    // registered derivatives.
    auto *asd = accessorDecl->getStorage();
    if (asd->hasStorage()) {
      diags.diagnose(originalName.Loc,
                     diag::derivative_attr_original_stored_property_unsupported,
                     originalName.Name);
      diags.diagnose(originalAFD->getLoc(), diag::decl_declared_here,
                     asd->getName());
      return true;
    }
    // Diagnose original class property and subscript setters.
    // TODO(SR-13096): Fix derivative function typing results regarding
    // class-typed function parameters.
    if (asd->getDeclContext()->getSelfClassDecl() &&
        accessorDecl->getAccessorKind() == AccessorKind::Set) {
      diags.diagnose(originalName.Loc,
                     diag::derivative_attr_class_setter_unsupported);
      diags.diagnose(originalAFD->getLoc(), diag::decl_declared_here,
                     asd->getName());
      return true;
    }
  }

  // Diagnose if original function has opaque result types.
  if (auto *opaqueResultTypeDecl = originalAFD->getOpaqueResultTypeDecl()) {
    diags.diagnose(
        attr->getLocation(),
        diag::autodiff_attr_opaque_result_type_unsupported);
    attr->setInvalid();
    return true;
  }

  // Diagnose if original function is an invalid class member.
  bool isOriginalClassMember =
      originalAFD->getDeclContext() &&
      originalAFD->getDeclContext()->getSelfClassDecl();
  if (isOriginalClassMember) {
    auto *classDecl = originalAFD->getDeclContext()->getSelfClassDecl();
    assert(classDecl);
    // Class members returning dynamic `Self` are not supported.
    // Dynamic `Self` is supported only as a single top-level result for class
    // members. JVP/VJP functions returning `(Self, ...)` tuples would not
    // type-check.
    bool diagnoseDynamicSelfResult = originalAFD->hasDynamicSelfResult();
    if (diagnoseDynamicSelfResult) {
      // Diagnose class initializers in non-final classes.
      if (isa<ConstructorDecl>(originalAFD)) {
        if (!classDecl->isSemanticallyFinal()) {
          diags.diagnose(attr->getLocation(),
                         diag::derivative_attr_nonfinal_class_init_unsupported,
                         classDecl->getDeclaredInterfaceType());
          return true;
        }
      }
      // Diagnose all other declarations returning dynamic `Self`.
      else {
        diags.diagnose(
            attr->getLocation(),
            diag::derivative_attr_class_member_dynamic_self_result_unsupported,
            DeclNameRef(originalAFD->getName()));
        return true;
      }
    }
  }

  attr->setOriginalFunction(originalAFD);

  // Returns true if:
  // - Original function and derivative function are static methods.
  // - Original function and derivative function are non-static methods.
  // - Original function is a Constructor declaration and derivative function is
  // a static method.
  auto compatibleStaticDecls = [&]() {
    return (isa<ConstructorDecl>(originalAFD) || originalAFD->isStatic()) ==
           derivative->isStatic();
  };

  // Diagnose if original function and derivative differ in terms of static declaration.
  if (!compatibleStaticDecls()) {
    bool derivativeMustBeStatic = !derivative->isStatic();
    diags
        .diagnose(attr->getOriginalFunctionName().Loc.getBaseNameLoc(),
                  diag::derivative_attr_static_method_mismatch_original,
                  originalAFD->getName(), derivative->getName(),
                  derivativeMustBeStatic)
        .highlight(attr->getOriginalFunctionName().Loc.getSourceRange());
    diags.diagnose(originalAFD->getNameLoc(),
                   diag::derivative_attr_static_method_mismatch_original_note,
                   originalAFD->getName(), derivativeMustBeStatic);
    auto fixItDiag =
        diags.diagnose(derivative->getStartLoc(),
                       diag::derivative_attr_static_method_mismatch_fix,
                       derivative->getName(), derivativeMustBeStatic);
    if (derivativeMustBeStatic) {
      fixItDiag.fixItInsert(derivative->getStartLoc(), "static ");
    } else {
      fixItDiag.fixItRemove(derivative->getStaticLoc());
    }
    return true;
  }

  // Returns true if:
  // - Original function and derivative function have the same access level.
  // - Original function is public and derivative function is internal
  //   `@usableFromInline`. This is the only special case.
  auto compatibleAccessLevels = [&]() {
    if (originalAFD->getFormalAccess() == derivative->getFormalAccess())
      return true;
    return originalAFD->getFormalAccess() == AccessLevel::Public &&
           (derivative->getFormalAccess() == AccessLevel::Public ||
            derivative->isUsableFromInline());
  };

  // Check access level compatibility for original and derivative functions.
  if (!compatibleAccessLevels()) {
    auto originalAccess = originalAFD->getFormalAccess();
    auto derivativeAccess =
        derivative->getFormalAccessScope().accessLevelForDiagnostics();
    diags.diagnose(originalName.Loc,
                   diag::derivative_attr_access_level_mismatch,
                   originalAFD->getName(), originalAccess,
                   derivative->getName(), derivativeAccess);
    auto fixItDiag =
        derivative->diagnose(diag::derivative_attr_fix_access, originalAccess);
    // If original access is public, suggest adding `@usableFromInline` to
    // derivative.
    if (originalAccess == AccessLevel::Public) {
      fixItDiag.fixItInsert(
          derivative->getAttributeInsertionLoc(/*forModifier*/ false),
          "@usableFromInline ");
    }
    // Otherwise, suggest changing derivative access level.
    else {
      fixItAccess(fixItDiag, derivative, originalAccess);
    }
    return true;
  }

