TypeCheckGeneric.cpp

//===--- TypeCheckGeneric.cpp - Generics ----------------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
//
// This file implements support for generics.
//
//===----------------------------------------------------------------------===//
#include "TypeChecker.h"
#include "TypeCheckType.h"
#include "swift/AST/DiagnosticsSema.h"
#include "swift/AST/ExistentialLayout.h"
#include "swift/AST/GenericEnvironment.h"
#include "swift/AST/GenericSignatureBuilder.h"
#include "swift/AST/ProtocolConformance.h"
#include "swift/AST/ParameterList.h"
#include "swift/AST/TypeCheckRequests.h"
#include "swift/AST/TypeResolutionStage.h"
#include "swift/AST/Types.h"
#include "swift/Basic/Defer.h"
#include "llvm/Support/ErrorHandling.h"

using namespace swift;

///
/// Common code for generic functions, generic types
///

std::string
TypeChecker::gatherGenericParamBindingsText(
                              ArrayRef<Type> types,
                              TypeArrayView<GenericTypeParamType> genericParams,
                              TypeSubstitutionFn substitutions) {
  llvm::SmallPtrSet<GenericTypeParamType *, 2> knownGenericParams;
  for (auto type : types) {
    if (type.isNull()) continue;

    type.visit([&](Type type) {
      if (auto gp = type->getAs<GenericTypeParamType>()) {
        knownGenericParams.insert(
            gp->getCanonicalType()->castTo<GenericTypeParamType>());
      }
    });
  }

  if (knownGenericParams.empty())
    return "";

  SmallString<128> result;
  for (auto gp : genericParams) {
    auto canonGP = gp->getCanonicalType()->castTo<GenericTypeParamType>();
    if (!knownGenericParams.count(canonGP))
      continue;

    if (result.empty())
      result += " [with ";
    else
      result += ", ";
    result += gp->getName().str();
    result += " = ";

    auto type = substitutions(canonGP);
    if (!type)
      return "";

    result += type.getString();
  }

  result += "]";
  return result.str().str();
}

//
// Generic functions
//

/// Get the opaque type representing the return type of a declaration, or
/// create it if it does not yet exist.
llvm::Expected<OpaqueTypeDecl *>
OpaqueResultTypeRequest::evaluate(Evaluator &evaluator,
                                  ValueDecl *originatingDecl) const {
  auto *repr = originatingDecl->getOpaqueResultTypeRepr();
  assert(repr && "Declaration does not have an opaque result type");
  auto *dc = originatingDecl->getInnermostDeclContext();
  auto &ctx = dc->getASTContext();

  // Protocol requirements can't have opaque return types.
  //
  // TODO: Maybe one day we could treat this as sugar for an associated type.
  if (isa<ProtocolDecl>(originatingDecl->getDeclContext())
      && originatingDecl->isProtocolRequirement()) {

    SourceLoc fixitLoc;
    if (auto vd = dyn_cast<VarDecl>(originatingDecl)) {
      fixitLoc = vd->getParentPatternBinding()->getStartLoc();
    } else {
      fixitLoc = originatingDecl->getStartLoc();
    }

    ctx.Diags.diagnose(repr->getLoc(),
                       diag::opaque_type_in_protocol_requirement)
      .fixItInsert(fixitLoc, "associatedtype <#AssocType#>\n")
      .fixItReplace(repr->getSourceRange(), "<#AssocType#>");
    
    return nullptr;
  }
  
  // Check the availability of the opaque type runtime support.
  if (!ctx.LangOpts.DisableAvailabilityChecking) {
    auto runningOS =
      TypeChecker::overApproximateAvailabilityAtLocation(
        repr->getLoc(),
        originatingDecl->getInnermostDeclContext());
    auto availability = ctx.getOpaqueTypeAvailability();
    if (!runningOS.isContainedIn(availability)) {
      TypeChecker::diagnosePotentialOpaqueTypeUnavailability(
        repr->getSourceRange(),
        originatingDecl->getInnermostDeclContext(),
        UnavailabilityReason::requiresVersionRange(availability.getOSVersion()));
    }
  }
  
  // Try to resolve the constraint repr. It should be some kind of existential
  // type.
  TypeResolutionOptions options(TypeResolverContext::GenericRequirement);
  TypeLoc constraintTypeLoc(repr->getConstraint());
  // Pass along the error type if resolving the repr failed.
  auto resolution = TypeResolution::forInterface(
    dc, dc->getGenericSignatureOfContext());
  bool validationError
    = TypeChecker::validateType(ctx, constraintTypeLoc, resolution, options);
  auto constraintType = constraintTypeLoc.getType();
  if (validationError)
    return nullptr;
  
