lib/Sema/TypeCheckType.cpp

//===--- TypeCheckType.cpp - Type Validation ------------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2018 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 validation for Swift types, emitting semantic errors as
// appropriate and checking default initializer values.
//
//===----------------------------------------------------------------------===//

#include "TypeChecker.h"
#include "TypeCheckAvailability.h"
#include "TypeCheckProtocol.h"
#include "TypeCheckType.h"
#include "TypoCorrection.h"

#include "swift/Strings.h"
#include "swift/AST/ASTDemangler.h"
#include "swift/AST/ASTVisitor.h"
#include "swift/AST/ASTWalker.h"
#include "swift/AST/DiagnosticsParse.h"
#include "swift/AST/ExistentialLayout.h"
#include "swift/AST/ForeignErrorConvention.h"
#include "swift/AST/GenericEnvironment.h"
#include "swift/AST/GenericSignatureBuilder.h"
#include "swift/AST/NameLookup.h"
#include "swift/AST/ParameterList.h"
#include "swift/AST/PrettyStackTrace.h"
#include "swift/AST/ProtocolConformance.h"
#include "swift/AST/TypeLoc.h"
#include "swift/AST/TypeResolutionStage.h"
#include "swift/Basic/SourceManager.h"
#include "swift/Basic/Statistic.h"
#include "swift/Basic/StringExtras.h"
#include "swift/ClangImporter/ClangImporter.h"
#include "llvm/ADT/APInt.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallString.h"
#include "llvm/ADT/StringSwitch.h"
#include "llvm/ADT/Twine.h"
#include "llvm/Support/SaveAndRestore.h"

using namespace swift;

#define DEBUG_TYPE "TypeCheckType"

/// Type resolution.

TypeResolution TypeResolution::forStructural(DeclContext *dc) {
  return TypeResolution(dc, TypeResolutionStage::Structural);
}

TypeResolution TypeResolution::forInterface(DeclContext *dc,
                                            LazyResolver *resolver) {
  return forInterface(dc, dc->getGenericSignatureOfContext(), resolver);
}

TypeResolution TypeResolution::forInterface(DeclContext *dc,
                                            GenericSignature *genericSig,
                                            LazyResolver *resolver) {
  TypeResolution result(dc, TypeResolutionStage::Interface);
  result.Resolver = resolver;
  result.complete.genericSig = genericSig;
  result.complete.builder = nullptr;
  return result;
}

TypeResolution TypeResolution::forContextual(DeclContext *dc) {
  return forContextual(dc, dc->getGenericEnvironmentOfContext());
}

TypeResolution TypeResolution::forContextual(DeclContext *dc,
                                             GenericEnvironment *genericEnv) {
  TypeResolution result(dc, TypeResolutionStage::Contextual);
  result.genericEnv = genericEnv;
  return result;
}

ASTContext &TypeResolution::getASTContext() const {
  return dc->getASTContext();
}

GenericSignatureBuilder *TypeResolution::getGenericSignatureBuilder() const {
  assert(stage == TypeResolutionStage::Interface);
  if (!complete.builder) {
    auto genericSig = getGenericSignature();
    ASTContext &ctx = genericSig->getASTContext();
    complete.builder = ctx.getOrCreateGenericSignatureBuilder(
                                          genericSig->getCanonicalSignature());

  }

  return complete.builder;
}

GenericSignature *TypeResolution::getGenericSignature() const {
  switch (stage) {
  case TypeResolutionStage::Contextual:
    return dc->getGenericSignatureOfContext();

  case TypeResolutionStage::Interface:
    if (complete.genericSig)
      return complete.genericSig;

    return dc->getGenericSignatureOfContext();

  case TypeResolutionStage::Structural:
    return nullptr;
  }
  llvm_unreachable("unhandled stage");
}

bool TypeResolution::usesArchetypes() const {
  switch (stage) {
  case TypeResolutionStage::Structural:
  case TypeResolutionStage::Interface:
    return false;

  case TypeResolutionStage::Contextual:
    return true;
  }
  llvm_unreachable("unhandled stage");
}

Type TypeResolution::mapTypeIntoContext(Type type) const {
  switch (stage) {
  case TypeResolutionStage::Structural:
  case TypeResolutionStage::Interface:
    return type;

  case TypeResolutionStage::Contextual:
    return GenericEnvironment::mapTypeIntoContext(genericEnv, type);
  }
  llvm_unreachable("unhandled stage");
}

Type TypeResolution::resolveDependentMemberType(
                                          Type baseTy, DeclContext *DC,
                                          SourceRange baseRange,
                                          ComponentIdentTypeRepr *ref) const {
  switch (stage) {
  case TypeResolutionStage::Structural:
    return DependentMemberType::get(baseTy, ref->getIdentifier());

  case TypeResolutionStage::Contextual:
    llvm_unreachable("Dependent type after archetype substitution");

  case TypeResolutionStage::Interface:
    // Handled below.
    break;
  }

  assert(stage == TypeResolutionStage::Interface);
  if (!getGenericSignature())
    return ErrorType::get(baseTy);

  auto builder = getGenericSignatureBuilder();
  auto baseEquivClass =
    builder->resolveEquivalenceClass(
                                baseTy,
                                ArchetypeResolutionKind::CompleteWellFormed);
  if (!baseEquivClass)
    return ErrorType::get(baseTy);

  ASTContext &ctx = baseTy->getASTContext();

  // Look for a nested type with the given name.
  if (auto nestedType =
          baseEquivClass->lookupNestedType(*builder, ref->getIdentifier())) {
    // Record the type we found.
    ref->setValue(nestedType, nullptr);
  } else {
    // Resolve the base to a potential archetype.
    // Perform typo correction.
    TypeChecker &tc = static_cast<TypeChecker &>(*ctx.getLazyResolver());
    TypoCorrectionResults corrections(tc, ref->getIdentifier(),
                                      DeclNameLoc(ref->getIdLoc()));
    tc.performTypoCorrection(DC, DeclRefKind::Ordinary,
                             MetatypeType::get(baseTy),
                             NameLookupFlags::ProtocolMembers,
                             corrections, builder);

    // Check whether we have a single type result.
    auto singleType = cast_or_null<TypeDecl>(
      corrections.getUniqueCandidateMatching([](ValueDecl *result) {
        return isa<TypeDecl>(result);
      }));

    // If we don't have a single result, complain and fail.
    if (!singleType) {
      Identifier name = ref->getIdentifier();
      SourceLoc nameLoc = ref->getIdLoc();
      ctx.Diags.diagnose(nameLoc, diag::invalid_member_type, name, baseTy)
        .highlight(baseRange);
      corrections.noteAllCandidates();

      return ErrorType::get(ctx);
    }

    // We have a single type result. Suggest it.
    ctx.Diags.diagnose(ref->getIdLoc(), diag::invalid_member_type_suggest,
                       baseTy, ref->getIdentifier(),
                       singleType->getBaseName().getIdentifier())
      .fixItReplace(ref->getIdLoc(),
                    singleType->getBaseName().userFacingName());

    // Correct to the single type result.
    ref->overwriteIdentifier(singleType->getBaseName().getIdentifier());
    ref->setValue(singleType, nullptr);
  }

  // If the nested type has been resolved to an associated type, use it.
  if (auto assocType = dyn_cast<AssociatedTypeDecl>(ref->getBoundDecl())) {
    return DependentMemberType::get(baseTy, assocType);
  }

  // Otherwise, the nested type comes from a concrete type,
  // or it's a typealias declared in protocol or protocol extension.
  // Substitute the base type into it.
  auto concrete = ref->getBoundDecl();
  auto lazyResolver = ctx.getLazyResolver();
  if (lazyResolver)
    lazyResolver->resolveDeclSignature(concrete);
  if (!concrete->hasInterfaceType())
    return ErrorType::get(ctx);

  // Make sure that base type didn't get replaced along the way.
  assert(baseTy->isTypeParameter());

  // There are two situations possible here:
  //
  // 1. Member comes from the protocol, which means that it has been
  //    found through a conformance constraint placed on base e.g. `T: P`.
  //    In this case member is a `typealias` declaration located in
  //    protocol or protocol extension.
  //
  // 2. Member comes from struct/enum/class type, which means that it
  //    has been found through same-type constraint on base e.g. `T == Q`.
  //
  // If this is situation #2 we need to make sure to switch base to
  // a concrete type (according to equivalence class) otherwise we'd
  // end up using incorrect generic signature while attempting to form
  // a substituted type for the member we found.
  if (!concrete->getDeclContext()->getSelfProtocolDecl()) {
    baseTy = baseEquivClass->concreteType ? baseEquivClass->concreteType
                                          : baseEquivClass->superclass;
    assert(baseTy);
  }

  return TypeChecker::substMemberTypeWithBase(DC->getParentModule(), concrete,
                                              baseTy);
}

Type TypeResolution::resolveSelfAssociatedType(Type baseTy,
                                               DeclContext *DC,
                                               Identifier name) const {
  switch (stage) {
  case TypeResolutionStage::Structural:
    return DependentMemberType::get(baseTy, name);

  case TypeResolutionStage::Contextual:
    llvm_unreachable("Dependent type after archetype substitution");

  case TypeResolutionStage::Interface:
    // Handled below.
    break;
  }

  assert(stage == TypeResolutionStage::Interface);
  auto builder = getGenericSignatureBuilder();
  auto baseEquivClass =
    builder->resolveEquivalenceClass(
                                baseTy,
                                ArchetypeResolutionKind::CompleteWellFormed);
  if (!baseEquivClass)
    return ErrorType::get(baseTy);

  // Look for a nested type with the given name.
  auto nestedType = baseEquivClass->lookupNestedType(*builder, name);
  assert(nestedType);

  // If the nested type has been resolved to an associated type, use it.
  if (auto assocType = dyn_cast<AssociatedTypeDecl>(nestedType)) {
    return DependentMemberType::get(baseTy, assocType);
  }

  if (nestedType->getDeclContext()->getSelfClassDecl()) {
    // We found a member of a class from a protocol or protocol
    // extension.
    //
    // Get the superclass of the 'Self' type parameter.
    baseTy = (baseEquivClass->concreteType
              ? baseEquivClass->concreteType
              : baseEquivClass->superclass);
    assert(baseTy);
  }

  return TypeChecker::substMemberTypeWithBase(DC->getParentModule(), nestedType,
                                              baseTy);
}

bool TypeResolution::areSameType(Type type1, Type type2) const {
  if (type1->isEqual(type2))
    return true;

  switch (stage) {
  case TypeResolutionStage::Structural:
  case TypeResolutionStage::Interface:
    // If neither type has a type parameter, we're done.
    if (!type1->hasTypeParameter() && !type2->hasTypeParameter())
      return false;

    break;

  case TypeResolutionStage::Contextual:
    // Contextual types have already been uniqued, so the isEqual() result
    // above is complete.
    return false;
  }

  // If we have a generic signature, canonicalize using it.
  if (auto genericSig = getGenericSignature()) {
    // If both are type parameters, we can use a cheaper check
    // that avoids transforming the type and computing anchors.
    if (type1->isTypeParameter() &&
        type2->isTypeParameter()) {
      return genericSig->areSameTypeParameterInContext(type1, type2);
    }
    return genericSig->getCanonicalTypeInContext(type1)
      == genericSig->getCanonicalTypeInContext(type2);
  }

  // Otherwise, perform a structural check.
  assert(stage == TypeResolutionStage::Structural);

  // FIXME: We should be performing a deeper equality check here.
  // If both refer to associated types with the same name, they'll implicitly
  // be considered equivalent.
  auto depMem1 = type1->getAs<DependentMemberType>();
  if (!depMem1) return false;

  auto depMem2 = type2->getAs<DependentMemberType>();
  if (!depMem2) return false;

  if (depMem1->getName() != depMem2->getName()) return false;

  return areSameType(depMem1->getBase(), depMem2->getBase());
}

Type TypeChecker::getArraySliceType(SourceLoc loc, Type elementType) {
  ASTContext &ctx = elementType->getASTContext();
  if (!ctx.getArrayDecl()) {
    ctx.Diags.diagnose(loc, diag::sugar_type_not_found, 0);
    return Type();
  }

  return ArraySliceType::get(elementType);
}

Type TypeChecker::getDictionaryType(SourceLoc loc, Type keyType, 
                                    Type valueType) {
  ASTContext &ctx = keyType->getASTContext();
  if (!ctx.getDictionaryDecl()) {
    ctx.Diags.diagnose(loc, diag::sugar_type_not_found, 3);
    return Type();
  }

  return DictionaryType::get(keyType, valueType);
}

Type TypeChecker::getOptionalType(SourceLoc loc, Type elementType) {
  ASTContext &ctx = elementType->getASTContext();
  if (!ctx.getOptionalDecl()) {
    ctx.Diags.diagnose(loc, diag::sugar_type_not_found, 1);
    return Type();
  }

  return OptionalType::get(elementType);
}

static Type getPointerType(TypeChecker &tc, SourceLoc loc, Type pointeeType,
                           PointerTypeKind kind) {
  auto pointerDecl = [&] {
    switch (kind) {
    case PTK_UnsafeMutableRawPointer:
    case PTK_UnsafeRawPointer:
      llvm_unreachable("these pointer types don't take arguments");
    case PTK_UnsafePointer:
      return tc.Context.getUnsafePointerDecl();
    case PTK_UnsafeMutablePointer:
      return tc.Context.getUnsafeMutablePointerDecl();
    case PTK_AutoreleasingUnsafeMutablePointer:
      return tc.Context.getAutoreleasingUnsafeMutablePointerDecl();
    }
    llvm_unreachable("bad kind");
  }();
  if (!pointerDecl) {
    tc.diagnose(loc, diag::pointer_type_not_found,
                kind == PTK_UnsafePointer ? 0 :
                kind == PTK_UnsafeMutablePointer ? 1 : 2);
    return Type();
  }

  tc.validateDecl(pointerDecl);
  if (pointerDecl->isInvalid())
    return Type();

  // TODO: validate generic signature?

  return BoundGenericType::get(pointerDecl, nullptr, pointeeType);
}

Type TypeChecker::getUnsafePointerType(SourceLoc loc, Type pointeeType) {
  return getPointerType(*this, loc, pointeeType, PTK_UnsafePointer);
}

Type TypeChecker::getUnsafeMutablePointerType(SourceLoc loc, Type pointeeType) {
  return getPointerType(*this, loc, pointeeType, PTK_UnsafeMutablePointer);
}

Type TypeChecker::getStringType(DeclContext *dc) {
  if (auto typeDecl = Context.getStringDecl())
    return typeDecl->getDeclaredInterfaceType();

  return Type();
}

Type TypeChecker::getSubstringType(DeclContext *dc) {
  if (auto typeDecl = Context.getSubstringDecl())
    return typeDecl->getDeclaredInterfaceType();

  return Type();
}

Type TypeChecker::getIntType(DeclContext *dc) {
  if (auto typeDecl = Context.getIntDecl())
    return typeDecl->getDeclaredInterfaceType();

  return Type();
}

Type TypeChecker::getInt8Type(DeclContext *dc) {
  if (auto typeDecl = Context.getInt8Decl())
    return typeDecl->getDeclaredInterfaceType();

  return Type();
}

Type TypeChecker::getUInt8Type(DeclContext *dc) {
  if (auto typeDecl = Context.getUInt8Decl())
    return typeDecl->getDeclaredInterfaceType();

  return Type();
}

/// Find the standard type of exceptions.
///
/// We call this the "exception type" to try to avoid confusion with
/// the AST's ErrorType node.
Type TypeChecker::getExceptionType(DeclContext *dc, SourceLoc loc) {
  return Context.getErrorDecl()->getDeclaredType();
}

Type
TypeChecker::getDynamicBridgedThroughObjCClass(DeclContext *dc,
                                               Type dynamicType,
                                               Type valueType) {
  // We can only bridge from class or Objective-C existential types.
  if (!dynamicType->satisfiesClassConstraint())
    return Type();

  // If the value type cannot be bridged, we're done.
  if (!valueType->isPotentiallyBridgedValueType())
    return Type();

  return Context.getBridgedToObjC(dc, valueType);
}

Type TypeChecker::resolveTypeInContext(
       TypeDecl *typeDecl,
       DeclContext *foundDC,
       TypeResolution resolution,
       TypeResolutionOptions options,
       bool isSpecialized) {

  auto fromDC = resolution.getDeclContext();
  ASTContext &ctx = fromDC->getASTContext();

  // If we found a generic parameter, map to the archetype if there is one.
  if (auto genericParam = dyn_cast<GenericTypeParamDecl>(typeDecl)) {
    return resolution.mapTypeIntoContext(
      genericParam->getDeclaredInterfaceType());
  }

  if (!isSpecialized) {
    // If we are referring to a type within its own context, and we have either
    // a generic type with no generic arguments or a non-generic type, use the
    // type within the context.
    if (auto *nominalType = dyn_cast<NominalTypeDecl>(typeDecl)) {
      for (auto *parentDC = fromDC;
           !parentDC->isModuleScopeContext();
           parentDC = parentDC->getParent()) {
        auto *parentNominal = parentDC->getSelfNominalTypeDecl();
        if (parentNominal == nominalType)
          return resolution.mapTypeIntoContext(
            parentDC->getDeclaredInterfaceType());
        if (isa<ExtensionDecl>(parentDC)) {
          auto *extendedType = parentNominal;
          while (extendedType != nullptr) {
            if (extendedType == nominalType)
              return resolution.mapTypeIntoContext(
                extendedType->getDeclaredInterfaceType());
            extendedType = extendedType->getParent()->getSelfNominalTypeDecl();
          }
        }
      }
    }