  // Get the resolved differentiability parameter indices.
  auto *resolvedDiffParamIndices = attr->getParameterIndices();

  // Get the parsed differentiability parameter indices, which have not yet been
  // resolved. Parsed differentiability parameter indices are defined only for
  // parsed attributes.
  auto parsedDiffParams = attr->getParsedParameters();

  // If differentiability parameter indices are not resolved, compute them.
  if (!resolvedDiffParamIndices)
    resolvedDiffParamIndices = computeDifferentiabilityParameters(
        parsedDiffParams, derivative, derivative->getGenericEnvironment(),
        attr->getAttrName(), attr->getLocation());
  if (!resolvedDiffParamIndices)
    return true;

  // Set the resolved differentiability parameter indices in the attribute.
  // Differentiability parameter indices verification is done by
  // `AnyFunctionType::getAutoDiffDerivativeFunctionLinearMapType` below.
  attr->setParameterIndices(resolvedDiffParamIndices);

  // Compute the expected differential/pullback type.
  auto expectedLinearMapTypeOrError =
      originalFnType->getAutoDiffDerivativeFunctionLinearMapType(
          resolvedDiffParamIndices, kind.getLinearMapKind(),
          LookUpConformanceInModule(derivative->getModuleContext()),
          /*makeSelfParamFirst*/ true);

  // Helper for diagnosing derivative function type errors.
  auto errorHandler = [&](const DerivativeFunctionTypeError &error) {
    attr->setInvalid();
    switch (error.kind) {
    case DerivativeFunctionTypeError::Kind::NoSemanticResults:
      diags
          .diagnose(attr->getLocation(),
                    diag::autodiff_attr_original_void_result,
                    originalAFD->getName())
          .highlight(attr->getOriginalFunctionName().Loc.getSourceRange());
      return;
    case DerivativeFunctionTypeError::Kind::MultipleSemanticResults:
      diags
          .diagnose(attr->getLocation(),
                    diag::autodiff_attr_original_multiple_semantic_results)
          .highlight(attr->getOriginalFunctionName().Loc.getSourceRange());
      return;
    case DerivativeFunctionTypeError::Kind::NoDifferentiabilityParameters:
      diags.diagnose(attr->getLocation(),
                     diag::diff_params_clause_no_inferred_parameters);
      return;
    case DerivativeFunctionTypeError::Kind::
        NonDifferentiableDifferentiabilityParameter: {
      auto nonDiffParam = error.getNonDifferentiableTypeAndIndex();
      SourceLoc loc = parsedDiffParams.empty()
                          ? attr->getLocation()
                          : parsedDiffParams[nonDiffParam.second].getLoc();
      diags.diagnose(loc, diag::diff_params_clause_param_not_differentiable,
                     nonDiffParam.first);
      return;
    }
    case DerivativeFunctionTypeError::Kind::NonDifferentiableResult:
      auto nonDiffResult = error.getNonDifferentiableTypeAndIndex();
      diags.diagnose(attr->getLocation(),
                     diag::autodiff_attr_result_not_differentiable,
                     nonDiffResult.first);
      return;
    }
  };
  // Diagnose any derivative function type errors.
  if (!expectedLinearMapTypeOrError) {
    auto error = expectedLinearMapTypeOrError.takeError();
    handleAllErrors(std::move(error), errorHandler);
    return true;
  }
  Type expectedLinearMapType = expectedLinearMapTypeOrError.get();
  if (expectedLinearMapType->hasTypeParameter())
    expectedLinearMapType =
        derivative->mapTypeIntoContext(expectedLinearMapType);
  if (expectedLinearMapType->hasArchetype())
    expectedLinearMapType = expectedLinearMapType->mapTypeOutOfContext();

  // Compute the actual differential/pullback type for comparison with the
  // expected type. We must canonicalize the derivative interface type before
  // extracting the differential/pullback type from it so that types are
  // simplified via the canonical generic signature.
  CanType canActualResultType = derivativeInterfaceType->getCanonicalType();
  while (isa<AnyFunctionType>(canActualResultType)) {
    canActualResultType =
        cast<AnyFunctionType>(canActualResultType).getResult();
  }
  CanType actualLinearMapType =
      cast<TupleType>(canActualResultType).getElementType(1);

  // Check if differential/pullback type matches expected type.
  if (!actualLinearMapType->isEqual(expectedLinearMapType)) {
    // Emit differential/pullback type mismatch error on attribute.
    diags.diagnose(attr->getLocation(),
                   diag::derivative_attr_result_func_type_mismatch,
                   funcResultElt.getName(), originalAFD->getName());
    // Emit note with expected differential/pullback type on actual type
    // location.
    auto *tupleReturnTypeRepr =
        cast<TupleTypeRepr>(derivative->getResultTypeRepr());
    auto *funcEltTypeRepr = tupleReturnTypeRepr->getElementType(1);
    diags
        .diagnose(funcEltTypeRepr->getStartLoc(),
                  diag::derivative_attr_result_func_type_mismatch_note,
                  funcResultElt.getName(), expectedLinearMapType)
        .highlight(funcEltTypeRepr->getSourceRange());
    // Emit note showing original function location, if possible.
    if (originalAFD->getLoc().isValid())
      diags.diagnose(originalAFD->getLoc(),
                     diag::derivative_attr_result_func_original_note,
                     originalAFD->getName());
    return true;
  }