  // Error out if the constraint type isn't a class or existential type.
  if (!constraintType->getClassOrBoundGenericClass()
      && !constraintType->isExistentialType()) {
    ctx.Diags.diagnose(repr->getConstraint()->getLoc(),
                       diag::opaque_type_invalid_constraint);
    return nullptr;
  }
  
  if (constraintType->hasArchetype())
    constraintType = constraintType->mapTypeOutOfContext();

  // Create a generic signature for the opaque environment. This is the outer
  // generic signature with an added generic parameter representing the opaque
  // type and its interface constraints.
  auto originatingDC = originatingDecl->getInnermostDeclContext();
  unsigned returnTypeDepth = 0;
  auto outerGenericSignature = originatingDC->getGenericSignatureOfContext();
  
  if (outerGenericSignature) {
    returnTypeDepth =
               outerGenericSignature->getGenericParams().back()->getDepth() + 1;
  }
  
  auto returnTypeParam = GenericTypeParamType::get(returnTypeDepth, 0, ctx);

  SmallVector<GenericTypeParamType *, 2> genericParamTypes;
  genericParamTypes.push_back(returnTypeParam);

  SmallVector<Requirement, 2> requirements;
  if (constraintType->getClassOrBoundGenericClass()) {
    requirements.push_back(Requirement(RequirementKind::Superclass,
                                       returnTypeParam, constraintType));
  } else {
    auto constraints = constraintType->getExistentialLayout();
    if (auto superclass = constraints.getSuperclass()) {
      requirements.push_back(Requirement(RequirementKind::Superclass,
                                         returnTypeParam, superclass));
    }
    for (auto protocol : constraints.getProtocols()) {
      requirements.push_back(Requirement(RequirementKind::Conformance,
                                         returnTypeParam, protocol));
    }
    if (auto layout = constraints.getLayoutConstraint()) {
      requirements.push_back(Requirement(RequirementKind::Layout,
                                         returnTypeParam, layout));
    }
  }
  
  auto interfaceSignature = evaluateOrDefault(
      ctx.evaluator,
      AbstractGenericSignatureRequest{
        outerGenericSignature.getPointer(),
        std::move(genericParamTypes),
        std::move(requirements)},
      GenericSignature());

  // Create the OpaqueTypeDecl for the result type.
  // It has the same parent context and generic environment as the originating
  // decl.
  auto parentDC = originatingDecl->getDeclContext();
  auto originatingGenericContext = originatingDecl->getAsGenericContext();
  GenericParamList *genericParams = originatingGenericContext
    ? originatingGenericContext->getGenericParams()
    : nullptr;

  auto opaqueDecl = new (ctx) OpaqueTypeDecl(originatingDecl,
                                             genericParams,
                                             parentDC,
                                             interfaceSignature,
                                             returnTypeParam);
  opaqueDecl->copyFormalAccessFrom(originatingDecl);
  if (auto originatingSig = originatingDC->getGenericSignatureOfContext()) {
    opaqueDecl->setGenericSignature(originatingSig);
  }
  
  // The declared interface type is an opaque ArchetypeType.
  SubstitutionMap subs;
  if (outerGenericSignature) {
    subs = outerGenericSignature->getIdentitySubstitutionMap();
  }
  auto opaqueTy = OpaqueTypeArchetypeType::get(opaqueDecl, subs);
  auto metatype = MetatypeType::get(opaqueTy);
  opaqueDecl->setInterfaceType(metatype);
  return opaqueDecl;
}

/// Determine whether the given type is \c Self, an associated type of \c Self,
/// or a concrete type.
static bool isSelfDerivedOrConcrete(Type protoSelf, Type type) {
  // Check for a concrete type.
  if (!type->hasTypeParameter())
    return true;

  if (type->isTypeParameter() &&
      type->getRootGenericParam()->isEqual(protoSelf))
    return true;

  return false;
}

// For a generic requirement in a protocol, make sure that the requirement
// set didn't add any requirements to Self or its associated types.
void TypeChecker::checkProtocolSelfRequirements(ValueDecl *decl) {
  // For a generic requirement in a protocol, make sure that the requirement
  // set didn't add any requirements to Self or its associated types.
  if (auto *proto = dyn_cast<ProtocolDecl>(decl->getDeclContext())) {
    auto protoSelf = proto->getSelfInterfaceType();
    auto sig = decl->getInnermostDeclContext()->getGenericSignatureOfContext();
    for (auto req : sig->getRequirements()) {
      // If one of the types in the requirement is dependent on a non-Self
      // type parameter, this requirement is okay.
      if (!isSelfDerivedOrConcrete(protoSelf, req.getFirstType()) ||
          !isSelfDerivedOrConcrete(protoSelf, req.getSecondType()))
        continue;