    // If we're inside an extension of a type alias, allow the type alias to be
    // referenced without generic arguments as well.
    if (auto *aliasDecl = dyn_cast<TypeAliasDecl>(typeDecl)) {
      for (auto *parentDC = fromDC;
            !parentDC->isModuleScopeContext();
            parentDC = parentDC->getParent()) {
        if (auto *ext = dyn_cast<ExtensionDecl>(parentDC)) {
          auto extendedType = ext->getExtendedType();
          if (auto *aliasType = dyn_cast<TypeAliasType>(extendedType.getPointer())) {
            if (aliasType->getDecl() == aliasDecl) {
              return resolution.mapTypeIntoContext(
                  aliasDecl->getDeclaredInterfaceType());
            }

            extendedType = aliasType->getParent();
            continue;
          }
        }
      }
    }
  }

  // Simple case -- the type is not nested inside of another type.
  // However, it might be nested inside another generic context, so
  // we do want to write the type in terms of interface types or
  // context archetypes, depending on the resolver given to us.
  if (!typeDecl->getDeclContext()->isTypeContext()) {
    if (auto *aliasDecl = dyn_cast<TypeAliasDecl>(typeDecl)) {
      // For a generic typealias, return the unbound generic form of the type.
      if (aliasDecl->getGenericParams())
        return aliasDecl->getUnboundGenericType();

      // Otherwise, simply return the underlying type.
      return resolution.mapTypeIntoContext(
        aliasDecl->getDeclaredInterfaceType());
    }

    // When a nominal type used outside its context, return the unbound
    // generic form of the type.
    if (auto *nominalDecl = dyn_cast<NominalTypeDecl>(typeDecl))
      return nominalDecl->getDeclaredType();

    assert(isa<ModuleDecl>(typeDecl));
    return typeDecl->getDeclaredInterfaceType();
  }

  assert(foundDC);

  // selfType is the self type of the context, unless the
  // context is a protocol type, in which case we might have
  // to use the existential type or superclass bound as a
  // parent type instead.
  Type selfType;
  if (isa<NominalTypeDecl>(typeDecl) &&
      typeDecl->getDeclContext()->getSelfProtocolDecl()) {
    // When looking up a nominal type declaration inside of a
    // protocol extension, always use the nominal type and
    // not the protocol 'Self' type.
    if (!foundDC->getDeclaredInterfaceType())
      return ErrorType::get(ctx);

    selfType = resolution.mapTypeIntoContext(
      foundDC->getDeclaredInterfaceType());
  } else {
    // Otherwise, we want the protocol 'Self' type for
    // substituting into alias types and associated types.
    selfType = resolution.mapTypeIntoContext(
      foundDC->getSelfInterfaceType());

    if (selfType->is<GenericTypeParamType>()) {
      if (typeDecl->getDeclContext()->getSelfProtocolDecl()) {
        if (isa<AssociatedTypeDecl>(typeDecl) ||
            (isa<TypeAliasDecl>(typeDecl) &&
             !cast<TypeAliasDecl>(typeDecl)->isGeneric())) {
          // FIXME: We should use this lookup method for the Interface
          // stage too, but right now that causes problems with
          // Sequence.SubSequence vs Collection.SubSequence; the former
          // is more canonical, but if we return that instead of the
          // latter, we infer the wrong associated type in some cases,
          // because we use the Sequence.SubSequence default instead of
          // the Collection.SubSequence default, even when the conforming
          // type wants to conform to Collection.
          if (resolution.getStage() == TypeResolutionStage::Structural) {
            return resolution.resolveSelfAssociatedType(
              selfType, foundDC, typeDecl->getName());
          } else if (auto assocType = dyn_cast<AssociatedTypeDecl>(typeDecl)) {
            typeDecl = assocType->getAssociatedTypeAnchor();
          }
        }
      }

      // FIXME: Remove this once the above FIXME is addressed.
      if (typeDecl->getDeclContext()->getSelfClassDecl()) {
        // We found a member of a class from a protocol or protocol
        // extension.
        //
        // Get the superclass of the 'Self' type parameter.
        auto *sig = foundDC->getGenericSignatureOfContext();
        if (!sig)
          return ErrorType::get(ctx);
        auto superclassType = sig->getSuperclassBound(selfType);
        if (!superclassType)
          return ErrorType::get(ctx);

        selfType = superclassType;
      }
    }
  }
  
  // Finally, substitute the base type into the member type.
  return substMemberTypeWithBase(fromDC->getParentModule(), typeDecl,
                                 selfType, resolution.usesArchetypes());
}

static TypeResolutionOptions
adjustOptionsForGenericArgs(TypeResolutionOptions options) {
  options.setContext(None);
  options -= TypeResolutionFlags::SILType;
  options -= TypeResolutionFlags::AllowUnavailableProtocol;

  return options;
}

/// This function checks if a bound generic type is UnsafePointer<Void> or
/// UnsafeMutablePointer<Void>. For these two type representations, we should
/// warn users that they are deprecated and replace them with more handy
/// UnsafeRawPointer and UnsafeMutableRawPointer, respectively.
static bool isPointerToVoid(ASTContext &Ctx, Type Ty, bool &IsMutable) {
  if (Ty.isNull())
    return false;
  auto *BGT = Ty->getAs<BoundGenericType>();
  if (!BGT)
    return false;
  if (BGT->getDecl() != Ctx.getUnsafePointerDecl() &&
      BGT->getDecl() != Ctx.getUnsafeMutablePointerDecl())
    return false;
  IsMutable = BGT->getDecl() == Ctx.getUnsafeMutablePointerDecl();
  assert(BGT->getGenericArgs().size() == 1);
  return BGT->getGenericArgs().front()->isVoid();
}

Type TypeChecker::applyGenericArguments(Type type,
                                        SourceLoc loc,
                                        TypeResolution resolution,
                                        GenericIdentTypeRepr *generic,
                                        TypeResolutionOptions options) {
  if (type->hasError()) {
    generic->setInvalid();
    return type;
  }

  auto dc = resolution.getDeclContext();
  auto &ctx = dc->getASTContext();
  auto &diags = ctx.Diags;

  // We must either have an unbound generic type, or a generic type alias.
  if (!type->is<UnboundGenericType>()) {
     if (!options.contains(TypeResolutionFlags::SilenceErrors)) {
      auto diag = diags.diagnose(loc, diag::not_a_generic_type, type);

      // Don't add fixit on module type; that isn't the right type regardless
      // of whether it had generic arguments.
      if (!type->is<ModuleType>()) {
        // When turning a SourceRange into CharSourceRange the closing angle
        // brackets on nested generics are lexed as one token.
        SourceRange angles = generic->getAngleBrackets();
        diag.fixItRemoveChars(angles.Start,
                              angles.End.getAdvancedLocOrInvalid(1));
      }

      generic->setInvalid();
    }
    return type;
  }

  auto *unboundType = type->castTo<UnboundGenericType>();
  auto *decl = unboundType->getDecl();

  // Make sure we have the right number of generic arguments.
  // FIXME: If we have fewer arguments than we need, that might be okay, if
  // we're allowed to deduce the remaining arguments from context.
  auto genericDecl = cast<GenericTypeDecl>(decl);
  auto genericArgs = generic->getGenericArgs();
  auto genericParams = genericDecl->getGenericParams();
  if (genericParams->size() != genericArgs.size()) {
    if (!options.contains(TypeResolutionFlags::SilenceErrors)) {
      diags.diagnose(loc, diag::type_parameter_count_mismatch, decl->getName(),
                     genericParams->size(), genericArgs.size(),
                     genericArgs.size() < genericParams->size())
          .highlight(generic->getAngleBrackets());
      decl->diagnose(diag::kind_declname_declared_here,
                     DescriptiveDeclKind::GenericType, decl->getName());
    }
    return ErrorType::get(ctx);
  }

  // In SIL mode, Optional<T> interprets T as a SIL type.
  if (options.contains(TypeResolutionFlags::SILType)) {
    if (auto nominal = dyn_cast<NominalTypeDecl>(decl)) {
      if (nominal->isOptionalDecl()) {
        // Validate the generic argument.
        Type objectType = resolution.resolveType(genericArgs[0], options);
        if (!objectType || objectType->hasError())
          return nullptr;

        return BoundGenericType::get(nominal, /*parent*/ Type(), objectType);
      }
    }  
  }

  // Cannot extend a bound generic type.
  if (options.is(TypeResolverContext::ExtensionBinding)) {
    if (!options.contains(TypeResolutionFlags::SilenceErrors)) {
      diags.diagnose(loc, diag::extension_specialization,
               genericDecl->getName())
        .highlight(generic->getSourceRange());
    }
    return ErrorType::get(ctx);
  }

  // FIXME: More principled handling of circularity.
  if (!genericDecl->hasValidSignature()) {
    diags.diagnose(loc, diag::recursive_decl_reference,
             genericDecl->getDescriptiveKind(), genericDecl->getName());
    genericDecl->diagnose(diag::kind_declared_here, DescriptiveDeclKind::Type);
    return ErrorType::get(ctx);
  }

  // Resolve the types of the generic arguments.
  options = adjustOptionsForGenericArgs(options);

  SmallVector<Type, 2> args;
  for (auto tyR : genericArgs) {
    // Propagate failure.
    Type substTy = resolution.resolveType(tyR, options);
    if (!substTy || substTy->hasError())
      return ErrorType::get(ctx);

    args.push_back(substTy);
  }

  auto result = applyUnboundGenericArguments(unboundType, genericDecl, loc,
                                             resolution, args);
  if (!result)
    return result;

  // Migration hack.
  bool isMutablePointer;
  if (isPointerToVoid(dc->getASTContext(), result, isMutablePointer)) {
    if (isMutablePointer)
      diags.diagnose(loc, diag::use_of_void_pointer, "Mutable").
        fixItReplace(generic->getSourceRange(), "UnsafeMutableRawPointer");
    else
      diags.diagnose(loc, diag::use_of_void_pointer, "").
        fixItReplace(generic->getSourceRange(), "UnsafeRawPointer");
  }
  return result;
}

/// Apply generic arguments to the given type.
Type TypeChecker::applyUnboundGenericArguments(
    UnboundGenericType *unboundType, GenericTypeDecl *decl,
    SourceLoc loc, TypeResolution resolution,
    ArrayRef<Type> genericArgs) {
  assert(genericArgs.size() == decl->getGenericParams()->size() &&
         "invalid arguments, use applyGenericArguments for diagnostic emitting");

  auto genericSig = decl->getGenericSignature();
  assert(genericSig != nullptr);

  TypeSubstitutionMap subs;

  // Get the interface type for the declaration. We will be substituting
  // type parameters that appear inside this type with the provided
  // generic arguments.
  auto resultType = decl->getDeclaredInterfaceType();

  bool hasTypeVariable = false;

  // Get the substitutions for outer generic parameters from the parent
  // type.
  if (auto parentType = unboundType->getParent()) {
    if (parentType->hasUnboundGenericType()) {
      // If we're working with a nominal type declaration, just construct
      // a bound generic type without checking the generic arguments.
      if (auto *nominalDecl = dyn_cast<NominalTypeDecl>(decl)) {
        return BoundGenericType::get(nominalDecl, parentType, genericArgs);
      }

      assert(!resultType->hasTypeParameter());
      return resultType;
    }

    subs = parentType->getContextSubstitutions(decl->getDeclContext());
    hasTypeVariable |= parentType->hasTypeVariable();
  }

  SourceLoc noteLoc = decl->getLoc();
  if (noteLoc.isInvalid())
    noteLoc = loc;

  // Realize the types of the generic arguments and add them to the
  // substitution map.
  for (unsigned i = 0, e = genericArgs.size(); i < e; i++) {
    auto origTy = genericSig->getInnermostGenericParams()[i];
    auto substTy = genericArgs[i];

    // Enter a substitution.
    subs[origTy->getCanonicalType()->castTo<GenericTypeParamType>()] =
      substTy;

    hasTypeVariable |= substTy->hasTypeVariable();
  }

  // Check the generic arguments against the requirements of the declaration's
  // generic signature.
  auto dc = resolution.getDeclContext();
  if (!hasTypeVariable &&
      resolution.getStage() > TypeResolutionStage::Structural) {
    auto result =
      checkGenericArguments(dc, loc, noteLoc, unboundType,
                            genericSig->getGenericParams(),
                            genericSig->getRequirements(),
                            QueryTypeSubstitutionMap{subs},
                            LookUpConformance(dc));

    switch (result) {
    case RequirementCheckResult::Failure:
    case RequirementCheckResult::SubstitutionFailure:
      return ErrorType::get(dc->getASTContext());
    case RequirementCheckResult::Success:
      break;
    }
  }

  // For a typealias, use the underlying type. We'll wrap up the result
  // later.
  auto typealias = dyn_cast<TypeAliasDecl>(decl);
  if (typealias) {
    resultType = typealias->getUnderlyingTypeLoc().getType();
  }

  // Apply the substitution map to the interface type of the declaration.
  resultType = resultType.subst(QueryTypeSubstitutionMap{subs},
                                LookUpConformance(dc),
                                SubstFlags::UseErrorType);

  // Form a sugared typealias reference.
  Type parentType = unboundType->getParent();
  if (typealias && (!parentType || !parentType->isAnyExistentialType())) {
    auto genericSig = typealias->getGenericSignature();
    auto subMap = SubstitutionMap::get(genericSig,
                                       QueryTypeSubstitutionMap{subs},
                                       LookUpConformance(dc));
    resultType = TypeAliasType::get(typealias, parentType,
                                    subMap, resultType);
  }

  return resultType;
}

/// Diagnose a use of an unbound generic type.
static void diagnoseUnboundGenericType(Type ty, SourceLoc loc) {
  auto unbound = ty->castTo<UnboundGenericType>();
  {
    auto &ctx = ty->getASTContext();
    InFlightDiagnostic diag = ctx.Diags.diagnose(loc,
        diag::generic_type_requires_arguments, ty);
    if (auto *genericD = unbound->getDecl()) {
      SmallString<64> genericArgsToAdd;
      if (TypeChecker::getDefaultGenericArgumentsString(genericArgsToAdd,
                                                        genericD))
        diag.fixItInsertAfter(loc, genericArgsToAdd);
    }
  }
  unbound->getDecl()->diagnose(diag::kind_declname_declared_here,
                               DescriptiveDeclKind::GenericType,
                               unbound->getDecl()->getName());
}

// Produce a diagnostic if the type we referenced was an
// associated type but the type itself was erroneous. We'll produce a
// diagnostic here if the diagnostic for the bad type witness would show up in
// a different context.
static void maybeDiagnoseBadConformanceRef(DeclContext *dc,
                                           Type parentTy,
                                           SourceLoc loc,
                                           TypeDecl *typeDecl) {
  auto protocol = dyn_cast<ProtocolDecl>(typeDecl->getDeclContext());
  if (!protocol)
    return;

  // If we weren't given a conformance, go look it up.
  ProtocolConformance *conformance = nullptr;
  if (auto conformanceRef = TypeChecker::conformsToProtocol(
          parentTy, protocol, dc,
          (ConformanceCheckFlags::InExpression |
           ConformanceCheckFlags::SuppressDependencyTracking |
           ConformanceCheckFlags::AllowUnavailableConditionalRequirements))) {
    if (conformanceRef->isConcrete())
      conformance = conformanceRef->getConcrete();
  }

  // If any errors have occurred, don't bother diagnosing this cross-file
  // issue.
  ASTContext &ctx = dc->getASTContext();
  if (ctx.Diags.hadAnyError())
    return;

  auto diagCode =
      (conformance && !conformance->getConditionalRequirementsIfAvailable())
          ? diag::unsupported_recursion_in_associated_type_reference
          : diag::broken_associated_type_witness;

  ctx.Diags.diagnose(loc, diagCode, isa<TypeAliasDecl>(typeDecl), typeDecl->getFullName(), parentTy);
}

/// Returns a valid type or ErrorType in case of an error.
static Type resolveTypeDecl(TypeDecl *typeDecl, SourceLoc loc,
                            DeclContext *foundDC,
                            TypeResolution resolution,
                            GenericIdentTypeRepr *generic,
                            TypeResolutionOptions options) {
  auto fromDC = resolution.getDeclContext();
  assert(fromDC && "No declaration context for type resolution?");

  ASTContext &ctx = typeDecl->getASTContext();
  auto &diags = ctx.Diags;
  auto lazyResolver = ctx.getLazyResolver();