  // Reject duplicate `@derivative` attributes.
  auto &derivativeAttrs = Ctx.DerivativeAttrs[std::make_tuple(
      originalAFD, resolvedDiffParamIndices, kind)];
  derivativeAttrs.insert(attr);
  if (derivativeAttrs.size() > 1) {
    diags.diagnose(attr->getLocation(),
                   diag::derivative_attr_original_already_has_derivative,
                   originalAFD->getName());
    for (auto *duplicateAttr : derivativeAttrs) {
      if (duplicateAttr == attr)
        continue;
      diags.diagnose(duplicateAttr->getLocation(),
                     diag::derivative_attr_duplicate_note);
    }
    return true;
  }

  // Register derivative function configuration.
  auto *resultIndices = IndexSubset::get(Ctx, 1, {0});
  originalAFD->addDerivativeFunctionConfiguration(
      {resolvedDiffParamIndices, resultIndices,
       derivative->getGenericSignature()});

  return false;
}

void AttributeChecker::visitDerivativeAttr(DerivativeAttr *attr) {
  if (typeCheckDerivativeAttr(Ctx, D, attr))
    attr->setInvalid();
}

AbstractFunctionDecl *
DerivativeAttrOriginalDeclRequest::evaluate(Evaluator &evaluator,
                                            DerivativeAttr *attr) const {
  // If the typechecker has resolved the original function, return it.
  if (auto *FD = attr->OriginalFunction.dyn_cast<AbstractFunctionDecl *>())
    return FD;

  // If the function can be lazily resolved, do so now.
  if (auto *Resolver = attr->OriginalFunction.dyn_cast<LazyMemberLoader *>())
    return Resolver->loadReferencedFunctionDecl(attr,
                                                attr->ResolverContextData);

  return nullptr;
}

/// Computes the linearity parameter indices from the given parsed linearity
/// parameters for the given transpose function. On error, emits diagnostics and
/// returns `nullptr`.
///
/// The attribute location is used in diagnostics.
static IndexSubset *
computeLinearityParameters(ArrayRef<ParsedAutoDiffParameter> parsedLinearParams,
                           AbstractFunctionDecl *transposeFunction,
                           SourceLoc attrLoc) {
  auto &ctx = transposeFunction->getASTContext();
  auto &diags = ctx.Diags;

  // Get the transpose function type.
  auto *transposeFunctionType =
      transposeFunction->getInterfaceType()->castTo<AnyFunctionType>();
  bool isCurried = transposeFunctionType->getResult()->is<AnyFunctionType>();

  // Get transposed result types.
  // The transpose function result type may be a singular type or a tuple type.
  ArrayRef<TupleTypeElt> transposeResultTypes;
  auto transposeResultType = transposeFunctionType->getResult();
  if (isCurried)
    transposeResultType =
        transposeResultType->castTo<AnyFunctionType>()->getResult();
  if (auto resultTupleType = transposeResultType->getAs<TupleType>()) {
    transposeResultTypes = resultTupleType->getElements();
  } else {
    transposeResultTypes = ArrayRef<TupleTypeElt>(transposeResultType);
  }

  // If `self` is a linearity parameter, the transpose function must be static.
  auto isStaticMethod = transposeFunction->isStatic();
  bool wrtSelf = false;
  if (!parsedLinearParams.empty())
    wrtSelf = parsedLinearParams.front().getKind() ==
              ParsedAutoDiffParameter::Kind::Self;
  if (wrtSelf && !isStaticMethod) {
    diags.diagnose(attrLoc, diag::transpose_attr_wrt_self_must_be_static);
    return nullptr;
  }

  // Build linearity parameter indices from parsed linearity parameters.
  auto numUncurriedParams = transposeFunctionType->getNumParams();
  if (isCurried) {
    auto *resultFnType =
        transposeFunctionType->getResult()->castTo<AnyFunctionType>();
    numUncurriedParams += resultFnType->getNumParams();
  }
  auto numParams =
      numUncurriedParams + parsedLinearParams.size() - 1 - (unsigned)wrtSelf;
  SmallBitVector parameterBits(numParams);
  int lastIndex = -1;
  for (unsigned i : indices(parsedLinearParams)) {
    auto paramLoc = parsedLinearParams[i].getLoc();
    switch (parsedLinearParams[i].getKind()) {
    case ParsedAutoDiffParameter::Kind::Named: {
      diags.diagnose(paramLoc, diag::transpose_attr_cannot_use_named_wrt_params,
                     parsedLinearParams[i].getName());
      return nullptr;
    }
    case ParsedAutoDiffParameter::Kind::Self: {
      // 'self' can only be the first in the list.
      if (i > 0) {
        diags.diagnose(paramLoc, diag::diff_params_clause_self_must_be_first);
        return nullptr;
      }
      parameterBits.set(parameterBits.size() - 1);
      break;
    }
    case ParsedAutoDiffParameter::Kind::Ordered: {
      auto index = parsedLinearParams[i].getIndex();
      if (index >= numParams) {
        diags.diagnose(paramLoc,
                       diag::diff_params_clause_param_index_out_of_range);
        return nullptr;
      }
      // Parameter names must be specified in the original order.
      if ((int)index <= lastIndex) {
        diags.diagnose(paramLoc,
                       diag::diff_params_clause_params_not_original_order);
        return nullptr;
      }
      parameterBits.set(index);
      lastIndex = index;
      break;
    }
    }
  }
  return IndexSubset::get(ctx, parameterBits);
}