      // The conformance of 'Self' to the protocol is okay.
      if (req.getKind() == RequirementKind::Conformance &&
          req.getSecondType()->getAs<ProtocolType>()->getDecl() == proto &&
          req.getFirstType()->is<GenericTypeParamType>())
        continue;

      diagnose(decl,
               diag::requirement_restricts_self,
               decl->getDescriptiveKind(), decl->getFullName(),
               req.getFirstType().getString(),
               static_cast<unsigned>(req.getKind()),
               req.getSecondType().getString());
    }
  }
}

/// All generic parameters of a generic function must be referenced in the
/// declaration's type, otherwise we have no way to infer them.
void TypeChecker::checkReferencedGenericParams(GenericContext *dc) {
  // Don't do this check for accessors: they're not used directly, so we
  // never need to infer their generic arguments.  This is mostly a
  // compile-time optimization, but it also avoids problems with accessors
  // like 'read' and 'modify' that would arise due to yields not being
  // part of the formal type.
  if (isa<AccessorDecl>(dc))
    return;

  auto *genericParams = dc->getGenericParams();
  auto genericSig = dc->getGenericSignatureOfContext();
  if (!genericParams)
    return;

  auto *decl = cast<ValueDecl>(dc->getInnermostDeclarationDeclContext());

  // A helper class to collect referenced generic type parameters
  // and dependent member types.
  class ReferencedGenericTypeWalker : public TypeWalker {
    SmallPtrSet<CanType, 4> ReferencedGenericParams;

  public:
    ReferencedGenericTypeWalker() {}
    Action walkToTypePre(Type ty) override {
      // Find generic parameters or dependent member types.
      // Once such a type is found, don't recurse into its children.
      if (!ty->hasTypeParameter())
        return Action::SkipChildren;
      if (ty->isTypeParameter()) {
        ReferencedGenericParams.insert(ty->getCanonicalType());
        return Action::SkipChildren;
      }
      return Action::Continue;
    }

    SmallPtrSetImpl<CanType> &getReferencedGenericParams() {
      return ReferencedGenericParams;
    }
  };

  // Collect all generic params referenced in parameter types and
  // return type.
  ReferencedGenericTypeWalker paramsAndResultWalker;
  auto *funcTy = decl->getInterfaceType()->castTo<GenericFunctionType>();
  for (const auto &param : funcTy->getParams())
    param.getPlainType().walk(paramsAndResultWalker);
  funcTy->getResult().walk(paramsAndResultWalker);

  // Set of generic params referenced in parameter types,
  // return type or requirements.
  auto &referencedGenericParams =
      paramsAndResultWalker.getReferencedGenericParams();

  // Check if at least one of the generic params in the requirement refers
  // to an already referenced generic parameter. If this is the case,
  // then the other type is also considered as referenced, because
  // it is used to put requirements on the first type.
  auto reqTypesVisitor = [&referencedGenericParams](Requirement req) -> bool {
    Type first;
    Type second;

    switch (req.getKind()) {
    case RequirementKind::Superclass:
    case RequirementKind::SameType:
      second = req.getSecondType();
      LLVM_FALLTHROUGH;

    case RequirementKind::Conformance:
    case RequirementKind::Layout:
      first = req.getFirstType();
      break;
    }

    // Collect generic parameter types referenced by types used in a requirement.
    ReferencedGenericTypeWalker walker;
    if (first && first->hasTypeParameter())
      first.walk(walker);
    if (second && second->hasTypeParameter())
      second.walk(walker);
    auto &genericParamsUsedByRequirementTypes =
        walker.getReferencedGenericParams();

    // If at least one of the collected generic types or a root generic
    // parameter of dependent member types is known to be referenced by
    // parameter types, return types or other types known to be "referenced",
    // then all the types used in the requirement are considered to be
    // referenced, because they are used to defined something that is known
    // to be referenced.
    bool foundNewReferencedGenericParam = false;
    if (std::any_of(genericParamsUsedByRequirementTypes.begin(),
                    genericParamsUsedByRequirementTypes.end(),
                    [&referencedGenericParams](CanType t) {
                      assert(t->isTypeParameter());
                      return referencedGenericParams.find(
                                 t->getRootGenericParam()
                                     ->getCanonicalType()) !=
                             referencedGenericParams.end();
                    })) {
      std::for_each(genericParamsUsedByRequirementTypes.begin(),
                    genericParamsUsedByRequirementTypes.end(),
                    [&referencedGenericParams,
                     &foundNewReferencedGenericParam](CanType t) {
                      // Add only generic type parameters, but ignore any
                      // dependent member types, because requirement
                      // on a dependent member type does not provide enough
                      // information to infer the base generic type
                      // parameter.
                      if (!t->is<GenericTypeParamType>())
                        return;
                      if (referencedGenericParams.insert(t).second)
                        foundNewReferencedGenericParam = true;
                    });
    }
    return foundNewReferencedGenericParam;
  };