  // Don't validate nominal type declarations during extension binding.
  if (!options.is(TypeResolverContext::ExtensionBinding) ||
      !isa<NominalTypeDecl>(typeDecl)) {
    // Validate the declaration.
    if (lazyResolver)
      lazyResolver->resolveDeclSignature(typeDecl);

    // If we were not able to validate recursively, bail out.
    if (!typeDecl->hasInterfaceType()) {
      diags.diagnose(loc, diag::recursive_decl_reference,
                  typeDecl->getDescriptiveKind(), typeDecl->getName());
      typeDecl->diagnose(diag::kind_declared_here,
                         DescriptiveDeclKind::Type);
      return ErrorType::get(ctx);
    }
  }

  // Resolve the type declaration to a specific type. How this occurs
  // depends on the current context and where the type was found.
  Type type =
    TypeChecker::resolveTypeInContext(typeDecl, foundDC, resolution, options,
                                      generic);

  if (type->is<UnboundGenericType>() && !generic &&
      !options.is(TypeResolverContext::TypeAliasDecl) &&
      !options.contains(TypeResolutionFlags::AllowUnboundGenerics)) {
    diagnoseUnboundGenericType(type, loc);
    return ErrorType::get(ctx);
  }

  if (type->hasError() && foundDC &&
      (isa<AssociatedTypeDecl>(typeDecl) || isa<TypeAliasDecl>(typeDecl))) {
    maybeDiagnoseBadConformanceRef(fromDC,
                                   foundDC->getDeclaredInterfaceType(),
                                   loc, typeDecl);
  }

  if (generic) {
    // Apply the generic arguments to the type.
    type = TypeChecker::applyGenericArguments(type, loc, resolution, generic,
                                              options);
    if (!type)
      return nullptr;
  }

  assert(type);
  return type;
}

static std::string getDeclNameFromContext(DeclContext *dc,
                                          NominalTypeDecl *nominal) {
  // We don't allow an unqualified reference to a type inside an
  // extension if the type is itself nested inside another type,
  // eg:
  //
  // extension A.B { ... B ... }
  //
  // Instead, you must write 'A.B'. Calculate the right name to use
  // for fixits.
  if (!isa<ExtensionDecl>(dc)) {
    SmallVector<Identifier, 2> idents;
    auto *parentNominal = nominal;
    while (parentNominal != nullptr) {
      idents.push_back(parentNominal->getName());
      parentNominal = parentNominal->getDeclContext()->getSelfNominalTypeDecl();
    }
    std::reverse(idents.begin(), idents.end());
    std::string result;
    for (auto ident : idents) {
      if (!result.empty())
        result += ".";
      result += ident.str();
    }
    return result;
  } else {
    return nominal->getName().str();
  }
}

//
// SE-0068 is "Expanding Swift Self to class members and value types"
// https://github.com/apple/swift-evolution/blob/master/proposals/0068-universal-self.md
//
static Type SelfAllowedBySE0068(TypeResolution resolution,
                                TypeResolutionOptions options) {
  auto dc = resolution.getDeclContext();
  ASTContext &ctx = dc->getASTContext();
  DeclContext *nominalDC = nullptr;
  NominalTypeDecl *nominal = nullptr;
  if ((nominalDC = dc->getInnermostTypeContext()) &&
      (nominal = nominalDC->getSelfNominalTypeDecl())) {
    assert(!isa<ProtocolDecl>(nominal) && "Cannot be a protocol");

    bool insideClass = nominalDC->getSelfClassDecl() != nullptr;
    AbstractFunctionDecl *methodDecl = dc->getInnermostMethodContext();
    bool declaringMethod = methodDecl &&
      methodDecl->getDeclContext() == dc->getParentForLookup();
    bool isMutablePropertyOrSubscriptOfClass = insideClass &&
      (options.is(TypeResolverContext::PatternBindingDecl) ||
       options.is(TypeResolverContext::FunctionResult));
    bool isTypeAliasInClass = insideClass &&
      options.is(TypeResolverContext::TypeAliasDecl);

    if (((!insideClass || !declaringMethod) &&
         !isMutablePropertyOrSubscriptOfClass && !isTypeAliasInClass &&
         !options.is(TypeResolverContext::GenericRequirement)) ||
        options.is(TypeResolverContext::ExplicitCastExpr)) {
      Type SelfType = nominal->getSelfInterfaceType();
      if (insideClass)
        SelfType = DynamicSelfType::get(SelfType, ctx);
      return resolution.mapTypeIntoContext(SelfType);
    }
  }

  return Type();
}

/// Diagnose a reference to an unknown type.
///
/// This routine diagnoses a reference to an unknown type, and
/// attempts to fix the reference via various means.
///
/// \returns either the corrected type, if possible, or an error type to
/// that correction failed.
static Type diagnoseUnknownType(TypeResolution resolution,
                                Type parentType,
                                SourceRange parentRange,
                                ComponentIdentTypeRepr *comp,
                                TypeResolutionOptions options,
                                NameLookupOptions lookupOptions) {
  auto dc = resolution.getDeclContext();
  ASTContext &ctx = dc->getASTContext();
  auto &diags = ctx.Diags;

  // Unqualified lookup case.
  if (parentType.isNull()) {
    if (comp->getIdentifier() == ctx.Id_Self &&
        !isa<GenericIdentTypeRepr>(comp)) {
      DeclContext *nominalDC = nullptr;
      NominalTypeDecl *nominal = nullptr;
      if ((nominalDC = dc->getInnermostTypeContext()) &&
          (nominal = nominalDC->getSelfNominalTypeDecl())) {
        // Attempt to refer to 'Self' within a non-protocol nominal
        // type. Fix this by replacing 'Self' with the nominal type name.
        assert(!isa<ProtocolDecl>(nominal) && "Cannot be a protocol");

        // Produce a Fix-It replacing 'Self' with the nominal type name.
        auto name = getDeclNameFromContext(dc, nominal);
        diags.diagnose(comp->getIdLoc(), diag::self_in_nominal, name)
          .fixItReplace(comp->getIdLoc(), name);

        // If this is a requirement, replacing 'Self' with a valid type will
        // result in additional unnecessary diagnostics (does not refer to a
        // generic parameter or associated type). Simply return an error type.
        if (options.is(TypeResolverContext::GenericRequirement))
          return ErrorType::get(ctx);

        auto type = resolution.mapTypeIntoContext(
          dc->getInnermostTypeContext()->getSelfInterfaceType());

        comp->overwriteIdentifier(nominal->getName());
        comp->setValue(nominal, nominalDC->getParent());
        return type;
      }
      // Attempt to refer to 'Self' from a free function.
      diags.diagnose(comp->getIdLoc(), diag::dynamic_self_non_method,
                     dc->getParent()->isLocalContext());

      return ErrorType::get(ctx);
    }

    // Try ignoring access control.
    DeclContext *lookupDC = dc;
    NameLookupOptions relookupOptions = lookupOptions;
    relookupOptions |= NameLookupFlags::KnownPrivate;
    relookupOptions |= NameLookupFlags::IgnoreAccessControl;
    auto inaccessibleResults =
    TypeChecker::lookupUnqualifiedType(lookupDC, comp->getIdentifier(),
                                       comp->getIdLoc(), relookupOptions);
    if (!inaccessibleResults.empty()) {
      // FIXME: What if the unviable candidates have different levels of access?
      auto first = cast<TypeDecl>(inaccessibleResults.front().getValueDecl());
      diags.diagnose(comp->getIdLoc(), diag::candidate_inaccessible,
                     comp->getIdentifier(), first->getFormalAccess());

      // FIXME: If any of the candidates (usually just one) are in the same
      // module we could offer a fix-it.
      for (auto lookupResult : inaccessibleResults)
        lookupResult.getValueDecl()->diagnose(diag::kind_declared_here,
                                              DescriptiveDeclKind::Type);

      // Don't try to recover here; we'll get more access-related diagnostics
      // downstream if we do.
      return ErrorType::get(ctx);
    }

    // Fallback.
    SourceLoc L = comp->getIdLoc();
    SourceRange R = SourceRange(comp->getIdLoc());

    // Check if the unknown type is in the type remappings.
    auto &Remapped = ctx.RemappedTypes;
    auto TypeName = comp->getIdentifier().str();
    auto I = Remapped.find(TypeName);
    if (I != Remapped.end()) {
      auto RemappedTy = I->second->getString();
      diags.diagnose(L, diag::use_undeclared_type_did_you_mean,
                     comp->getIdentifier(), RemappedTy)
        .highlight(R)
        .fixItReplace(R, RemappedTy);

      // Replace the computed type with the suggested type.
      comp->overwriteIdentifier(ctx.getIdentifier(RemappedTy));

      // HACK: 'NSUInteger' suggests both 'UInt' and 'Int'.
      if (TypeName == ctx.getSwiftName(KnownFoundationEntity::NSUInteger)) {
        diags.diagnose(L, diag::note_remapped_type, "UInt")
          .fixItReplace(R, "UInt");
      }

      return I->second;
    }

    diags.diagnose(L, diag::use_undeclared_type,
                comp->getIdentifier())
      .highlight(R);

    return ErrorType::get(ctx);
  }

  // Qualified lookup case.
  if (!parentType->mayHaveMembers()) {
    diags.diagnose(comp->getIdLoc(), diag::invalid_member_type,
                   comp->getIdentifier(), parentType)
        .highlight(parentRange);
    return ErrorType::get(ctx);
  }

  // Try ignoring access control.
  NameLookupOptions relookupOptions = lookupOptions;
  relookupOptions |= NameLookupFlags::KnownPrivate;
  relookupOptions |= NameLookupFlags::IgnoreAccessControl;
  auto inaccessibleMembers =
    TypeChecker::lookupMemberType(dc, parentType, comp->getIdentifier(),
                                  relookupOptions);
  if (inaccessibleMembers) {
    // FIXME: What if the unviable candidates have different levels of access?
    const TypeDecl *first = inaccessibleMembers.front().Member;
    diags.diagnose(comp->getIdLoc(), diag::candidate_inaccessible,
                   comp->getIdentifier(), first->getFormalAccess());

    // FIXME: If any of the candidates (usually just one) are in the same module
    // we could offer a fix-it.
    for (auto lookupResult : inaccessibleMembers)
      lookupResult.Member->diagnose(diag::kind_declared_here,
                                    DescriptiveDeclKind::Type);

    // Don't try to recover here; we'll get more access-related diagnostics
    // downstream if we do.
    return ErrorType::get(ctx);
  }

  // FIXME: Typo correction!

  // Lookup into a type.
  if (auto moduleType = parentType->getAs<ModuleType>()) {
    diags.diagnose(comp->getIdLoc(), diag::no_module_type,
                   comp->getIdentifier(), moduleType->getModule()->getName());
  } else {
    LookupResult memberLookup;
    // Let's try to lookup given identifier as a member of the parent type,
    // this allows for more precise diagnostic, which distinguishes between
    // identifier not found as a member type vs. not found at all.
    NameLookupOptions memberLookupOptions = lookupOptions;
    memberLookupOptions |= NameLookupFlags::IgnoreAccessControl;
    memberLookupOptions |= NameLookupFlags::KnownPrivate;

    memberLookup = TypeChecker::lookupMember(dc, parentType,
                                             comp->getIdentifier(),
                                             memberLookupOptions);

    // Looks like this is not a member type, but simply a member of parent type.
    if (!memberLookup.empty()) {
      auto member = memberLookup[0].getValueDecl();
      diags.diagnose(comp->getIdLoc(), diag::invalid_member_reference,
                     member->getDescriptiveKind(), comp->getIdentifier(),
                     parentType)
          .highlight(parentRange);
    } else {
      diags.diagnose(comp->getIdLoc(), diag::invalid_member_type,
                     comp->getIdentifier(), parentType)
        .highlight(parentRange);
    }
  }
  return ErrorType::get(ctx);
}

/// Resolve the given identifier type representation as an unqualified type,
/// returning the type it references.
///
/// \returns Either the resolved type or a null type, the latter of
/// which indicates that some dependencies were unsatisfied.
static Type
resolveTopLevelIdentTypeComponent(TypeResolution resolution,
                                  ComponentIdentTypeRepr *comp,
                                  TypeResolutionOptions options) {
  // Short-circuiting.
  ASTContext &ctx = resolution.getASTContext();
  auto &diags = ctx.Diags;
  if (comp->isInvalid()) return ErrorType::get(ctx);

  // If the component has already been bound to a declaration, handle
  // that now.
  if (auto *typeDecl = comp->getBoundDecl()) {
    // Resolve the type declaration within this context.
    return resolveTypeDecl(typeDecl, comp->getIdLoc(),
                           comp->getDeclContext(), resolution,
                           dyn_cast<GenericIdentTypeRepr>(comp), options);
  }

  // Resolve the first component, which is the only one that requires
  // unqualified name lookup.
  auto DC = resolution.getDeclContext();
  DeclContext *lookupDC = DC;

  // Dynamic 'Self' in the result type of a function body.
  if (options.getBaseContext() == TypeResolverContext::DynamicSelfResult &&
      comp->getIdentifier() == ctx.Id_Self) {
    auto func = cast<FuncDecl>(DC);
    assert(func->hasDynamicSelf() && "Not marked as having dynamic Self?");

    // FIXME: The passed-in TypeRepr should get 'typechecked' as well.
    // The issue is though that ComponentIdentTypeRepr only accepts a ValueDecl
    // while the 'Self' type is more than just a reference to a TypeDecl.

    auto selfType = resolution.mapTypeIntoContext(
      func->getDeclContext()->getSelfInterfaceType());
    return DynamicSelfType::get(selfType, ctx);
  }

  auto id = comp->getIdentifier();

  NameLookupOptions lookupOptions = defaultUnqualifiedLookupOptions;
  if (options.contains(TypeResolutionFlags::KnownNonCascadingDependency))
    lookupOptions |= NameLookupFlags::KnownPrivate;
  auto globals = TypeChecker::lookupUnqualifiedType(lookupDC,
                                                    id,
                                                    comp->getIdLoc(),
                                                    lookupOptions);

  // Process the names we found.
  Type current;
  TypeDecl *currentDecl = nullptr;
  DeclContext *currentDC = nullptr;
  bool isAmbiguous = false;
  for (const auto entry : globals) {
    auto *foundDC = entry.getDeclContext();
    auto *typeDecl = cast<TypeDecl>(entry.getValueDecl());

    Type type = resolveTypeDecl(typeDecl, comp->getIdLoc(),
                                foundDC, resolution,
                                dyn_cast<GenericIdentTypeRepr>(comp), options);

    if (!type)
      return type;

    if (type->is<ErrorType>())
      return type;

    // If this is the first result we found, record it.
    if (current.isNull()) {
      current = type;
      currentDecl = typeDecl;
      currentDC = foundDC;
      continue;
    }

    // Otherwise, check for an ambiguity.
    if (!resolution.areSameType(current, type)) {
      isAmbiguous = true;
      break;
    }

    // We have a found multiple type aliases that refer to the same thing.
    // Ignore the duplicate.
  }

  // Complain about any ambiguities we detected.
  // FIXME: We could recover by looking at later components.
  if (isAmbiguous) {
    if (!options.contains(TypeResolutionFlags::SilenceErrors)) {
      diags.diagnose(comp->getIdLoc(), diag::ambiguous_type_base,
                     comp->getIdentifier())
        .highlight(comp->getIdLoc());
      for (auto entry : globals) {
        entry.getValueDecl()->diagnose(diag::found_candidate);
      }
    }

    comp->setInvalid();
    return ErrorType::get(ctx);
  }

  // If we found nothing, complain and give ourselves a chance to recover.
  if (current.isNull()) {
    // If we're not allowed to complain or we couldn't fix the
    // source, bail out.
    if (options.contains(TypeResolutionFlags::SilenceErrors))
      return ErrorType::get(ctx);

    if (id == ctx.Id_Self)
      if (auto SelfType = SelfAllowedBySE0068(resolution, options))
        return SelfType;

    return diagnoseUnknownType(resolution, nullptr, SourceRange(), comp,
                               options, lookupOptions);
  }

  comp->setValue(currentDecl, currentDC);
  return current;
}

static void diagnoseAmbiguousMemberType(Type baseTy, SourceRange baseRange,
                                        Identifier name, SourceLoc nameLoc,
                                        LookupTypeResult &lookup) {
  ASTContext &ctx = baseTy->getASTContext();
  auto &diags = ctx.Diags;
  if (auto moduleTy = baseTy->getAs<ModuleType>()) {
    diags.diagnose(nameLoc, diag::ambiguous_module_type, name,
                   moduleTy->getModule()->getName())
      .highlight(baseRange);
  } else {
    diags.diagnose(nameLoc, diag::ambiguous_member_type, name, baseTy)
      .highlight(baseRange);
  }
  for (const auto &member : lookup) {
    member.Member->diagnose(diag::found_candidate_type, member.MemberType);
  }
}