/// Checks if the given linearity parameter types are valid for the given
/// original function in the given derivative generic environment and module
/// context. Returns true on error.
///
/// The parsed differentiability parameters and attribute location are used in
/// diagnostics.
static bool checkLinearityParameters(
    AbstractFunctionDecl *originalAFD,
    SmallVector<AnyFunctionType::Param, 4> linearParams,
    GenericEnvironment *derivativeGenEnv, ModuleDecl *module,
    ArrayRef<ParsedAutoDiffParameter> parsedLinearParams, SourceLoc attrLoc) {
  auto &ctx = module->getASTContext();
  auto &diags = ctx.Diags;

  // Check that linearity parameters have allowed types.
  for (unsigned i : range(linearParams.size())) {
    auto linearParamType = linearParams[i].getPlainType();
    if (!linearParamType->hasTypeParameter())
      linearParamType = linearParamType->mapTypeOutOfContext();
    if (derivativeGenEnv)
      linearParamType = derivativeGenEnv->mapTypeIntoContext(linearParamType);
    else
      linearParamType = originalAFD->mapTypeIntoContext(linearParamType);
    SourceLoc loc =
        parsedLinearParams.empty() ? attrLoc : parsedLinearParams[i].getLoc();
    // Parameter must conform to `Differentiable` and satisfy
    // `Self == Self.TangentVector`.
    if (!conformsToDifferentiable(linearParamType, module,
                                  /*tangentVectorEqualsSelf*/ true)) {
      diags.diagnose(loc,
                     diag::transpose_attr_invalid_linearity_parameter_or_result,
                     linearParamType.getString(), /*isParameter*/ true);
      return true;
    }
  }
  return false;
}

/// Given a transpose function type where `self` is a linearity parameter,
/// sets `staticSelfType` and `instanceSelfType` and returns true if they are
/// equals. Otherwise, returns false.
static bool
doTransposeStaticAndInstanceSelfTypesMatch(AnyFunctionType *transposeType,
                                           Type &staticSelfType,
                                           Type &instanceSelfType) {
  // Transpose type should have the form:
  // `(StaticSelf) -> (...) -> (InstanceSelf, ...)`.
  auto methodType = transposeType->getResult()->castTo<AnyFunctionType>();
  auto transposeResult = methodType->getResult();

  // Get transposed result types.
  // The transpose function result type may be a singular type or a tuple type.
  SmallVector<TupleTypeElt, 4> transposeResultTypes;
  if (auto transposeResultTupleType = transposeResult->getAs<TupleType>()) {
    transposeResultTypes.append(transposeResultTupleType->getElements().begin(),
                                transposeResultTupleType->getElements().end());
  } else {
    transposeResultTypes.push_back(transposeResult);
  }
  assert(!transposeResultTypes.empty());

  // Get the static and instance `Self` types.
  staticSelfType = transposeType->getParams()
                       .front()
                       .getPlainType()
                       ->getMetatypeInstanceType();
  instanceSelfType = transposeResultTypes.front().getType();

  // Return true if static and instance `Self` types are equal.
  return staticSelfType->isEqual(instanceSelfType);
}

void AttributeChecker::visitTransposeAttr(TransposeAttr *attr) {
  auto *transpose = cast<FuncDecl>(D);
  auto *module = transpose->getParentModule();
  auto originalName = attr->getOriginalFunctionName();
  auto *transposeInterfaceType =
      transpose->getInterfaceType()->castTo<AnyFunctionType>();
  bool isCurried = transposeInterfaceType->getResult()->is<AnyFunctionType>();

  // Get the linearity parameter indices.
  auto *linearParamIndices = attr->getParameterIndices();

  // Get the parsed linearity parameter indices, which have not yet been
  // resolved. Parsed linearity parameter indices are defined only for parsed
  // attributes.
  auto parsedLinearParams = attr->getParsedParameters();

  // If linearity parameter indices are not resolved, compute them.
  if (!linearParamIndices)
    linearParamIndices = computeLinearityParameters(
        parsedLinearParams, transpose, attr->getLocation());
  if (!linearParamIndices) {
    attr->setInvalid();
    return;
  }

  // Diagnose empty linearity parameter indices. This occurs when no `wrt:`
  // clause is declared and no linearity parameters can be inferred.
  if (linearParamIndices->isEmpty()) {
    diagnoseAndRemoveAttr(attr,
                          diag::diff_params_clause_no_inferred_parameters);
    return;
  }

  bool wrtSelf = false;
  if (!parsedLinearParams.empty())
    wrtSelf = parsedLinearParams.front().getKind() ==
              ParsedAutoDiffParameter::Kind::Self;

  // If the transpose function is curried and `self` is a linearity parameter,
  // check that the instance and static `Self` types are equal.
  Type staticSelfType, instanceSelfType;
  bool doSelfTypesMatch = false;
  if (isCurried && wrtSelf) {
    doSelfTypesMatch = doTransposeStaticAndInstanceSelfTypesMatch(
        transposeInterfaceType, staticSelfType, instanceSelfType);
    if (!doSelfTypesMatch) {
      diagnose(attr->getLocation(),
               diag::transpose_attr_wrt_self_must_be_static);
      diagnose(attr->getLocation(),
               diag::transpose_attr_wrt_self_self_type_mismatch_note,
               staticSelfType, instanceSelfType);
      attr->setInvalid();
      return;
    }
  }

  auto *expectedOriginalFnType = getTransposeOriginalFunctionType(
      transposeInterfaceType, linearParamIndices, wrtSelf);

  // `R` result type must conform to `Differentiable` and satisfy
  // `Self == Self.TangentVector`.
  auto expectedOriginalResultType = expectedOriginalFnType->getResult();
  if (isCurried)
    expectedOriginalResultType =
        expectedOriginalResultType->castTo<AnyFunctionType>()->getResult();
  if (expectedOriginalResultType->hasTypeParameter())
    expectedOriginalResultType = transpose->mapTypeIntoContext(
        expectedOriginalResultType);
  if (!conformsToDifferentiable(expectedOriginalResultType, module,
                                /*tangentVectorEqualsSelf*/ true)) {
    diagnoseAndRemoveAttr(
        attr, diag::transpose_attr_invalid_linearity_parameter_or_result,
        expectedOriginalResultType.getString(), /*isParameter*/ false);
    return;
  }

  auto isValidOriginalCandidate = [&](AbstractFunctionDecl *originalCandidate)
      -> Optional<AbstractFunctionDeclLookupErrorKind> {
    // Error if the original candidate does not have the expected type.
    if (!checkFunctionSignature(
            cast<AnyFunctionType>(expectedOriginalFnType->getCanonicalType()),
            originalCandidate->getInterfaceType()->getCanonicalType()))
      return AbstractFunctionDeclLookupErrorKind::CandidateTypeMismatch;
    return None;
  };

  Type baseType;
  if (attr->getBaseTypeRepr()) {
    baseType = TypeResolution::resolveContextualType(
        attr->getBaseTypeRepr(), transpose->getDeclContext(), None,
        /*unboundTyOpener*/ nullptr,
        /*placeholderHandler*/ nullptr);
  }
  auto lookupOptions =
      (attr->getBaseTypeRepr() ? defaultMemberLookupOptions
                               : defaultUnqualifiedLookupOptions) |
      NameLookupFlags::IgnoreAccessControl;
  auto transposeTypeCtx = transpose->getInnermostTypeContext();
  if (!transposeTypeCtx) transposeTypeCtx = transpose->getParent();
  assert(transposeTypeCtx);

  // Look up original function.
  auto funcLoc = originalName.Loc.getBaseNameLoc();
  if (attr->getBaseTypeRepr())
    funcLoc = attr->getBaseTypeRepr()->getLoc();
  auto *originalAFD = findAutoDiffOriginalFunctionDecl(
      attr, baseType, originalName, transposeTypeCtx, lookupOptions,
      isValidOriginalCandidate, expectedOriginalFnType);
  if (!originalAFD) {
    attr->setInvalid();
    return;
  }
  attr->setOriginalFunction(originalAFD);

  // Diagnose if original function has opaque result types.
  if (auto *opaqueResultTypeDecl = originalAFD->getOpaqueResultTypeDecl()) {
    diagnose(attr->getLocation(),
             diag::autodiff_attr_opaque_result_type_unsupported);
    attr->setInvalid();
    return;
  }

  // Get the linearity parameter types.
  SmallVector<AnyFunctionType::Param, 4> linearParams;
  expectedOriginalFnType->getSubsetParameters(linearParamIndices, linearParams,
                                              /*reverseCurryLevels*/ true);

  // Check if linearity parameter indices are valid.
  if (checkLinearityParameters(originalAFD, linearParams,
                               transpose->getGenericEnvironment(),
                               transpose->getModuleContext(),
                               parsedLinearParams, attr->getLocation())) {
    D->getAttrs().removeAttribute(attr);
    attr->setInvalid();
    return;
  }

  // Returns true if:
  // - Original function and transpose function are static methods.
  // - Original function and transpose function are non-static methods.
  // - Original function is a Constructor declaration and transpose function is
  // a static method.
  auto compatibleStaticDecls = [&]() {
    return (isa<ConstructorDecl>(originalAFD) || originalAFD->isStatic()) ==
           transpose->isStatic();
  };

  // Diagnose if original function and transpose differ in terms of static declaration.
  if (!doSelfTypesMatch && !compatibleStaticDecls()) {
    bool transposeMustBeStatic = !transpose->isStatic();
    diagnose(attr->getOriginalFunctionName().Loc.getBaseNameLoc(),
             diag::transpose_attr_static_method_mismatch_original,
             originalAFD->getName(), transpose->getName(),
             transposeMustBeStatic)
        .highlight(attr->getOriginalFunctionName().Loc.getSourceRange());
    diagnose(originalAFD->getNameLoc(),
             diag::transpose_attr_static_method_mismatch_original_note,
             originalAFD->getName(), transposeMustBeStatic);
    auto fixItDiag = diagnose(transpose->getStartLoc(),
                              diag::transpose_attr_static_method_mismatch_fix,
                              transpose->getName(), transposeMustBeStatic);
    if (transposeMustBeStatic) {
      fixItDiag.fixItInsert(transpose->getStartLoc(), "static ");
    } else {
      fixItDiag.fixItRemove(transpose->getStaticLoc());
    }
    return;
  }

  // Set the resolved linearity parameter indices in the attribute.
  attr->setParameterIndices(linearParamIndices);
}

void AttributeChecker::visitActorAttr(ActorAttr *attr) {
  auto classDecl = dyn_cast<ClassDecl>(D);
  if (!classDecl)
    return; // already diagnosed