/// Resolve the given identifier type representation as a qualified
/// lookup within the given parent type, returning the type it
/// references.
static Type resolveNestedIdentTypeComponent(
              TypeResolution resolution,
              Type parentTy,
              SourceRange parentRange,
              ComponentIdentTypeRepr *comp,
              TypeResolutionOptions options) {
  auto DC = resolution.getDeclContext();
  auto &ctx = DC->getASTContext();
  auto &diags = ctx.Diags;

  auto maybeApplyGenericArgs = [&](Type memberType) {
    // If there are generic arguments, apply them now.
    if (auto genComp = dyn_cast<GenericIdentTypeRepr>(comp)) {
      return TypeChecker::applyGenericArguments(memberType, comp->getIdLoc(),
                                                resolution, genComp, options);
    }

    if (memberType->is<UnboundGenericType>() &&
        !options.is(TypeResolverContext::TypeAliasDecl) &&
        !options.contains(TypeResolutionFlags::AllowUnboundGenerics)) {
      diagnoseUnboundGenericType(memberType, comp->getLoc());
      return ErrorType::get(ctx);
    }

    return memberType;
  };

  auto maybeDiagnoseBadMemberType = [&](TypeDecl *member, Type memberType,
                                        AssociatedTypeDecl *inferredAssocType) {
    // Diagnose invalid cases.
    if (TypeChecker::isUnsupportedMemberTypeAccess(parentTy, member)) {
      if (!options.contains(TypeResolutionFlags::SilenceErrors)) {
        if (parentTy->is<UnboundGenericType>())
          diagnoseUnboundGenericType(parentTy, parentRange.End);
        else if (parentTy->isExistentialType() &&
                 isa<AssociatedTypeDecl>(member)) {
          diags.diagnose(comp->getIdLoc(), diag::assoc_type_outside_of_protocol,
                         comp->getIdentifier());
        } else if (parentTy->isExistentialType() &&
                   isa<TypeAliasDecl>(member)) {
          diags.diagnose(comp->getIdLoc(), diag::typealias_outside_of_protocol,
                         comp->getIdentifier());
        }
      }

      return ErrorType::get(ctx);
    }

    // Only the last component of the underlying type of a type alias may
    // be an unbound generic.
    if (options.is(TypeResolverContext::TypeAliasDecl)) {
      if (parentTy->is<UnboundGenericType>()) {
        if (!options.contains(TypeResolutionFlags::SilenceErrors))
          diagnoseUnboundGenericType(parentTy, parentRange.End);

        return ErrorType::get(ctx);
      }
    }

    // Diagnose a bad conformance reference if we need to.
    if (!options.contains(TypeResolutionFlags::SilenceErrors) &&
        inferredAssocType && memberType && memberType->hasError()) {
      maybeDiagnoseBadConformanceRef(DC, parentTy, comp->getLoc(),
                                     inferredAssocType);
    }

    // If we found a reference to an associated type or other member type that
    // was marked invalid, just return ErrorType to silence downstream errors.
    if (member->isInvalid())
      return ErrorType::get(ctx);

    // At this point, we need to have resolved the type of the member.
    if (!memberType || memberType->hasError())
      return memberType;

    // If there are generic arguments, apply them now.
    return maybeApplyGenericArgs(memberType);
  };

  // Short-circuiting.
  if (comp->isInvalid()) return ErrorType::get(ctx);

  // If the parent is a type parameter, the member is a dependent member,
  // and we skip much of the work below.
  if (parentTy->isTypeParameter()) {
    if (auto memberType = resolution.resolveDependentMemberType(
            parentTy, DC, parentRange, comp)) {
      // Hack -- if we haven't resolved this to a declaration yet, don't
      // attempt to apply generic arguments, since this will emit a
      // diagnostic, and its possible that this type will become a concrete
      // type later on.
      if (!memberType->is<DependentMemberType>() ||
          memberType->castTo<DependentMemberType>()->getAssocType()) {
        return maybeApplyGenericArgs(memberType);
      }

      return memberType;
    }
  }

  // Phase 2: If a declaration has already been bound, use it.
  if (auto *typeDecl = comp->getBoundDecl()) {
    auto memberType =
      TypeChecker::substMemberTypeWithBase(DC->getParentModule(), typeDecl,
                                           parentTy);
    return maybeDiagnoseBadMemberType(typeDecl, memberType, nullptr);
  }

  // Phase 1: Find and bind the component decl.

  // Look for member types with the given name.
  bool isKnownNonCascading = options.contains(TypeResolutionFlags::KnownNonCascadingDependency);
  if (!isKnownNonCascading && options.isAnyExpr()) {
    // Expressions cannot affect a function's signature.
    isKnownNonCascading = isa<AbstractFunctionDecl>(DC);
  }

  NameLookupOptions lookupOptions = defaultMemberLookupOptions;
  if (isKnownNonCascading)
    lookupOptions |= NameLookupFlags::KnownPrivate;
  if (options.is(TypeResolverContext::ExtensionBinding))
    lookupOptions -= NameLookupFlags::ProtocolMembers;
  LookupTypeResult memberTypes;
  if (parentTy->mayHaveMembers())
    memberTypes = TypeChecker::lookupMemberType(DC, parentTy,
                                                comp->getIdentifier(),
                                                lookupOptions);

  // Name lookup was ambiguous. Complain.
  // FIXME: Could try to apply generic arguments first, and see whether
  // that resolves things. But do we really want that to succeed?
  if (memberTypes.size() > 1) {
    if (!options.contains(TypeResolutionFlags::SilenceErrors))
      diagnoseAmbiguousMemberType(parentTy, parentRange, comp->getIdentifier(),
                                  comp->getIdLoc(), memberTypes);
    return ErrorType::get(ctx);
  }

  // If we didn't find anything, complain.
  Type memberType;
  TypeDecl *member = nullptr;
  AssociatedTypeDecl *inferredAssocType = nullptr;
  if (!memberTypes) {
    // If we're not allowed to complain or we couldn't fix the
    // source, bail out.
    if (options.contains(TypeResolutionFlags::SilenceErrors))
      return ErrorType::get(ctx);

    memberType = diagnoseUnknownType(resolution, parentTy, parentRange,
                                     comp, options, lookupOptions);
    member = comp->getBoundDecl();
    if (!member)
      return ErrorType::get(ctx);
  } else {
    memberType = memberTypes.back().MemberType;
    member = memberTypes.back().Member;
    inferredAssocType = memberTypes.back().InferredAssociatedType;
    comp->setValue(member, nullptr);
  }

  return maybeDiagnoseBadMemberType(member, memberType, inferredAssocType);
}

static Type resolveIdentTypeComponent(
              TypeResolution resolution,
              ArrayRef<ComponentIdentTypeRepr *> components,
              TypeResolutionOptions options) {
  auto comp = components.back();

  // The first component uses unqualified lookup.
  auto parentComps = components.slice(0, components.size()-1);
  if (parentComps.empty()) {
    return resolveTopLevelIdentTypeComponent(resolution, comp, options);
  }

  // All remaining components use qualified lookup.

  // Resolve the parent type.
  Type parentTy = resolveIdentTypeComponent(resolution, parentComps, options);
  if (!parentTy || parentTy->hasError()) return parentTy;
  
  SourceRange parentRange(parentComps.front()->getIdLoc(),
                          parentComps.back()->getSourceRange().End);

  // Resolve the nested type.
  return resolveNestedIdentTypeComponent(resolution, parentTy,
                                         parentRange, comp,
                                         options);
}

static bool diagnoseAvailability(IdentTypeRepr *IdType,
                                 DeclContext *DC,
                                 bool AllowPotentiallyUnavailableProtocol) {
  DeclAvailabilityFlags flags =
    DeclAvailabilityFlag::ContinueOnPotentialUnavailability;
  if (AllowPotentiallyUnavailableProtocol)
    flags |= DeclAvailabilityFlag::AllowPotentiallyUnavailableProtocol;
  ASTContext &ctx = DC->getASTContext();
  auto componentRange = IdType->getComponentRange();
  for (auto comp : componentRange) {
    if (auto *typeDecl = comp->getBoundDecl()) {
      assert(ctx.getLazyResolver() && "Must have a type checker!");
      TypeChecker &tc = static_cast<TypeChecker &>(*ctx.getLazyResolver());
      if (diagnoseDeclAvailability(typeDecl, tc, DC, comp->getIdLoc(), flags)) {
        return true;
      }
    }
  }

  return false;
}

// Hack to apply context-specific @escaping to an AST function type.
static Type applyNonEscapingFromContext(DeclContext *DC,
                                        Type ty,
                                        TypeResolutionOptions options) {
  // Remember whether this is a function parameter.
  bool defaultNoEscape = options.is(TypeResolverContext::FunctionInput) &&
                         !options.hasBase(TypeResolverContext::EnumElementDecl);

  // Desugar here
  auto *funcTy = ty->castTo<FunctionType>();
  auto extInfo = funcTy->getExtInfo();
  if (defaultNoEscape && !extInfo.isNoEscape()) {
    extInfo = extInfo.withNoEscape();

    // We lost the sugar to flip the isNoEscape bit.
    //
    // FIXME: It would be better to add a new AttributedType sugared type,
    // which would wrap the TypeAliasType or ParenType, and apply the
    // isNoEscape bit when de-sugaring.
    // <https://bugs.swift.org/browse/SR-2520>
    return FunctionType::get(funcTy->getParams(), funcTy->getResult(), extInfo);
  }

  // Note: original sugared type
  return ty;
}

/// Returns a valid type or ErrorType in case of an error.
Type TypeChecker::resolveIdentifierType(
       TypeResolution resolution,
       IdentTypeRepr *IdType,
       TypeResolutionOptions options) {
  auto DC = resolution.getDeclContext();
  ASTContext &ctx = DC->getASTContext();
  auto &diags = ctx.Diags;
  auto ComponentRange = IdType->getComponentRange();
  auto Components = llvm::makeArrayRef(ComponentRange.begin(),
                                       ComponentRange.end());
  Type result = resolveIdentTypeComponent(resolution, Components, options);
  if (!result) return nullptr;

  if (auto moduleTy = result->getAs<ModuleType>()) {
    if (!options.contains(TypeResolutionFlags::SilenceErrors)) {
      auto moduleName = moduleTy->getModule()->getName();
      diags.diagnose(Components.back()->getIdLoc(),
                     diag::use_undeclared_type, moduleName);
      diags.diagnose(Components.back()->getIdLoc(),
                     diag::note_module_as_type, moduleName);
    }
    Components.back()->setInvalid();
    return ErrorType::get(ctx);
  }

  // Hack to apply context-specific @escaping to a typealias with an underlying
  // function type.
  if (result->is<FunctionType>())
    result = applyNonEscapingFromContext(DC, result, options);

  // Check the availability of the type.

  // We allow a type to conform to a protocol that is less available than
  // the type itself. This enables a type to retroactively model or directly
  // conform to a protocol only available on newer OSes and yet still be used on
  // older OSes.
  // To support this, inside inheritance clauses we allow references to
  // protocols that are unavailable in the current type refinement context.

  if (!options.contains(TypeResolutionFlags::SilenceErrors) &&
      !options.contains(TypeResolutionFlags::AllowUnavailable) &&
      diagnoseAvailability(IdType, DC,
             options.contains(TypeResolutionFlags::AllowUnavailableProtocol))) {
    Components.back()->setInvalid();
    return ErrorType::get(ctx);
  }
  
  return result;
}

/// Validate whether type associated with @autoclosure attribute is correct,
/// it supposed to be a function type with no parameters.
/// \returns true if there was an error, false otherwise.
static bool validateAutoClosureAttr(TypeChecker &TC, const SourceLoc &loc,
                                    Type paramType) {
  if (auto *fnType = paramType->getAs<FunctionType>()) {
    if (fnType->getNumParams() != 0) {
      TC.diagnose(loc, diag::autoclosure_function_input_nonunit);
      return true;
    }
    // A function type with no parameters.
    return false;
  }

  TC.diagnose(loc, diag::autoclosure_function_type);
  return true;
}

/// Check whether the type associated with particular source location
/// has `@autoclosure` attribute, and if so, validate that such use is correct.
/// \returns true if there was an error, false otherwise.
static bool validateAutoClosureAttributeUse(TypeChecker &TC, const TypeLoc &loc,
                                            Type type,
                                            TypeResolutionOptions options) {
  auto *TR = loc.getTypeRepr();
  if (!TR || TR->isInvalid())
    return false;

  // If is a parameter declaration marked as @autoclosure.
  if (options.is(TypeResolverContext::FunctionInput)) {
    if (auto *ATR = dyn_cast<AttributedTypeRepr>(TR)) {
      const auto attrLoc = ATR->getAttrs().getLoc(TAK_autoclosure);
      if (attrLoc.isValid())
        return validateAutoClosureAttr(TC, attrLoc, type);
    }
  }

  // Otherwise, let's dig into the type and see if there are any
  // functions with parameters marked as @autoclosure,
  // such would be a part of expressions like:
  // `let _: (@autoclosure () -> Int) -> Void = ...`.
  bool isValid = true;
  type.visit([&](Type subType) {
    if (auto *fnType = subType->getAs<FunctionType>()) {
      isValid &= llvm::none_of(
          fnType->getParams(), [&](const FunctionType::Param &param) {
            return param.isAutoClosure() &&
                   validateAutoClosureAttr(TC, loc.getLoc(),
                                           param.getPlainType());
          });
    }
  });

  return !isValid;
}

bool TypeChecker::validateType(TypeLoc &Loc, TypeResolution resolution,
                               TypeResolutionOptions options) {
  // If we've already validated this type, don't do so again.
  if (Loc.wasValidated())
    return Loc.isError();

  if (Context.Stats)
    Context.Stats->getFrontendCounters().NumTypesValidated++;

  Type type = Loc.getType();
  if (type.isNull()) {
    type = resolution.resolveType(Loc.getTypeRepr(), options);
    if (!type) {
      type = ErrorType::get(Context);
      // Diagnose types that are illegal in SIL.
    } else if (options.contains(TypeResolutionFlags::SILType)
               && !type->isLegalSILType()) {
      diagnose(Loc.getLoc(), diag::illegal_sil_type, type);
      Loc.setInvalidType(Context);
      return true;
    } else if (validateAutoClosureAttributeUse(*this, Loc, type, options)) {
      type = ErrorType::get(Context);
    }
  }

  Loc.setType(type);
  if (!type->hasError()) {
    const DeclContext *DC = resolution.getDeclContext();
    if (options.isAnyExpr() || DC->getParent()->isLocalContext())
      if (DC->getResilienceExpansion() == ResilienceExpansion::Minimal)
        diagnoseGenericTypeExportability(Loc, DC);
  }

  return type->hasError();
}

namespace {
  const auto DefaultParameterConvention = ParameterConvention::Direct_Unowned;
  const auto DefaultResultConvention = ResultConvention::Unowned;

  class TypeResolver {
    ASTContext &Context;
    TypeResolution resolution;
    DeclContext *DC;

  public:
    explicit TypeResolver(TypeResolution resolution)
      : Context(resolution.getDeclContext()->getASTContext()),
        resolution(resolution),
        DC(resolution.getDeclContext())
    {
    }

    Type resolveType(TypeRepr *repr, TypeResolutionOptions options);

  private:
    template<typename ...ArgTypes>
    InFlightDiagnostic diagnose(ArgTypes &&...Args) const {
      auto &diags = Context.Diags;
      return diags.diagnose(std::forward<ArgTypes>(Args)...);
    }

    Type resolveAttributedType(AttributedTypeRepr *repr,
                               TypeResolutionOptions options);
    Type resolveAttributedType(TypeAttributes &attrs, TypeRepr *repr,
                               TypeResolutionOptions options);
    Type resolveASTFunctionType(FunctionTypeRepr *repr,
                                TypeResolutionOptions options,
                                FunctionType::ExtInfo extInfo
                                  = FunctionType::ExtInfo());
    bool
    resolveASTFunctionTypeParams(TupleTypeRepr *inputRepr,
                                 TypeResolutionOptions options,
                                 bool requiresMappingOut,
                                 // SWIFT_ENABLE_TENSORFLOW
                                 bool isDifferentiable,
                                 SmallVectorImpl<AnyFunctionType::Param> &ps);