  (void)classDecl->isActor();
}

void AttributeChecker::visitDistributedActorAttr(DistributedActorAttr *attr) {
  auto dc = D->getDeclContext();

  // distributed can be applied to actor definitions and their methods
  if (auto varDecl = dyn_cast<VarDecl>(D)) {
    // distributed can not be applied to stored properties
    diagnoseAndRemoveAttr(attr, diag::distributed_actor_property);
    return;
  }

  // distributed can only be declared on an `actor`
  if (auto classDecl = dyn_cast<ClassDecl>(D)) {
    if (!classDecl->isActor()) {
      diagnoseAndRemoveAttr(attr, diag::distributed_actor_not_actor);
      return;
    } else {
      // good: `distributed actor`
      return;
    }
  } else if (dyn_cast<StructDecl>(D) || dyn_cast<EnumDecl>(D)) {
    diagnoseAndRemoveAttr(
        attr, diag::distributed_actor_func_not_in_distributed_actor);
    return;
  }

  if (auto funcDecl = dyn_cast<AbstractFunctionDecl>(D)) {
    // distributed functions must not be static
    if (funcDecl->isStatic()) {
      diagnoseAndRemoveAttr(attr, diag::distributed_actor_func_static);
      return;
    }

    // distributed func cannot be simultaneously nonisolated
    if (auto nonisolated =
            funcDecl->getAttrs().getAttribute<NonisolatedAttr>()) {
      diagnoseAndRemoveAttr(nonisolated,
                            diag::distributed_actor_func_nonisolated,
                            funcDecl->getName());
      return;
    }

    // distributed func must be declared inside an distributed actor
    if (dc->getSelfClassDecl() &&
        !dc->getSelfClassDecl()->isDistributedActor()) {
      diagnoseAndRemoveAttr(
          attr, diag::distributed_actor_func_not_in_distributed_actor);
      return;
    } else if (auto protoDecl = dc->getSelfProtocolDecl()){
      if (!protoDecl->inheritsFromDistributedActor()) {
        auto diag = diagnoseAndRemoveAttr(
            attr, diag::distributed_actor_func_not_in_distributed_actor);
        diagnoseDistributedFunctionInNonDistributedActorProtocol(
            protoDecl, diag);
        return;
      }
    } else if (dc->getSelfStructDecl() || dc->getSelfEnumDecl()) {
      diagnoseAndRemoveAttr(
          attr, diag::distributed_actor_func_not_in_distributed_actor);
      return;
    }
  }
}

void AttributeChecker::visitNonisolatedAttr(NonisolatedAttr *attr) {
  // 'nonisolated' can be applied to global and static/class variables
  // that do not have storage.
  auto dc = D->getDeclContext();

  if (auto var = dyn_cast<VarDecl>(D)) {
    // stored properties have limitations as to when they can be nonisolated.
    if (var->hasStorage()) {
      auto nominal = dyn_cast<NominalTypeDecl>(dc);

      // 'nonisolated' can not be applied to stored properties inside
      // distributed actors. Attempts of nonisolated access would be
      // cross-actor, and that means they might be accessing on a remote actor,
      // in which case the stored property storage does not exist.
      //
      // The synthesized "id" and "actorSystem" are the only exceptions,
      // because the implementation mirrors them.
      if (nominal && nominal->isDistributedActor() &&
          !(var->isImplicit() &&
            (var->getName() == Ctx.Id_id ||
             var->getName() == Ctx.Id_actorSystem))) {
        diagnoseAndRemoveAttr(attr,
                              diag::nonisolated_distributed_actor_storage);
        return;
      }

      // 'nonisolated' can not be applied to mutable stored properties.
      if (var->supportsMutation()) {
        diagnoseAndRemoveAttr(attr, diag::nonisolated_mutable_storage);
        return;
      }
    }

    // nonisolated can not be applied to local properties.
    if (dc->isLocalContext()) {
      diagnoseAndRemoveAttr(attr, diag::nonisolated_local_var);
      return;
    }

    // If this is a static or global variable, we're all set.
    if (dc->isModuleScopeContext() ||
        (dc->isTypeContext() && var->isStatic())) {
      return;
    }
  }
  
  // `nonisolated` on most actor initializers is invalid or redundant.
  if (auto ctor = dyn_cast<ConstructorDecl>(D)) {
    if (auto nominal = dyn_cast<NominalTypeDecl>(dc)) {
      if (nominal->isAnyActor()) {
        if (ctor->isConvenienceInit()) {
          // all convenience inits are `nonisolated` by default
          diagnoseAndRemoveAttr(attr, diag::nonisolated_actor_convenience_init)
            .warnUntilSwiftVersion(6);
          return;
          
        } else if (!ctor->hasAsync()) {
          // the isolation for a synchronous init cannot be `nonisolated`.
          diagnoseAndRemoveAttr(attr, diag::nonisolated_actor_sync_init)
            .warnUntilSwiftVersion(6);
          return;
        }
      }
    }
  }

  if (auto VD = dyn_cast<ValueDecl>(D)) {
    (void)getActorIsolation(VD);
  }
}

void AttributeChecker::visitGlobalActorAttr(GlobalActorAttr *attr) {
  auto nominal = dyn_cast<NominalTypeDecl>(D);
  if (!nominal)
    return; // already diagnosed

  (void)nominal->isGlobalActor();
}

void AttributeChecker::visitAsyncAttr(AsyncAttr *attr) {
  auto var = dyn_cast<VarDecl>(D);
  if (!var)
    return;

  auto patternBinding = var->getParentPatternBinding();
  if (!patternBinding)
    return; // already diagnosed

  // "Async" modifier can only be applied to local declarations.
  if (!patternBinding->getDeclContext()->isLocalContext()) {
    diagnoseAndRemoveAttr(attr, diag::async_let_not_local);
    return;
  }

  // Check each of the pattern binding entries.
  bool diagnosedVar = false;
  for (unsigned index : range(patternBinding->getNumPatternEntries())) {
    auto pattern = patternBinding->getPattern(index);

    // Look for variables bound by this pattern.
    bool foundAnyVariable = false;
    bool isLet = true;
    pattern->forEachVariable([&](VarDecl *var) {
      if (!var->isLet())
        isLet = false;
      foundAnyVariable = true;
    });

    // Each entry must bind at least one named variable, so that there is
    // something to "await".
    if (!foundAnyVariable) {
      diagnose(pattern->getLoc(), diag::async_let_no_variables);
      attr->setInvalid();
      return;
    }

    // Async can only be used on an "async let".
    if (!isLet && !diagnosedVar) {
      diagnose(patternBinding->getLoc(), diag::async_not_let)
        .fixItReplace(patternBinding->getLoc(), "let");
      diagnosedVar = true;
    }

    // Each pattern entry must have an initializer expression.
    if (patternBinding->getEqualLoc(index).isInvalid()) {
      diagnose(pattern->getLoc(), diag::async_let_not_initialized);
      attr->setInvalid();
      return;
    }
  }
}

void AttributeChecker::visitMarkerAttr(MarkerAttr *attr) {
  auto proto = dyn_cast<ProtocolDecl>(D);
  if (!proto)
    return;

  // A marker protocol cannot inherit a non-marker protocol.
  for (auto inheritedProto : proto->getInheritedProtocols()) {
    if (!inheritedProto->isMarkerProtocol()) {
      proto->diagnose(
          diag::marker_protocol_inherit_nonmarker,
          proto->getName(), inheritedProto->getName());
      inheritedProto->diagnose(
          diag::decl_declared_here, inheritedProto->getName());
    }
  }

  if (Type superclass = proto->getSuperclass()) {
    proto->diagnose(
        diag::marker_protocol_inherit_class,
        proto->getName(), superclass);
  }

  // A marker protocol cannot have any requirements.
  for (auto member : proto->getAllMembers()) {
    auto value = dyn_cast<ValueDecl>(member);
    if (!value)
      continue;

    if (value->isProtocolRequirement()) {
      value->diagnose(diag::marker_protocol_requirement, proto->getName());
      break;
    }
  }
}

void AttributeChecker::visitReasyncAttr(ReasyncAttr *attr) {
  // Make sure the function takes a 'throws' function argument or a
  // conformance to a '@rethrows' protocol.
  auto fn = dyn_cast<AbstractFunctionDecl>(D);
  if (fn->getPolymorphicEffectKind(EffectKind::Async)
        != PolymorphicEffectKind::Invalid) {
    return;
  }

  diagnose(attr->getLocation(), diag::reasync_without_async_parameter);
  attr->setInvalid();
}

void AttributeChecker::visitUnavailableFromAsyncAttr(
    UnavailableFromAsyncAttr *attr) {
  if (DeclContext *dc = dyn_cast<DeclContext>(D)) {
    if (dc->isAsyncContext()) {
      if (ValueDecl *vd = dyn_cast<ValueDecl>(D)) {
        D->getASTContext().Diags.diagnose(
            D->getLoc(), diag::async_named_decl_must_be_available_from_async,
            D->getDescriptiveKind(), vd->getName());
      } else {
        D->getASTContext().Diags.diagnose(
            D->getLoc(), diag::async_decl_must_be_available_from_async,
            D->getDescriptiveKind());
      }
    }
  }
}

namespace {

class ClosureAttributeChecker
    : public AttributeVisitor<ClosureAttributeChecker> {
  ASTContext &ctx;
  ClosureExpr *closure;
public:
  ClosureAttributeChecker(ClosureExpr *closure)
    : ctx(closure->getASTContext()), closure(closure) { }

  void visitDeclAttribute(DeclAttribute *attr) {
    ctx.Diags.diagnose(
        attr->getLocation(), diag::unsupported_closure_attr,
        attr->isDeclModifier(), attr->getAttrName())
      .fixItRemove(attr->getRangeWithAt());
    attr->setInvalid();
  }

  void visitSendableAttr(SendableAttr *attr) {
    // Nothing else to check.
  }

  void visitCustomAttr(CustomAttr *attr) {
    // Check whether this custom attribute is the global actor attribute.
    auto globalActorAttr = evaluateOrDefault(
        ctx.evaluator, GlobalActorAttributeRequest{closure}, None);
    if (globalActorAttr && globalActorAttr->first == attr)
      return;

    // Otherwise, it's an error.
    std::string typeName;
    if (auto typeRepr = attr->getTypeRepr()) {
      llvm::raw_string_ostream out(typeName);
      typeRepr->print(out);
    } else {
      typeName = attr->getType().getString();
    }

    ctx.Diags.diagnose(
        attr->getLocation(), diag::unsupported_closure_attr,
        attr->isDeclModifier(), typeName)
      .fixItRemove(attr->getRangeWithAt());
    attr->setInvalid();
  }
};