    Type resolveSILFunctionType(FunctionTypeRepr *repr,
                                TypeResolutionOptions options,
                                SILCoroutineKind coroutineKind
                                  = SILCoroutineKind::None,
                                SILFunctionType::ExtInfo extInfo
                                  = SILFunctionType::ExtInfo(),
                                ParameterConvention calleeConvention
                                  = DefaultParameterConvention,
                                TypeRepr *witnessmethodProtocol = nullptr);
    SILParameterInfo resolveSILParameter(TypeRepr *repr,
                                         TypeResolutionOptions options);
    SILYieldInfo resolveSILYield(TypeAttributes &remainingAttrs,
                                 TypeRepr *repr, TypeResolutionOptions options);
    bool resolveSILResults(TypeRepr *repr, TypeResolutionOptions options,
                           SmallVectorImpl<SILYieldInfo> &yields,
                           SmallVectorImpl<SILResultInfo> &results,
                           Optional<SILResultInfo> &errorResult);
    bool resolveSingleSILResult(TypeRepr *repr, TypeResolutionOptions options,
                                SmallVectorImpl<SILYieldInfo> &yields,
                                SmallVectorImpl<SILResultInfo> &results,
                                Optional<SILResultInfo> &errorResult);
    Type resolveSpecifierTypeRepr(SpecifierTypeRepr *repr,
                                  TypeResolutionOptions options);
    Type resolveArrayType(ArrayTypeRepr *repr,
                          TypeResolutionOptions options);
    Type resolveDictionaryType(DictionaryTypeRepr *repr,
                               TypeResolutionOptions options);
    Type resolveOptionalType(OptionalTypeRepr *repr,
                             TypeResolutionOptions options);
    Type resolveImplicitlyUnwrappedOptionalType(ImplicitlyUnwrappedOptionalTypeRepr *repr,
                                                TypeResolutionOptions options,
                                                bool isDirect);
    Type resolveTupleType(TupleTypeRepr *repr,
                          TypeResolutionOptions options);
    Type resolveCompositionType(CompositionTypeRepr *repr,
                                TypeResolutionOptions options);
    Type resolveMetatypeType(MetatypeTypeRepr *repr,
                             TypeResolutionOptions options);
    Type resolveProtocolType(ProtocolTypeRepr *repr,
                             TypeResolutionOptions options);
    Type resolveSILBoxType(SILBoxTypeRepr *repr,
                           TypeResolutionOptions options);

    Type buildMetatypeType(MetatypeTypeRepr *repr,
                           Type instanceType,
                           Optional<MetatypeRepresentation> storedRepr);
    Type buildProtocolType(ProtocolTypeRepr *repr,
                           Type instanceType,
                           Optional<MetatypeRepresentation> storedRepr);
    
    Type resolveOpaqueReturnType(TypeRepr *repr, StringRef mangledName,
                                 unsigned ordinal,
                                 TypeResolutionOptions options);

    // SWIFT_ENABLE_TENSORFLOW
    bool isDifferentiableType(Type ty);
  };
} // end anonymous namespace

Type TypeResolution::resolveType(TypeRepr *TyR,
                              TypeResolutionOptions options) {
  FrontendStatsTracer StatsTracer(getASTContext().Stats, "resolve-type", TyR);
  PrettyStackTraceTypeRepr stackTrace(getASTContext(), "resolving", TyR);

  TypeResolver typeResolver(*this);
  auto result = typeResolver.resolveType(TyR, options);
  
  // If we resolved down to an error, make sure to mark the typeRepr as invalid
  // so we don't produce a redundant diagnostic.
  if (result && result->hasError())
    TyR->setInvalid();
  return result;
}

Type TypeResolver::resolveType(TypeRepr *repr, TypeResolutionOptions options) {
  assert(repr && "Cannot validate null TypeReprs!");

  // If we know the type representation is invalid, just return an
  // error type.
  if (repr->isInvalid()) return ErrorType::get(Context);

  // Strip the "is function input" bits unless this is a type that knows about
  // them.
  if (!isa<SpecifierTypeRepr>(repr) && !isa<TupleTypeRepr>(repr) &&
      !isa<AttributedTypeRepr>(repr) && !isa<FunctionTypeRepr>(repr) &&
      !isa<IdentTypeRepr>(repr) &&
      !isa<ImplicitlyUnwrappedOptionalTypeRepr>(repr)) {
    options.setContext(None);
  }


  if (Context.LangOpts.DisableAvailabilityChecking)
    options |= TypeResolutionFlags::AllowUnavailable;

  bool isDirect = false;
  if ((options & TypeResolutionFlags::Direct) && !isa<SpecifierTypeRepr>(repr)){
    isDirect = true;
    options -= TypeResolutionFlags::Direct;
  }

  switch (repr->getKind()) {
  case TypeReprKind::Error:
    return ErrorType::get(Context);

  case TypeReprKind::Attributed:
    return resolveAttributedType(cast<AttributedTypeRepr>(repr), options);
  case TypeReprKind::InOut:
  case TypeReprKind::Shared:
  case TypeReprKind::Owned:
    return resolveSpecifierTypeRepr(cast<SpecifierTypeRepr>(repr), options);

  case TypeReprKind::SimpleIdent:
  case TypeReprKind::GenericIdent:
  case TypeReprKind::CompoundIdent:
    return TypeChecker::resolveIdentifierType(resolution,
                                              cast<IdentTypeRepr>(repr),
                                              options);

  case TypeReprKind::Function: {
    if (!(options & TypeResolutionFlags::SILType)) {
      // Default non-escaping for closure parameters
      auto result =
          resolveASTFunctionType(cast<FunctionTypeRepr>(repr), options);
      if (result && result->is<FunctionType>())
        return applyNonEscapingFromContext(DC, result, options);
      return result;
    }
    return resolveSILFunctionType(cast<FunctionTypeRepr>(repr), options);
  }
  case TypeReprKind::SILBox:
    assert((options & TypeResolutionFlags::SILType) && "SILBox repr in non-SIL type context?!");
    return resolveSILBoxType(cast<SILBoxTypeRepr>(repr), options);

  case TypeReprKind::Array:
    return resolveArrayType(cast<ArrayTypeRepr>(repr), options);

  case TypeReprKind::Dictionary:
    return resolveDictionaryType(cast<DictionaryTypeRepr>(repr), options);

  case TypeReprKind::Optional:
    return resolveOptionalType(cast<OptionalTypeRepr>(repr), options);

  case TypeReprKind::ImplicitlyUnwrappedOptional: {
    auto iuoRepr = cast<ImplicitlyUnwrappedOptionalTypeRepr>(repr);
    return resolveImplicitlyUnwrappedOptionalType(iuoRepr, options, isDirect);
  }

  case TypeReprKind::Tuple:
    return resolveTupleType(cast<TupleTypeRepr>(repr), options);

  case TypeReprKind::Composition:
    return resolveCompositionType(cast<CompositionTypeRepr>(repr), options);

  case TypeReprKind::Metatype:
    return resolveMetatypeType(cast<MetatypeTypeRepr>(repr), options);

  case TypeReprKind::Protocol:
    return resolveProtocolType(cast<ProtocolTypeRepr>(repr), options);
      
  case TypeReprKind::OpaqueReturn: {
    // Only valid as the return type of a function, which should be handled
    // during function decl type checking.
    auto opaqueRepr = cast<OpaqueReturnTypeRepr>(repr);
    if (!(options & TypeResolutionFlags::SilenceErrors)) {
      diagnose(opaqueRepr->getOpaqueLoc(),
               diag::unsupported_opaque_type);
    }
    
    // Try to resolve the constraint upper bound type as a placeholder.
    options |= TypeResolutionFlags::SilenceErrors;
    auto constraintType = resolveType(opaqueRepr->getConstraint(),
                                      options);
    
    return constraintType && !constraintType->hasError()
      ? ErrorType::get(constraintType) : ErrorType::get(Context);
  }

  case TypeReprKind::Fixed:
    return cast<FixedTypeRepr>(repr)->getType();
  }
  llvm_unreachable("all cases should be handled");
}

static Type rebuildWithDynamicSelf(ASTContext &Context, Type ty) {
  if (auto metatypeTy = ty->getAs<MetatypeType>()) {
    return MetatypeType::get(
        rebuildWithDynamicSelf(Context, metatypeTy->getInstanceType()),
        metatypeTy->getRepresentation());
  } else if (auto optionalTy = ty->getOptionalObjectType()) {
    return OptionalType::get(rebuildWithDynamicSelf(Context, optionalTy));
  } else {
    return DynamicSelfType::get(ty, Context);
  }
}

Type TypeResolver::resolveAttributedType(AttributedTypeRepr *repr,
                                         TypeResolutionOptions options) {
  // Copy the attributes, since we're about to start hacking on them.
  TypeAttributes attrs = repr->getAttrs();
  assert(!attrs.empty());

  return resolveAttributedType(attrs, repr->getTypeRepr(), options);
}

Type TypeResolver::resolveAttributedType(TypeAttributes &attrs,
                                         TypeRepr *repr,
                                         TypeResolutionOptions options) {
  // Convenience to grab the source range of a type attribute.
  auto getTypeAttrRangeWithAt = [](ASTContext &ctx, SourceLoc attrLoc) {
    return SourceRange(attrLoc.getAdvancedLoc(-1),
                       Lexer::getLocForEndOfToken(ctx.SourceMgr, attrLoc));

  };

  // Remember whether this is a function parameter.
  bool isParam = options.is(TypeResolverContext::FunctionInput);

  bool isVariadicFunctionParam =
      options.is(TypeResolverContext::VariadicFunctionInput) &&
      !options.hasBase(TypeResolverContext::EnumElementDecl);

  // The type we're working with, in case we want to build it differently
  // based on the attributes we see.
  Type ty;
  
  // If this is a reference to an opaque return type, resolve it.
  if (auto &opaque = attrs.OpaqueReturnTypeOf) {
    return resolveOpaqueReturnType(repr, opaque->mangledName, opaque->index,
                                   options);
  }
  
  // In SIL *only*, allow @thin, @thick, or @objc_metatype to apply to
  // a metatype.
  if (attrs.has(TAK_thin) || attrs.has(TAK_thick) || 
      attrs.has(TAK_objc_metatype)) {
    if (auto SF = DC->getParentSourceFile()) {
      if (SF->Kind == SourceFileKind::SIL) {
        TypeRepr *base;
        if (auto metatypeRepr = dyn_cast<MetatypeTypeRepr>(repr)) {
          base = metatypeRepr->getBase();
        } else if (auto protocolRepr = dyn_cast<ProtocolTypeRepr>(repr)) {
          base = protocolRepr->getBase();
        } else {
          base = nullptr;
        }

        if (base) {
          Optional<MetatypeRepresentation> storedRepr;
          // The instance type is not a SIL type.
          auto instanceOptions = options;
          instanceOptions.setContext(None);
          instanceOptions -= TypeResolutionFlags::SILType;

          auto instanceTy = resolveType(base, instanceOptions);
          if (!instanceTy || instanceTy->hasError())
            return instanceTy;

          // Check for @thin.
          if (attrs.has(TAK_thin)) {
            storedRepr = MetatypeRepresentation::Thin;
            attrs.clearAttribute(TAK_thin);
          }

          // Check for @thick.
          if (attrs.has(TAK_thick)) {
            if (storedRepr)
              diagnose(repr->getStartLoc(), diag::sil_metatype_multiple_reprs);
              
            storedRepr = MetatypeRepresentation::Thick;
            attrs.clearAttribute(TAK_thick);
          }

          // Check for @objc_metatype.
          if (attrs.has(TAK_objc_metatype)) {
            if (storedRepr)
              diagnose(repr->getStartLoc(), diag::sil_metatype_multiple_reprs);
              
            storedRepr = MetatypeRepresentation::ObjC;
            attrs.clearAttribute(TAK_objc_metatype);
          }

          if (instanceTy->hasError()) {
            ty = instanceTy;
          } else if (auto metatype = dyn_cast<MetatypeTypeRepr>(repr)) {
            ty = buildMetatypeType(metatype, instanceTy, storedRepr);
          } else {
            ty = buildProtocolType(cast<ProtocolTypeRepr>(repr),
                                   instanceTy, storedRepr);
          }
        }
      }
    }
  }

  // Pass down the variable function type attributes to the
  // function-type creator.
  static const TypeAttrKind FunctionAttrs[] = {
    TAK_convention, TAK_pseudogeneric,
    TAK_callee_owned, TAK_callee_guaranteed, TAK_noescape, TAK_autoclosure,
    // SWIFT_ENABLE_TENSORFLOW
    TAK_escaping, TAK_autodiff, TAK_differentiable, TAK_yield_once, TAK_yield_many
  };

  auto checkUnsupportedAttr = [&](TypeAttrKind attr) {
    if (attrs.has(attr)) {
      diagnose(attrs.getLoc(attr), diag::unknown_attribute,
               TypeAttributes::getAttrName(attr));
      attrs.clearAttribute(attr);
    }
  };
  
  // Some function representation attributes are not supported at source level;
  // only SIL knows how to handle them.  Reject them unless this is a SIL input.
  if (!(options & TypeResolutionFlags::SILType)) {
    for (auto silOnlyAttr : {TAK_pseudogeneric,
                             TAK_callee_owned,
                             TAK_callee_guaranteed,
                             TAK_noescape,
                             TAK_yield_once,
                             TAK_yield_many}) {
      checkUnsupportedAttr(silOnlyAttr);
    }
  }  

  // Other function representation attributes are not normally supported at
  // source level, but we want to support them there in SIL files.
  auto SF = DC->getParentSourceFile();
  if (!SF || SF->Kind != SourceFileKind::SIL) {
    for (auto silOnlyAttr : {TAK_thin, TAK_thick}) {
      checkUnsupportedAttr(silOnlyAttr);
    }
  }

  bool hasFunctionAttr =
      llvm::any_of(FunctionAttrs, [&attrs](const TypeAttrKind &attr) {
        return attrs.has(attr);
      });

  // Function attributes require a syntactic function type.
  auto *fnRepr = dyn_cast<FunctionTypeRepr>(repr);

  if (fnRepr && hasFunctionAttr) {
    if (options & TypeResolutionFlags::SILType) {
      SILFunctionType::Representation rep;
      TypeRepr *witnessMethodProtocol = nullptr;

      auto coroutineKind = SILCoroutineKind::None;
      if (attrs.has(TAK_yield_once)) {
        coroutineKind = SILCoroutineKind::YieldOnce;
      } else if (attrs.has(TAK_yield_many)) {
        coroutineKind = SILCoroutineKind::YieldMany;
      }

      auto calleeConvention = ParameterConvention::Direct_Unowned;
      if (attrs.has(TAK_callee_owned)) {
        if (attrs.has(TAK_callee_guaranteed)) {
          diagnose(attrs.getLoc(TAK_callee_owned),
                   diag::sil_function_repeat_convention, /*callee*/ 2);
        }
        calleeConvention = ParameterConvention::Direct_Owned;
      } else if (attrs.has(TAK_callee_guaranteed)) {
        calleeConvention = ParameterConvention::Direct_Guaranteed;
      }

      if (!attrs.hasConvention()) {
        rep = SILFunctionType::Representation::Thick;
      } else {
        auto convention = attrs.getConvention();
        // SIL exposes a greater number of conventions than Swift source.
        auto parsedRep =
            llvm::StringSwitch<Optional<SILFunctionType::Representation>>(
                convention)
                .Case("thick", SILFunctionType::Representation::Thick)
                .Case("block", SILFunctionType::Representation::Block)
                .Case("thin", SILFunctionType::Representation::Thin)
                .Case("c", SILFunctionType::Representation::CFunctionPointer)
                .Case("method", SILFunctionType::Representation::Method)
                .Case("objc_method",
                      SILFunctionType::Representation::ObjCMethod)
                .Case("witness_method",
                      SILFunctionType::Representation::WitnessMethod)
                .Default(None);
        if (!parsedRep) {
          diagnose(attrs.getLoc(TAK_convention),
                   diag::unsupported_sil_convention, attrs.getConvention());
          rep = SILFunctionType::Representation::Thin;
        } else {
          rep = *parsedRep;
        }

        if (rep == SILFunctionType::Representation::WitnessMethod) {
          auto protocolName = *attrs.conventionWitnessMethodProtocol;
          witnessMethodProtocol = new (Context) SimpleIdentTypeRepr(
              SourceLoc(), Context.getIdentifier(protocolName));
        }
      }

      // Resolve the function type directly with these attributes.
      SILFunctionType::ExtInfo extInfo(rep, attrs.has(TAK_pseudogeneric),
                                       // SWIFT_ENABLE_TENSORFLOW
                                       attrs.has(TAK_noescape),
                                       attrs.has(TAK_autodiff) || attrs.has(TAK_differentiable));

      ty = resolveSILFunctionType(fnRepr, options, coroutineKind, extInfo,
                                  calleeConvention, witnessMethodProtocol);
      if (!ty || ty->hasError())
        return ty;
    } else {
      FunctionType::Representation rep = FunctionType::Representation::Swift;
      if (attrs.hasConvention()) {
        auto parsedRep =
            llvm::StringSwitch<Optional<FunctionType::Representation>>(
                attrs.getConvention())
                .Case("swift", FunctionType::Representation::Swift)
                .Case("block", FunctionType::Representation::Block)
                .Case("thin", FunctionType::Representation::Thin)
                .Case("c", FunctionType::Representation::CFunctionPointer)
                .Default(None);
        if (!parsedRep) {
          diagnose(attrs.getLoc(TAK_convention), diag::unsupported_convention,
                   attrs.getConvention());
          rep = FunctionType::Representation::Swift;
        } else {
          rep = *parsedRep;
        }
      }

      // @autoclosure is only valid on parameters.
      if (!isParam && attrs.has(TAK_autoclosure)) {
        diagnose(attrs.getLoc(TAK_autoclosure),
                 isVariadicFunctionParam ? diag::attr_not_on_variadic_parameters
                                         : diag::attr_only_on_parameters,
                 "@autoclosure");
        attrs.clearAttribute(TAK_autoclosure);
      }

      // Resolve the function type directly with these attributes.
      FunctionType::ExtInfo extInfo(rep, /*noescape=*/false,
                                    // SWIFT_ENABLE_TENSORFLOW
                                    fnRepr->throws(),
                                    attrs.has(TAK_autodiff) || attrs.has(TAK_differentiable));

      ty = resolveASTFunctionType(fnRepr, options, extInfo);
      if (!ty || ty->hasError())
        return ty;
    }
  }

  auto instanceOptions = options;
  instanceOptions.setContext(None);

  // If we didn't build the type differently above, we might have
  // a typealias pointing at a function type with the @escaping
  // attribute. Resolve the type as if it were in non-parameter
  // context, and then set isNoEscape if @escaping is not present.
  if (!ty) ty = resolveType(repr, instanceOptions);
  if (!ty || ty->hasError()) return ty;

  // Type aliases inside protocols are not yet resolved in the structural
  // stage of type resolution
  if (ty->is<DependentMemberType>() &&
      resolution.getStage() == TypeResolutionStage::Structural) {
    return ty;
  }

  // Handle @escaping
  if (ty->is<FunctionType>()) {
    if (attrs.has(TAK_escaping)) {
      // The attribute is meaningless except on non-variadic parameter types.
      if (!isParam || options.getBaseContext() == TypeResolverContext::EnumElementDecl) {
        auto loc = attrs.getLoc(TAK_escaping);
        auto attrRange = getTypeAttrRangeWithAt(Context, loc);

        diagnose(loc, diag::escaping_non_function_parameter)
          .fixItRemove(attrRange);

        // Try to find a helpful note based on how the type is being used
        if (options.is(TypeResolverContext::ImmediateOptionalTypeArgument)) {
          diagnose(repr->getLoc(), diag::escaping_optional_type_argument);
        }
      }

      attrs.clearAttribute(TAK_escaping);
    } else {
      // No attribute; set the isNoEscape bit if we're in parameter context.
      ty = applyNonEscapingFromContext(DC, ty, options);
    }
  }

  if (hasFunctionAttr && !fnRepr) {
    if (attrs.has(TAK_autoclosure)) {
      // @autoclosure is going to be diagnosed when type of
      // the parameter is validated, because that attribute
      // applies to the declaration now.
      attrs.clearAttribute(TAK_autoclosure);
    }

    for (auto i : FunctionAttrs) {
      if (!attrs.has(i))
        continue;

      auto diag = diagnose(attrs.getLoc(i),
                           diag::attribute_requires_function_type,
                           TypeAttributes::getAttrName(i));

      // If we see @escaping among the attributes on this type, because it isn't
      // a function type, we'll remove it.
      if (i == TAK_escaping) {
        diag.fixItRemove(getTypeAttrRangeWithAt(Context,
                                                attrs.getLoc(TAK_escaping)));
        // Specialize the diagnostic for Optionals.
        if (ty->getOptionalObjectType()) {
          diag.flush();
          diagnose(repr->getLoc(), diag::escaping_optional_type_argument);
        }
      }
      attrs.clearAttribute(i);
    }
  } else if (hasFunctionAttr && fnRepr) {
    // Remove the function attributes from the set so that we don't diagnose.
    for (auto i : FunctionAttrs)
      attrs.clearAttribute(i);
    attrs.convention = None;
  }
  
  // SWIFT_ENABLE_TENSORFLOW
  // @nondiff is only valid on parameters.
  if (attrs.has(TAK_nondiff)) {
    if (!isParam)
        diagnose(attrs.getLoc(TAK_nondiff),
                 isVariadicFunctionParam
                     ? diag::attr_not_on_variadic_parameters
                     : diag::attr_not_on_variadic_parameters, "@nondiff");
    attrs.clearAttribute(TAK_nondiff);
  }
  
  // In SIL, handle @opened (n), which creates an existential archetype.
  if (attrs.has(TAK_opened)) {
    if (!ty->isExistentialType()) {
      diagnose(attrs.getLoc(TAK_opened), diag::opened_non_protocol, ty);
    } else {
      ty = OpenedArchetypeType::get(ty, attrs.OpenedID);
    }
    attrs.clearAttribute(TAK_opened);
  }

  // In SIL files *only*, permit @weak and @unowned to apply directly to types.
  if (attrs.hasOwnership()) {
    if (auto SF = DC->getParentSourceFile()) {
      if (SF->Kind == SourceFileKind::SIL) {
        if (((attrs.has(TAK_sil_weak) || attrs.has(TAK_sil_unmanaged)) &&
             ty->getOptionalObjectType()) ||
            (!attrs.has(TAK_sil_weak) && ty->hasReferenceSemantics())) {
          ty = ReferenceStorageType::get(ty, attrs.getOwnership(), Context);
          attrs.clearOwnership();
        }
      }
    }
  }
  
  // In SIL *only*, allow @block_storage to specify a block storage type.
  if ((options & TypeResolutionFlags::SILType) && attrs.has(TAK_block_storage)) {
    ty = SILBlockStorageType::get(ty->getCanonicalType());
    attrs.clearAttribute(TAK_block_storage);
  }
  
  // In SIL *only*, allow @box to specify a box type.
  if ((options & TypeResolutionFlags::SILType) && attrs.has(TAK_box)) {
    ty = SILBoxType::get(ty->getCanonicalType());
    attrs.clearAttribute(TAK_box);
  }

  // In SIL *only*, allow @dynamic_self to specify a dynamic Self type.
  if ((options & TypeResolutionFlags::SILMode) && attrs.has(TAK_dynamic_self)) {
    ty = rebuildWithDynamicSelf(Context, ty);
    attrs.clearAttribute(TAK_dynamic_self);
  }

  for (unsigned i = 0; i != TypeAttrKind::TAK_Count; ++i)
    if (attrs.has((TypeAttrKind)i))
      diagnose(attrs.getLoc((TypeAttrKind)i),
               diag::attribute_does_not_apply_to_type);

  return ty;
}

bool TypeResolver::resolveASTFunctionTypeParams(
    TupleTypeRepr *inputRepr, TypeResolutionOptions options,
    // SWIFT_ENABLE_TENSORFLOW
    bool requiresMappingOut, bool isDifferentiable,
    SmallVectorImpl<AnyFunctionType::Param> &elements) {
  elements.reserve(inputRepr->getNumElements());

  auto elementOptions = options.withoutContext(true);
  elementOptions.setContext(TypeResolverContext::FunctionInput);
  for (unsigned i = 0, end = inputRepr->getNumElements(); i != end; ++i) {
    auto *eltTypeRepr = inputRepr->getElementType(i);

    // If the element is a variadic parameter, resolve the parameter type as if
    // it were in non-parameter position, since we want functions to be
    // @escaping in this case.
    auto thisElementOptions = elementOptions;
    bool variadic = false;
    if (inputRepr->hasEllipsis() &&
        elements.size() == inputRepr->getEllipsisIndex()) {
      thisElementOptions = elementOptions.withoutContext();
      thisElementOptions.setContext(TypeResolverContext::VariadicFunctionInput);
      variadic = true;
    }

    Type ty = resolveType(eltTypeRepr, thisElementOptions);
    if (!ty) return true;

    if (ty->hasError()) {
      elements.emplace_back(ErrorType::get(Context));
      continue;
    }

    // Parameters of polymorphic functions speak in terms of interface types.
    if (requiresMappingOut) {
      ty = ty->mapTypeOutOfContext();
    }

    bool autoclosure = false;
    if (auto *ATR = dyn_cast<AttributedTypeRepr>(eltTypeRepr))
      autoclosure = ATR->getAttrs().has(TAK_autoclosure);

    ValueOwnership ownership;

    auto *nestedRepr = eltTypeRepr;

    // Look through parens here; other than parens, specifiers
    // must appear at the top level of a parameter type.
    while (auto *tupleRepr = dyn_cast<TupleTypeRepr>(nestedRepr)) {
      if (!tupleRepr->isParenType())
        break;
      nestedRepr = tupleRepr->getElementType(0);
    }

    switch (nestedRepr->getKind()) {
    case TypeReprKind::Shared:
      ownership = ValueOwnership::Shared;
      break;
    case TypeReprKind::InOut:
      ownership = ValueOwnership::InOut;
      break;
    case TypeReprKind::Owned:
      ownership = ValueOwnership::Owned;
      break;
    default:
      ownership = ValueOwnership::Default;
      break;
    }

    // SWIFT_ENABLE_TENSORFLOW
    bool nondiff = false;
    if (auto *attrTypeRepr = dyn_cast<AttributedTypeRepr>(eltTypeRepr)) {
      if (attrTypeRepr->getAttrs().has(TAK_nondiff)) {
        if (!isDifferentiable)
          diagnose(eltTypeRepr->getLoc(),
                   diag::nondiff_attr_invalid_on_nondifferentiable_function)
              .highlight(eltTypeRepr->getSourceRange());
        else
          nondiff = true;
      }
    }

    if (isDifferentiable &&
        resolution.getStage() != TypeResolutionStage::Structural) {
      if (!nondiff && !isDifferentiableType(ty)) {
        diagnose(eltTypeRepr->getLoc(),
                 diag::autodiff_attr_argument_not_differentiable)
            .fixItInsert(eltTypeRepr->getLoc(), "@nondiff ");
      }
    }

    // SWIFT_ENABLE_TENSORFLOW
    ParameterTypeFlags paramFlags =
        ParameterTypeFlags::fromParameterType(ty, variadic, autoclosure,
                                              ownership, nondiff);
    elements.emplace_back(ty, Identifier(), paramFlags);
  }

  return false;
}

Type TypeResolver::resolveOpaqueReturnType(TypeRepr *repr,
                                           StringRef mangledName,
                                           unsigned ordinal,
                                           TypeResolutionOptions options) {
  // The type repr should be a generic identifier type. We don't really use
  // the identifier for anything, but we do resolve the generic arguments
  // to instantiate the possibly-generic opaque type.
  SmallVector<Type, 4> TypeArgs;
  if (auto generic = dyn_cast<GenericIdentTypeRepr>(repr)) {
    for (auto argRepr : generic->getGenericArgs()) {
      auto argTy = resolveType(argRepr, options);
      if (!argTy)
        return Type();
      TypeArgs.push_back(argTy);
    }
  }
  
  // Use type reconstruction to summon the opaque type decl.
  Demangler demangle;
  auto definingDeclNode = demangle.demangleSymbol(mangledName);
  if (!definingDeclNode)
    return Type();
  ASTBuilder builder(Context);
  builder.Resolver = resolution.Resolver;
  auto opaqueNode =
    builder.getNodeFactory().createNode(Node::Kind::OpaqueReturnTypeOf);
  opaqueNode->addChild(definingDeclNode, builder.getNodeFactory());
  
  auto ty = builder.resolveOpaqueType(opaqueNode, TypeArgs, ordinal);
  if (!ty) {
    diagnose(repr->getLoc(), diag::no_opaque_return_type_of);
  }
  return ty;
}

Type TypeResolver::resolveASTFunctionType(FunctionTypeRepr *repr,
                                          TypeResolutionOptions parentOptions,
                                          FunctionType::ExtInfo extInfo) {
  TypeResolutionOptions options = None;
  options |= parentOptions.withoutContext().getFlags();

  SmallVector<AnyFunctionType::Param, 8> params;
  if (resolveASTFunctionTypeParams(repr->getArgsTypeRepr(), options,
                                   // SWIFT_ENABLE_TENSORFLOW
                                   repr->getGenericEnvironment() != nullptr,
                                   extInfo.isDifferentiable(), params)) {
    return Type();
  }

  Type outputTy = resolveType(repr->getResultTypeRepr(), options);
  if (!outputTy || outputTy->hasError()) return outputTy;

  extInfo = extInfo.withThrows(repr->throws());

  // If this is a function type without parens around the parameter list,
  // diagnose this and produce a fixit to add them.
  if (!repr->isWarnedAbout()) {
    // If someone wrote (Void) -> () in Swift 3, they probably meant
    // () -> (), but (Void) -> () is (()) -> () so emit a warning
    // asking if they meant () -> ().
    auto args = repr->getArgsTypeRepr();
    if (args->getNumElements() == 1) {
      if (const auto Void =
          dyn_cast<SimpleIdentTypeRepr>(args->getElementType(0))) {
        if (Void->getIdentifier().str() == "Void") {
          diagnose(args->getStartLoc(), diag::paren_void_probably_void)
            .fixItReplace(args->getSourceRange(), "()");
          repr->setWarned();
        }
      }
    }
  }

  // SIL uses polymorphic function types to resolve overloaded member functions.
  if (auto genericEnv = repr->getGenericEnvironment()) {
    outputTy = outputTy->mapTypeOutOfContext();
    return GenericFunctionType::get(genericEnv->getGenericSignature(),
                                    params, outputTy, extInfo);
  }

  auto fnTy = FunctionType::get(params, outputTy, extInfo);
  // If the type is a block or C function pointer, it must be representable in
  // ObjC.
  switch (auto rep = extInfo.getRepresentation()) {
  case AnyFunctionType::Representation::Block:
  case AnyFunctionType::Representation::CFunctionPointer:
    if (!fnTy->isRepresentableIn(ForeignLanguage::ObjectiveC, DC)) {
      StringRef strName =
        rep == AnyFunctionType::Representation::Block ? "block" : "c";
      auto extInfo2 =
        extInfo.withRepresentation(AnyFunctionType::Representation::Swift);
      auto simpleFnTy = FunctionType::get(params, outputTy, extInfo2);
      diagnose(repr->getStartLoc(), diag::objc_convention_invalid,
               simpleFnTy, strName);
    }
    break;

  case AnyFunctionType::Representation::Thin:
  case AnyFunctionType::Representation::Swift:
    break;
  }

  // SWIFT_ENABLE_TENSORFLOW
  // If the function is marked as `@differentiable`, the result must be a
  // differentiable type.
  if (extInfo.isDifferentiable() &&
      resolution.getStage() != TypeResolutionStage::Structural) {
    if (!isDifferentiableType(outputTy)) {
      diagnose(repr->getResultTypeRepr()->getLoc(),
               diag::autodiff_attr_result_not_differentiable)
          .highlight(repr->getResultTypeRepr()->getSourceRange());
    }
  }

  return fnTy;
}

// SWIFT_ENABLE_TENSORFLOW
bool TypeResolver::isDifferentiableType(Type ty) {
  if (resolution.getStage() != TypeResolutionStage::Contextual) {
    ty = DC->mapTypeIntoContext(ty);
  }
  return ty
      ->getAutoDiffAssociatedVectorSpace(
          AutoDiffAssociatedVectorSpaceKind::Tangent,
          LookUpConformanceInModule(DC->getParentModule()))
      .hasValue();
}

Type TypeResolver::resolveSILBoxType(SILBoxTypeRepr *repr,
                                     TypeResolutionOptions options) {
  // Resolve the field types.
  SmallVector<SILField, 4> fields;
  {
    // Resolve field types using the box type's generic environment, if it
    // has one. (TODO: Field types should never refer to generic parameters
    // outside the box's own environment; we should really validate that...)
    Optional<TypeResolution> resolveSILBoxGenericParams;
    Optional<llvm::SaveAndRestore<TypeResolution>>
      useSILBoxGenericEnv;
    if (auto env = repr->getGenericEnvironment()) {
      resolveSILBoxGenericParams = TypeResolution::forContextual(DC, env);
      useSILBoxGenericEnv.emplace(resolution, *resolveSILBoxGenericParams);
    }
    
    for (auto &fieldRepr : repr->getFields()) {
      auto fieldTy = resolveType(fieldRepr.getFieldType(), options);
      fields.push_back({fieldTy->getCanonicalType(), fieldRepr.isMutable()});
    }
  }

  // Substitute out parsed context types into interface types.
  CanGenericSignature genericSig;
  if (auto *genericEnv = repr->getGenericEnvironment()) {
    genericSig = genericEnv->getGenericSignature()->getCanonicalSignature();
    
    for (auto &field : fields) {
      auto transTy = field.getLoweredType()->mapTypeOutOfContext();
      field = {transTy->getCanonicalType(), field.isMutable()};
    }
  }
  
  // Resolve the generic arguments.
  // Start by building a TypeSubstitutionMap.
  SubstitutionMap subMap;
  if (genericSig) {
    TypeSubstitutionMap genericArgMap;

    auto params = genericSig->getGenericParams();
    if (repr->getGenericArguments().size()
          != genericSig->getGenericParams().size()) {
      diagnose(repr->getLoc(), diag::sil_box_arg_mismatch);
      return ErrorType::get(Context);
    }
  
    for (unsigned i : indices(params)) {
      auto argTy = resolveType(repr->getGenericArguments()[i], options);
      genericArgMap.insert({params[i], argTy->getCanonicalType()});
    }
    
    bool ok = true;
    subMap = SubstitutionMap::get(
      genericSig,
      QueryTypeSubstitutionMap{genericArgMap},
      [&](CanType depTy, Type replacement, ProtocolDecl *proto)
      -> ProtocolConformanceRef {
        auto result = TypeChecker::conformsToProtocol(
                                            replacement, proto, DC,
                                            ConformanceCheckOptions());
        // TODO: getSubstitutions callback ought to return Optional.
        if (!result) {
          ok = false;
          return ProtocolConformanceRef(proto);
        }
        
        return *result;
      });

    if (!ok)
      return ErrorType::get(Context);
  }
  
  auto layout = SILLayout::get(Context, genericSig, fields);
  return SILBoxType::get(Context, layout, subMap);
}