}

void TypeChecker::checkClosureAttributes(ClosureExpr *closure) {
  ClosureAttributeChecker checker(closure);
  for (auto attr : closure->getAttrs()) {
    checker.visit(attr);
  }
}

static bool renameCouldMatch(const ValueDecl *original,
                             const ValueDecl *candidate,
                             bool originalIsObjCVisible,
                             AccessLevel minAccess) {
  // Can't match itself
  if (original == candidate)
    return false;

  // Kinds have to match, but we want to allow eg. an accessor to match
  // a function
  if (candidate->getKind() != original->getKind() &&
      !(isa<FuncDecl>(candidate) && isa<FuncDecl>(original)))
    return false;

  // Instance can't match static/class function
  if (candidate->isInstanceMember() != original->isInstanceMember())
    return false;

  // If the original is ObjC visible then the rename must be as well
  if (originalIsObjCVisible &&
      !objc_translation::isVisibleToObjC(candidate, minAccess))
    return false;

  // @available is intended for public interfaces, so an implementation-only
  // decl shouldn't match
  if (candidate->getAttrs().hasAttribute<ImplementationOnlyAttr>())
    return false;

  return true;
}

static bool parametersMatch(const AbstractFunctionDecl *a,
                            const AbstractFunctionDecl *b) {
  auto aParams = a->getParameters();
  auto bParams = b->getParameters();

  if (aParams->size() != bParams->size())
    return false;

  for (auto index : indices(*aParams)) {
    auto aParamType = aParams->get(index)->getType();
    auto bParamType = bParams->get(index)->getType();
    if (!aParamType->matchesParameter(bParamType, TypeMatchOptions()))
      return false;
  }
  return true;
}

ValueDecl *RenamedDeclRequest::evaluate(Evaluator &evaluator,
                                        const ValueDecl *attached,
                                        const AvailableAttr *attr) const {
  if (!attached || !attr)
    return nullptr;

  if (attr->RenameDecl)
    return attr->RenameDecl;

  if (attr->Rename.empty())
    return nullptr;

  auto attachedContext = attached->getDeclContext();
  auto parsedName = parseDeclName(attr->Rename);
  auto nameRef = parsedName.formDeclNameRef(attached->getASTContext());

  // Handle types separately
  if (isa<NominalTypeDecl>(attached)) {
    if (!parsedName.ContextName.empty())
      return nullptr;

    SmallVector<ValueDecl *, 1> lookupResults;
    attachedContext->lookupQualified(attachedContext->getParentModule(),
                                     nameRef.withoutArgumentLabels(),
                                     NL_OnlyTypes, lookupResults);
    if (lookupResults.size() == 1)
      return lookupResults[0];
    return nullptr;
  }

  auto minAccess = AccessLevel::Private;
  if (attached->getModuleContext()->isExternallyConsumed())
    minAccess = AccessLevel::Public;
  bool attachedIsObjcVisible =
      objc_translation::isVisibleToObjC(attached, minAccess);

  SmallVector<ValueDecl *, 4> lookupResults;
  SmallVector<AbstractFunctionDecl *, 4> asyncResults;
  lookupReplacedDecl(nameRef, attr, attached, lookupResults);

  ValueDecl *renamedDecl = nullptr;
  auto attachedFunc = dyn_cast<AbstractFunctionDecl>(attached);
  for (auto candidate : lookupResults) {
    // If the name is a getter or setter, grab the underlying accessor (if any)
    if (parsedName.IsGetter || parsedName.IsSetter) {
      auto *VD = dyn_cast<VarDecl>(candidate);
      if (!VD)
        continue;

      candidate = VD->getAccessor(parsedName.IsGetter ? AccessorKind::Get :
                                                        AccessorKind::Set);
      if (!candidate)
        continue;
    }

    if (!renameCouldMatch(attached, candidate, attachedIsObjcVisible,
                          minAccess))
      continue;

    if (auto *candidateFunc = dyn_cast<AbstractFunctionDecl>(candidate)) {
      // Require both functions to be async/not. Async alternatives are handled
      // below if there's no other matches
      if (attachedFunc->hasAsync() != candidateFunc->hasAsync()) {
        if (candidateFunc->hasAsync())
          asyncResults.push_back(candidateFunc);
        continue;
      }

      // Require matching parameters for functions, unless there's only a single
      // match
      if (lookupResults.size() > 1 &&
          !parametersMatch(attachedFunc, candidateFunc))
        continue;
    }

    // Do not match if there are any duplicates
    if (renamedDecl) {
      renamedDecl = nullptr;
      break;
    }
    renamedDecl = candidate;
  }

  // Try to match up an async alternative instead (ie. one where the
  // completion handler has been removed).
  if (!renamedDecl && !asyncResults.empty()) {
    for (AbstractFunctionDecl *candidate : asyncResults) {
      Optional<unsigned> completionHandler =
          attachedFunc->findPotentialCompletionHandlerParam(candidate);
      if (!completionHandler)
        continue;

      // TODO: Check the result of the async function matches the parameters
      //       of the completion handler?

      // Do not match if there are any duplicates
      if (renamedDecl) {
        renamedDecl = nullptr;
        break;
      }
      renamedDecl = candidate;
    }
  }

  return renamedDecl;
}