Type TypeResolver::resolveSILFunctionType(FunctionTypeRepr *repr,
                                          TypeResolutionOptions options,
                                          SILCoroutineKind coroutineKind,
                                          SILFunctionType::ExtInfo extInfo,
                                          ParameterConvention callee,
                                          TypeRepr *witnessMethodProtocol) {
  options.setContext(None);

  bool hasError = false;

  // Resolve parameter and result types using the function's generic
  // environment.
  SmallVector<SILParameterInfo, 4> params;
  SmallVector<SILYieldInfo, 4> yields;
  SmallVector<SILResultInfo, 4> results;
  Optional<SILResultInfo> errorResult;
  {
    Optional<TypeResolution> resolveSILFunctionGenericParams;
    Optional<llvm::SaveAndRestore<TypeResolution>> useSILFunctionGenericEnv;
    
    // Resolve generic params using the function's generic environment, if it
    // has one.
    if (auto env = repr->getGenericEnvironment()) {
      resolveSILFunctionGenericParams = TypeResolution::forContextual(DC, env);
      useSILFunctionGenericEnv.emplace(resolution,
                                       *resolveSILFunctionGenericParams);
    }
    
    auto argsTuple = repr->getArgsTypeRepr();
    // SIL functions cannot be variadic.
    if (argsTuple->hasEllipsis()) {
      diagnose(argsTuple->getEllipsisLoc(), diag::sil_function_ellipsis);
    }
    // SIL functions cannot have parameter names.
    for (auto &element : argsTuple->getElements()) {
      if (element.UnderscoreLoc.isValid())
        diagnose(element.UnderscoreLoc, diag::sil_function_input_label);
    }

    for (auto elt : argsTuple->getElements()) {
      auto elementOptions = options;
      elementOptions.setContext(TypeResolverContext::FunctionInput);
      auto param = resolveSILParameter(elt.Type, elementOptions);
      params.push_back(param);
      if (!param.getType()) return nullptr;

      if (param.getType()->hasError())
        hasError = true;
    }

    {
      // FIXME: Deal with unsatisfied dependencies.
      if (resolveSILResults(repr->getResultTypeRepr(), options, yields,
                            results, errorResult)) {
        hasError = true;
      }

      // Diagnose non-coroutines that declare yields.
      if (coroutineKind == SILCoroutineKind::None && !yields.empty()) {
        diagnose(repr->getResultTypeRepr()->getLoc(),
                 diag::sil_non_coro_yields);
        hasError = true;
      }
    }
  } // restore generic type resolution

  if (hasError) {
    return ErrorType::get(Context);
  }

  // FIXME: Remap the parsed context types to interface types.
  CanGenericSignature genericSig;
  SmallVector<SILParameterInfo, 4> interfaceParams;
  SmallVector<SILYieldInfo, 4> interfaceYields;
  SmallVector<SILResultInfo, 4> interfaceResults;
  Optional<SILResultInfo> interfaceErrorResult;
  if (auto *genericEnv = repr->getGenericEnvironment()) {
    genericSig = genericEnv->getGenericSignature()->getCanonicalSignature();
 
    for (auto &param : params) {
      auto transParamType = param.getType()->mapTypeOutOfContext()
          ->getCanonicalType();
      interfaceParams.push_back(param.getWithType(transParamType));
    }
    for (auto &yield : yields) {
      auto transYieldType = yield.getType()->mapTypeOutOfContext()
          ->getCanonicalType();
      interfaceYields.push_back(yield.getWithType(transYieldType));
    }
    for (auto &result : results) {
      auto transResultType = result.getType()->mapTypeOutOfContext()
          ->getCanonicalType();
      interfaceResults.push_back(result.getWithType(transResultType));
    }

    if (errorResult) {
      auto transErrorResultType = errorResult->getType()->mapTypeOutOfContext()
          ->getCanonicalType();
      interfaceErrorResult =
        errorResult->getWithType(transErrorResultType);
    }
  } else {
    interfaceParams = params;
    interfaceYields = yields;
    interfaceResults = results;
    interfaceErrorResult = errorResult;
  }
  Optional<ProtocolConformanceRef> witnessMethodConformance;
  if (witnessMethodProtocol) {
    auto resolved = resolveType(witnessMethodProtocol, options);
    if (resolved->hasError())
      return resolved;

    auto protocolType = resolved->getAs<ProtocolType>();
    if (!protocolType)
      return ErrorType::get(Context);

    Type selfType = params.back().getType();
    // The Self type can be nested in a few layers of metatypes (etc.).
    while (auto metatypeType = selfType->getAs<MetatypeType>()) {
      auto next = metatypeType->getInstanceType();
      if (next->isEqual(selfType))
        break;
      selfType = next;
    }

    witnessMethodConformance = TypeChecker::conformsToProtocol(
        selfType, protocolType->getDecl(), DC, ConformanceCheckOptions());
    assert(witnessMethodConformance &&
           "found witness_method without matching conformance");
  }

  return SILFunctionType::get(genericSig, extInfo, coroutineKind,
                              callee,
                              interfaceParams, interfaceYields,
                              interfaceResults, interfaceErrorResult,
                              Context, witnessMethodConformance);
}

SILYieldInfo TypeResolver::resolveSILYield(TypeAttributes &attrs,
                                           TypeRepr *repr,
                                           TypeResolutionOptions options) {
  AttributedTypeRepr attrRepr(attrs, repr);
  options.setContext(TypeResolverContext::FunctionInput);
  SILParameterInfo paramInfo = resolveSILParameter(&attrRepr, options);
  return SILYieldInfo(paramInfo.getType(), paramInfo.getConvention());
}

SILParameterInfo TypeResolver::resolveSILParameter(
                                 TypeRepr *repr,
                                 TypeResolutionOptions options) {
  assert(options.is(TypeResolverContext::FunctionInput) &&
         "Parameters should be marked as inputs");
  auto convention = DefaultParameterConvention;
  Type type;
  bool hadError = false;

  // SWIFT_ENABLE_TENSORFLOW
  auto differentiability =
      SILParameterDifferentiability::DifferentiableOrNotApplicable;

  if (auto attrRepr = dyn_cast<AttributedTypeRepr>(repr)) {
    auto attrs = attrRepr->getAttrs();

    auto checkFor = [&](TypeAttrKind tak, ParameterConvention attrConv) {
      if (!attrs.has(tak)) return;
      if (convention != DefaultParameterConvention) {
        diagnose(attrs.getLoc(tak), diag::sil_function_repeat_convention,
                 /*input*/ 0);
        hadError = true;
      }
      attrs.clearAttribute(tak);
      convention = attrConv;
    };
    checkFor(TypeAttrKind::TAK_in_guaranteed,
             ParameterConvention::Indirect_In_Guaranteed);
    checkFor(TypeAttrKind::TAK_in, ParameterConvention::Indirect_In);
    checkFor(TypeAttrKind::TAK_in_constant,
             ParameterConvention::Indirect_In_Constant);
    checkFor(TypeAttrKind::TAK_inout, ParameterConvention::Indirect_Inout);
    checkFor(TypeAttrKind::TAK_inout_aliasable,
             ParameterConvention::Indirect_InoutAliasable);
    checkFor(TypeAttrKind::TAK_owned, ParameterConvention::Direct_Owned);
    checkFor(TypeAttrKind::TAK_guaranteed,
             ParameterConvention::Direct_Guaranteed);

    // SWIFT_ENABLE_TENSORFLOW
    if (attrs.has(TAK_nondiff)) {
      attrs.clearAttribute(TAK_nondiff);
      differentiability = SILParameterDifferentiability::NotDifferentiable;
    }

    type = resolveAttributedType(attrs, attrRepr->getTypeRepr(), options);
  } else {
    type = resolveType(repr, options);
  }

  if (!type || type->hasError()) {
    hadError = true;

  // Diagnose types that are illegal in SIL.
  } else if (!type->isLegalSILType()) {
    diagnose(repr->getLoc(), diag::illegal_sil_type, type);
    hadError = true;
  }

  if (hadError) type = ErrorType::get(Context);
  // SWIFT_ENABLE_TENSORFLOW
  return SILParameterInfo(type->getCanonicalType(), convention,
                          differentiability);
}

bool TypeResolver::resolveSingleSILResult(TypeRepr *repr,
                                          TypeResolutionOptions options,
                                          SmallVectorImpl<SILYieldInfo> &yields,
                              SmallVectorImpl<SILResultInfo> &ordinaryResults,
                                       Optional<SILResultInfo> &errorResult) {
  Type type;
  auto convention = DefaultResultConvention;
  bool isErrorResult = false;

  if (auto attrRepr = dyn_cast<AttributedTypeRepr>(repr)) {
    // Copy the attributes out; we're going to destructively modify them.
    auto attrs = attrRepr->getAttrs();

    // Recognize @yields.
    if (attrs.has(TypeAttrKind::TAK_yields)) {
      attrs.clearAttribute(TypeAttrKind::TAK_yields);

      // The treatment from this point on is basically completely different.
      auto yield = resolveSILYield(attrs, attrRepr->getTypeRepr(), options);
      if (yield.getType()->hasError())
        return true;

      yields.push_back(yield);
      return false;
    }

    // Recognize @error.
    if (attrs.has(TypeAttrKind::TAK_error)) {
      attrs.clearAttribute(TypeAttrKind::TAK_error);
      isErrorResult = true;

      // Error results are always implicitly @owned.
      convention = ResultConvention::Owned;
    }

    // Recognize result conventions.
    bool hadError = false;
    auto checkFor = [&](TypeAttrKind tak, ResultConvention attrConv) {
      if (!attrs.has(tak)) return;
      if (convention != DefaultResultConvention) {
        diagnose(attrs.getLoc(tak), diag::sil_function_repeat_convention,
                 /*result*/ 1);
        hadError = true;
      }
      attrs.clearAttribute(tak);
      convention = attrConv;
    };
    checkFor(TypeAttrKind::TAK_out, ResultConvention::Indirect);
    checkFor(TypeAttrKind::TAK_owned, ResultConvention::Owned);
    checkFor(TypeAttrKind::TAK_unowned_inner_pointer,
             ResultConvention::UnownedInnerPointer);
    checkFor(TypeAttrKind::TAK_autoreleased, ResultConvention::Autoreleased);
    if (hadError) return true;

    type = resolveAttributedType(attrs, attrRepr->getTypeRepr(), options);
  } else {
    type = resolveType(repr, options);
  }

  // Propagate type-resolution errors out.
  if (!type || type->hasError()) return true;

  // Diagnose types that are illegal in SIL.
  if (!type->isLegalSILType()) {
    diagnose(repr->getStartLoc(), diag::illegal_sil_type, type);
    return false;
  }

  assert(!isErrorResult || convention == ResultConvention::Owned);
  SILResultInfo resolvedResult(type->getCanonicalType(), convention);

  if (!isErrorResult) {
    ordinaryResults.push_back(resolvedResult);
    return false;
  }

  // Error result types must have pointer-like representation.
  // FIXME: check that here?

  // We don't expect to have a reason to support multiple independent
  // error results.  (Would this be disjunctive or conjunctive?)
  if (errorResult.hasValue()) {
    diagnose(repr->getStartLoc(),
             diag::sil_function_multiple_error_results);
    return true;
  }

  errorResult = resolvedResult;
  return false;
}

bool TypeResolver::resolveSILResults(TypeRepr *repr,
                                     TypeResolutionOptions options,
                                SmallVectorImpl<SILYieldInfo> &yields,
                                SmallVectorImpl<SILResultInfo> &ordinaryResults,
                                Optional<SILResultInfo> &errorResult) {
  if (auto tuple = dyn_cast<TupleTypeRepr>(repr)) {
    bool hadError = false;
    for (auto &element : tuple->getElements()) {
      if (element.UnderscoreLoc.isValid())
        diagnose(element.UnderscoreLoc, diag::sil_function_output_label);
    }
    for (auto elt : tuple->getElements()) {
      if (resolveSingleSILResult(elt.Type, options,
                                 yields, ordinaryResults, errorResult))
        hadError = true;
    }
    return hadError;
  }

  return resolveSingleSILResult(repr, options,
                                yields, ordinaryResults, errorResult);
}

Type TypeResolver::resolveSpecifierTypeRepr(SpecifierTypeRepr *repr,
                                            TypeResolutionOptions options) {
  // inout is only valid for (non-Subscript and non-EnumCaseDecl)
  // function parameters.
  if (!options.is(TypeResolverContext::FunctionInput) ||
      options.hasBase(TypeResolverContext::SubscriptDecl) ||
      options.hasBase(TypeResolverContext::EnumElementDecl)) {

    decltype(diag::attr_only_on_parameters) diagID;
    if (options.getBaseContext() == TypeResolverContext::SubscriptDecl) {
      diagID = diag::attr_not_on_subscript_parameters;
    } else if (options.is(TypeResolverContext::VariadicFunctionInput)) {
      diagID = diag::attr_not_on_variadic_parameters;
    } else {
      diagID = diag::attr_only_on_parameters;
    }
    StringRef name;
    switch (repr->getKind()) {
    case TypeReprKind::InOut:
      name = "inout";
      break;
    case TypeReprKind::Shared:
      name = "__shared";
      break;
    case TypeReprKind::Owned:
      name = "__owned";
      break;
    default:
      llvm_unreachable("unknown SpecifierTypeRepr kind");
    }
    diagnose(repr->getSpecifierLoc(), diagID, name);
    repr->setInvalid();
    return ErrorType::get(Context);
  }

  if (!isa<ImplicitlyUnwrappedOptionalTypeRepr>(repr->getBase())) {
    // Anything within the inout isn't a parameter anymore.
    options.setContext(None);
  }

  return resolveType(repr->getBase(), options);
}


Type TypeResolver::resolveArrayType(ArrayTypeRepr *repr,
                                    TypeResolutionOptions options) {
  Type baseTy = resolveType(repr->getBase(), options.withoutContext());
  if (!baseTy || baseTy->hasError()) return baseTy;

  auto sliceTy =
    TypeChecker::getArraySliceType(repr->getBrackets().Start, baseTy);
  if (!sliceTy)
    return ErrorType::get(Context);

  return sliceTy;
}

Type TypeResolver::resolveDictionaryType(DictionaryTypeRepr *repr,
                                         TypeResolutionOptions options) {
  options = adjustOptionsForGenericArgs(options);

  Type keyTy = resolveType(repr->getKey(), options.withoutContext());
  if (!keyTy || keyTy->hasError()) return keyTy;

  Type valueTy = resolveType(repr->getValue(), options.withoutContext());
  if (!valueTy || valueTy->hasError()) return valueTy;

  auto dictDecl = Context.getDictionaryDecl();

  if (auto dictTy = TypeChecker::getDictionaryType(repr->getBrackets().Start,
                                                   keyTy, valueTy)) {
    // Check the requirements on the generic arguments.
    if (auto lazyResolver = Context.getLazyResolver())
      lazyResolver->resolveDeclSignature(dictDecl);

    auto unboundTy = dictDecl->getDeclaredType()->castTo<UnboundGenericType>();

    Type args[] = {keyTy, valueTy};

    if (!TypeChecker::applyUnboundGenericArguments(
            unboundTy, dictDecl, repr->getStartLoc(), resolution, args)) {
      return nullptr;
    }

    return dictTy;
  }

  return ErrorType::get(Context);
}

Type TypeResolver::resolveOptionalType(OptionalTypeRepr *repr,
                                       TypeResolutionOptions options) {
  TypeResolutionOptions elementOptions = options.withoutContext(true);
  elementOptions.setContext(TypeResolverContext::ImmediateOptionalTypeArgument);

  // The T in T? is a generic type argument and therefore always an AST type.
  // FIXME: diagnose non-materializability of element type!
  Type baseTy = resolveType(repr->getBase(), elementOptions);
  if (!baseTy || baseTy->hasError()) return baseTy;

  auto optionalTy = TypeChecker::getOptionalType(repr->getQuestionLoc(),
                                                 baseTy);
  if (!optionalTy) return ErrorType::get(Context);

  return optionalTy;
}

Type TypeResolver::resolveImplicitlyUnwrappedOptionalType(
      ImplicitlyUnwrappedOptionalTypeRepr *repr,
      TypeResolutionOptions options,
      bool isDirect) {
  TypeResolutionFlags allowIUO = TypeResolutionFlags::SILType;

  bool doDiag = false;
  switch (options.getContext()) {
  case TypeResolverContext::None:
    if (!isDirect || !(options & allowIUO))
      doDiag = true;
    break;
  case TypeResolverContext::FunctionInput:
  case TypeResolverContext::FunctionResult:
  case TypeResolverContext::DynamicSelfResult:
  case TypeResolverContext::PatternBindingDecl:
    doDiag = !isDirect;
    break;
  case TypeResolverContext::VariadicFunctionInput:
  case TypeResolverContext::ProtocolWhereClause:
  case TypeResolverContext::ForEachStmt:
  case TypeResolverContext::ExtensionBinding:
  case TypeResolverContext::ExplicitCastExpr:
  case TypeResolverContext::SubscriptDecl:
  case TypeResolverContext::EnumElementDecl:
  case TypeResolverContext::EnumPatternPayload:
  case TypeResolverContext::TypeAliasDecl:
  case TypeResolverContext::GenericTypeAliasDecl:
  case TypeResolverContext::GenericRequirement:
  case TypeResolverContext::ImmediateOptionalTypeArgument:
  case TypeResolverContext::InExpression:
  case TypeResolverContext::EditorPlaceholderExpr:
  case TypeResolverContext::AbstractFunctionDecl:
  case TypeResolverContext::ClosureExpr:
    doDiag = true;
    break;
  }

  if (doDiag) {
    // Prior to Swift 5, we allow 'as T!' and turn it into a disjunction.
    if (Context.isSwiftVersionAtLeast(5)) {
      diagnose(repr->getStartLoc(),
               diag::implicitly_unwrapped_optional_in_illegal_position)
          .fixItReplace(repr->getExclamationLoc(), "?");
    } else if (options.is(TypeResolverContext::ExplicitCastExpr)) {
      diagnose(
          repr->getStartLoc(),
          diag::implicitly_unwrapped_optional_deprecated_in_this_position);
    } else {
      diagnose(
          repr->getStartLoc(),
          diag::implicitly_unwrapped_optional_in_illegal_position_interpreted_as_optional)
          .fixItReplace(repr->getExclamationLoc(), "?");
    }
  }

  TypeResolutionOptions elementOptions = options.withoutContext(true);
  elementOptions.setContext(TypeResolverContext::ImmediateOptionalTypeArgument);

  // The T in T! is a generic type argument and therefore always an AST type.
  // FIXME: diagnose non-materializability of element type!
  Type baseTy = resolveType(repr->getBase(), elementOptions);
  if (!baseTy || baseTy->hasError()) return baseTy;

  Type uncheckedOptionalTy;
  uncheckedOptionalTy = TypeChecker::getOptionalType(repr->getExclamationLoc(),
                                                     baseTy);

  if (!uncheckedOptionalTy)
    return ErrorType::get(Context);

  return uncheckedOptionalTy;
}

Type TypeResolver::resolveTupleType(TupleTypeRepr *repr,
                                    TypeResolutionOptions options) {
  SmallVector<TupleTypeElt, 8> elements;
  elements.reserve(repr->getNumElements());
  
  auto elementOptions = options;
  if (!repr->isParenType()) {
    elementOptions = elementOptions.withoutContext(true);
  }

  // Variadic tuples are not permitted.
  bool complained = false;
  if (repr->hasEllipsis()) {
    diagnose(repr->getEllipsisLoc(), diag::tuple_ellipsis);
    repr->removeEllipsis();
    complained = true;
  }

  bool hadError = false;
  for (unsigned i = 0, end = repr->getNumElements(); i != end; ++i) {
    auto *tyR = repr->getElementType(i);

    Type ty = resolveType(tyR, elementOptions);
    if (!ty || ty->hasError())
      hadError = true;

    elements.emplace_back(ty, repr->getElementName(i), ParameterTypeFlags());
  }

  if (hadError)
    return ErrorType::get(Context);

  // Single-element labeled tuples are not permitted outside of declarations
  // or SIL, either.
  if (elements.size() == 1 && elements[0].hasName()
      && !(options & TypeResolutionFlags::SILType)) {
    if (!complained) {
      diagnose(repr->getElementNameLoc(0), diag::tuple_single_element)
        .fixItRemoveChars(repr->getElementNameLoc(0),
                          repr->getElementType(0)->getStartLoc());
    }

    elements[0] = TupleTypeElt(elements[0].getType());
  }

  return TupleType::get(elements, Context);
}

Type TypeResolver::resolveCompositionType(CompositionTypeRepr *repr,
                                          TypeResolutionOptions options) {

  // Note that the superclass type will appear as part of one of the
  // types in 'Members', so it's not used when constructing the
  // fully-realized type below -- but we just record it to make sure
  // there is only one superclass.
  Type SuperclassType;
  SmallVector<Type, 4> Members;

  // Whether we saw at least one protocol. A protocol composition
  // must either be empty (in which case it is Any or AnyObject),
  // or if it has a superclass constraint, have at least one protocol.
  bool HasProtocol = false;

  auto checkSuperclass = [&](SourceLoc loc, Type t) -> bool {
    if (SuperclassType && !SuperclassType->isEqual(t)) {
      diagnose(loc, diag::protocol_composition_one_class, t,
               SuperclassType);
      return true;
    }

    SuperclassType = t;
    return false;
  };

  for (auto tyR : repr->getTypes()) {
    Type ty = resolveType(tyR, options.withoutContext());
    if (!ty || ty->hasError()) return ty;

    auto nominalDecl = ty->getAnyNominal();
    if (nominalDecl && isa<ClassDecl>(nominalDecl)) {
      if (checkSuperclass(tyR->getStartLoc(), ty))
        continue;

      Members.push_back(ty);
      continue;
    }

    if (ty->isExistentialType()) {
      auto layout = ty->getExistentialLayout();
      if (auto superclass = layout.explicitSuperclass)
        if (checkSuperclass(tyR->getStartLoc(), superclass))
          continue;
      if (!layout.getProtocols().empty())
        HasProtocol = true;

      Members.push_back(ty);
      continue;
    }

    diagnose(tyR->getStartLoc(),
             diag::invalid_protocol_composition_member,
             ty);
  }

  // Avoid confusing diagnostics ('MyClass' not convertible to 'MyClass',
  // etc) by collapsing a composition consisting of a single class down
  // to the class itself.
  if (SuperclassType && !HasProtocol)
    return SuperclassType;

  // In user-written types, AnyObject constraints always refer to the
  // AnyObject type in the standard library.
  return ProtocolCompositionType::get(Context, Members,
                                      /*HasExplicitAnyObject=*/false);
}

Type TypeResolver::resolveMetatypeType(MetatypeTypeRepr *repr,
                                       TypeResolutionOptions options) {
  // The instance type of a metatype is always abstract, not SIL-lowered.
  Type ty = resolveType(repr->getBase(), options.withoutContext());
  if (!ty || ty->hasError()) return ty;

  Optional<MetatypeRepresentation> storedRepr;
  
  // In SIL mode, a metatype must have a @thin, @thick, or
  // @objc_metatype attribute, so metatypes should have been lowered
  // in resolveAttributedType.
  if (options & TypeResolutionFlags::SILType) {
    diagnose(repr->getStartLoc(), diag::sil_metatype_without_repr);
    storedRepr = MetatypeRepresentation::Thick;
  }

  return buildMetatypeType(repr, ty, storedRepr);
}

Type TypeResolver::buildMetatypeType(
       MetatypeTypeRepr *repr,
       Type instanceType,
       Optional<MetatypeRepresentation> storedRepr) {
  if (instanceType->isAnyExistentialType()) {
    // TODO: diagnose invalid representations?
    return ExistentialMetatypeType::get(instanceType, storedRepr);
  } else {
    return MetatypeType::get(instanceType, storedRepr);
  }
}

Type TypeResolver::resolveProtocolType(ProtocolTypeRepr *repr,
                                       TypeResolutionOptions options) {
  // The instance type of a metatype is always abstract, not SIL-lowered.
  Type ty = resolveType(repr->getBase(), options.withoutContext());
  if (!ty || ty->hasError()) return ty;

  Optional<MetatypeRepresentation> storedRepr;
  
  // In SIL mode, a metatype must have a @thin, @thick, or
  // @objc_metatype attribute, so metatypes should have been lowered
  // in resolveAttributedType.
  if (options & TypeResolutionFlags::SILType) {
    diagnose(repr->getStartLoc(), diag::sil_metatype_without_repr);
    storedRepr = MetatypeRepresentation::Thick;
  }

  return buildProtocolType(repr, ty, storedRepr);
}

Type TypeResolver::buildProtocolType(
       ProtocolTypeRepr *repr,
       Type instanceType,
       Optional<MetatypeRepresentation> storedRepr) {
  if (!instanceType->isAnyExistentialType()) {
    diagnose(repr->getProtocolLoc(), diag::dot_protocol_on_non_existential,
             instanceType);
    return ErrorType::get(Context);
  }

  return MetatypeType::get(instanceType, storedRepr);
}

Type TypeChecker::substMemberTypeWithBase(ModuleDecl *module,
                                          TypeDecl *member,
                                          Type baseTy,
                                          bool useArchetypes) {
  Type sugaredBaseTy = baseTy;

  // For type members of a base class, make sure we use the right
  // derived class as the parent type. If the base type is an error
  // type, we have an invalid extension, so do nothing.
  if (!baseTy->is<ErrorType>()) {
    if (auto *ownerClass = member->getDeclContext()->getSelfClassDecl()) {
      baseTy = baseTy->getSuperclassForDecl(ownerClass, useArchetypes);
    }
  }

  if (baseTy->is<ModuleType>()) {
    baseTy = Type();
    sugaredBaseTy = Type();
  }

  // The declared interface type for a generic type will have the type
  // arguments; strip them off.
  if (auto *nominalDecl = dyn_cast<NominalTypeDecl>(member)) {
    // If the base type is not a nominal type, we might be looking up a
    // nominal member of a generic parameter. This is not supported right
    // now, but at least don't crash.
    if (member->getDeclContext()->getSelfProtocolDecl())
      return nominalDecl->getDeclaredType();

    if (!isa<ProtocolDecl>(nominalDecl) &&
        nominalDecl->getGenericParams()) {
      return UnboundGenericType::get(
          nominalDecl, baseTy,
          nominalDecl->getASTContext());
    }

    if (baseTy && baseTy->is<ErrorType>())
      return baseTy;

    return NominalType::get(
        nominalDecl, baseTy,
        nominalDecl->getASTContext());
  }

  auto *aliasDecl = dyn_cast<TypeAliasDecl>(member);
  if (aliasDecl) {
    if (aliasDecl->getGenericParams()) {
      return UnboundGenericType::get(
          aliasDecl, baseTy,
          aliasDecl->getASTContext());
    }

    // FIXME: If this is a protocol typealias and we haven't built the
    // protocol's generic environment yet, do so now, to ensure the
    // typealias's underlying type has fully resolved dependent
    // member types.
    if (auto *protoDecl = dyn_cast<ProtocolDecl>(aliasDecl->getDeclContext())) {
      ASTContext &ctx = protoDecl->getASTContext();
      ctx.getLazyResolver()->resolveProtocolEnvironment(protoDecl);
    }
  }

  Type resultType;
  auto memberType = aliasDecl ? aliasDecl->getUnderlyingTypeLoc().getType()
                              : member->getDeclaredInterfaceType();
  SubstitutionMap subs;
  if (baseTy) {
    // Cope with the presence of unbound generic types, which are ill-formed
    // at this point but break the invariants of getContextSubstitutionMap().
    if (baseTy->hasUnboundGenericType()) {
      if (memberType->hasTypeParameter())
        return ErrorType::get(memberType);

      return memberType;
    }

    if (baseTy->is<ErrorType>())
      return ErrorType::get(memberType);

    subs = baseTy->getContextSubstitutionMap(module, member->getDeclContext());
    resultType = memberType.subst(subs, SubstFlags::UseErrorType);
  } else {
    resultType = memberType;
  }

  // If we're referring to a typealias within a generic context, build
  // a sugared alias type.
  if (aliasDecl && (!sugaredBaseTy || !sugaredBaseTy->isAnyExistentialType())) {
    resultType = TypeAliasType::get(aliasDecl, sugaredBaseTy, subs, resultType);
  }

  return resultType;
}

namespace {

class UnsupportedProtocolVisitor
  : public TypeReprVisitor<UnsupportedProtocolVisitor>, public ASTWalker
{
  TypeChecker &TC;
  bool checkStatements;
  bool hitTopStmt;
    
public:
  UnsupportedProtocolVisitor(TypeChecker &tc, bool checkStatements)
    : TC(tc), checkStatements(checkStatements), hitTopStmt(false) { }

  bool walkToTypeReprPre(TypeRepr *T) override {
    if (T->isInvalid())
      return false;
    if (auto compound = dyn_cast<CompoundIdentTypeRepr>(T)) {
      // Only visit the last component to check, because nested typealiases in
      // existentials are okay.
      visit(compound->getComponentRange().back());
      return false;
    }
    // Arbitrary protocol constraints are OK on opaque types.
    if (isa<OpaqueReturnTypeRepr>(T))
      return false;
    
    visit(T);
    return true;
  }

  std::pair<bool, Stmt*> walkToStmtPre(Stmt *S) override {
    if (checkStatements && !hitTopStmt) {
      hitTopStmt = true;
      return { true, S };
    }

    return { false, S };
  }

  bool walkToDeclPre(Decl *D) override {
    return !checkStatements;
  }

  void visitIdentTypeRepr(IdentTypeRepr *T) {
    if (T->isInvalid())
      return;
    
    auto comp = T->getComponentRange().back();
    if (auto *proto = dyn_cast_or_null<ProtocolDecl>(comp->getBoundDecl())) {
      if (!proto->existentialTypeSupported(&TC)) {
        TC.diagnose(comp->getIdLoc(), diag::unsupported_existential_type,
                    proto->getName());
        T->setInvalid();
      }
    } else if (auto *alias = dyn_cast_or_null<TypeAliasDecl>(comp->getBoundDecl())) {
      if (!alias->hasInterfaceType())
        return;
      auto type = Type(alias->getDeclaredInterfaceType()->getDesugaredType());
      type.findIf([&](Type type) -> bool {
        if (T->isInvalid())
          return false;
        if (type->isExistentialType()) {
          auto layout = type->getExistentialLayout();
          for (auto *proto : layout.getProtocols()) {
            auto *protoDecl = proto->getDecl();

            if (protoDecl->existentialTypeSupported(&TC))
              continue;
            
            TC.diagnose(comp->getIdLoc(), diag::unsupported_existential_type,
                        protoDecl->getName());
            T->setInvalid();
          }
        }
        return false;
      });
    }
  }

  void visitRequirements(ArrayRef<RequirementRepr> reqts) {
    for (auto reqt : reqts) {
      if (reqt.getKind() == RequirementReprKind::SameType) {
        if (auto *repr = reqt.getFirstTypeLoc().getTypeRepr())
          repr->walk(*this);
        if (auto *repr = reqt.getSecondTypeLoc().getTypeRepr())
          repr->walk(*this);
      }
    }
  }
};

} // end anonymous namespace

void TypeChecker::checkUnsupportedProtocolType(Decl *decl) {
  if (!decl || decl->isInvalid())
    return;

  if (auto *protocolDecl = dyn_cast<ProtocolDecl>(decl))
    checkUnsupportedProtocolType(protocolDecl->getTrailingWhereClause());
  else if (auto *genericDecl = dyn_cast<GenericTypeDecl>(decl))
    checkUnsupportedProtocolType(genericDecl->getGenericParams());
  else if (auto *assocType = dyn_cast<AssociatedTypeDecl>(decl))
    checkUnsupportedProtocolType(assocType->getTrailingWhereClause());
  else if (auto *extDecl = dyn_cast<ExtensionDecl>(decl))
    checkUnsupportedProtocolType(extDecl->getTrailingWhereClause());
  else if (auto *subscriptDecl = dyn_cast<SubscriptDecl>(decl))
    checkUnsupportedProtocolType(subscriptDecl->getGenericParams());
  else if (auto *funcDecl = dyn_cast<AbstractFunctionDecl>(decl)) {
    if (!isa<AccessorDecl>(funcDecl))
      checkUnsupportedProtocolType(funcDecl->getGenericParams());
  }

  if (isa<TypeDecl>(decl) || isa<ExtensionDecl>(decl))
    return;

  UnsupportedProtocolVisitor visitor(*this, /*checkStatements=*/false);
  decl->walk(visitor);
}

void TypeChecker::checkUnsupportedProtocolType(Stmt *stmt) {
  if (!stmt)
    return;

  UnsupportedProtocolVisitor visitor(*this, /*checkStatements=*/true);
  stmt->walk(visitor);
}

void TypeChecker::checkUnsupportedProtocolType(TrailingWhereClause *whereClause) {
  if (whereClause == nullptr)
    return;

  UnsupportedProtocolVisitor visitor(*this, /*checkStatements=*/false);
  visitor.visitRequirements(whereClause->getRequirements());
}

void TypeChecker::checkUnsupportedProtocolType(GenericParamList *genericParams) {
  if (genericParams  == nullptr)
    return;

  UnsupportedProtocolVisitor visitor(*this, /*checkStatements=*/false);
  visitor.visitRequirements(genericParams->getRequirements());
}