  ArrayRef<Requirement> requirements;

  auto FindReferencedGenericParamsInRequirements =
    [&requirements, genericSig, &reqTypesVisitor] {
    requirements = genericSig->getRequirements();
    // Try to find new referenced generic parameter types in requirements until
    // we reach a fix point. We need to iterate until a fix point, because we
    // may have e.g. chains of same-type requirements like:
    // not-yet-referenced-T1 == not-yet-referenced-T2.DepType2,
    // not-yet-referenced-T2 == not-yet-referenced-T3.DepType3,
    // not-yet-referenced-T3 == referenced-T4.DepType4.
    // When we process the first of these requirements, we don't know yet that
    // T2
    // will be referenced, because T3 will be referenced,
    // because T3 == T4.DepType4.
    while (true) {
      bool foundNewReferencedGenericParam = false;
      for (auto req : requirements) {
        if (reqTypesVisitor(req))
          foundNewReferencedGenericParam = true;
      }
      if (!foundNewReferencedGenericParam)
        break;
    }
  };

  // Find the depth of the function's own generic parameters.
  unsigned fnGenericParamsDepth = genericParams->getParams().front()->getDepth();

  // Check that every generic parameter type from the signature is
  // among referencedGenericParams.
  for (auto *genParam : genericSig->getGenericParams()) {
    auto *paramDecl = genParam->getDecl();
    if (paramDecl->getDepth() != fnGenericParamsDepth)
      continue;
    if (!referencedGenericParams.count(genParam->getCanonicalType())) {
      // Lazily search for generic params that are indirectly used in the
      // function signature. Do it only if there is a generic parameter
      // that is not known to be referenced yet.
      if (requirements.empty()) {
        FindReferencedGenericParamsInRequirements();
        // Nothing to do if this generic parameter is considered to be
        // referenced after analyzing the requirements from the generic
        // signature.
        if (referencedGenericParams.count(genParam->getCanonicalType()))
          continue;
      }
      // Produce an error that this generic parameter cannot be bound.
      diagnose(paramDecl->getLoc(), diag::unreferenced_generic_parameter,
               paramDecl->getNameStr());
      decl->setInterfaceType(ErrorType::get(Context));
      decl->setInvalid();
    }
  }
}

///
/// Generic types
///

GenericSignature TypeChecker::checkGenericSignature(
                      GenericParamList *genericParamList,
                      DeclContext *dc,
                      GenericSignature parentSig,
                      bool allowConcreteGenericParams,
                      SmallVector<Requirement, 2> additionalRequirements,
                      SmallVector<TypeLoc, 2> inferenceSources) {
  assert(genericParamList && "Missing generic parameters?");

  auto request = InferredGenericSignatureRequest{
    dc->getParentModule(), parentSig.getPointer(), genericParamList,
    additionalRequirements, inferenceSources,
    allowConcreteGenericParams};
  auto sig = evaluateOrDefault(dc->getASTContext().evaluator,
                               request, nullptr);

  // Debugging of the generic signature builder and generic signature
  // generation.
  if (dc->getASTContext().LangOpts.DebugGenericSignatures) {
    if (auto *VD = dyn_cast_or_null<ValueDecl>(dc->getAsDecl())) {
      VD->dumpRef(llvm::errs());
    } else {
      dc->printContext(llvm::errs());
    }
    llvm::errs() << "\n";
    llvm::errs() << "Generic signature: ";
    sig->print(llvm::errs());
    llvm::errs() << "\n";
    llvm::errs() << "Canonical generic signature: ";
    sig->getCanonicalSignature()->print(llvm::errs());
    llvm::errs() << "\n";
  }

  return sig;
}

/// Form the interface type of an extension from the raw type and the
/// extension's list of generic parameters.
static Type formExtensionInterfaceType(
                         ExtensionDecl *ext, Type type,
                         GenericParamList *genericParams,
                         SmallVectorImpl<Requirement> &sameTypeReqs,
                         bool &mustInferRequirements) {
  if (type->is<ErrorType>())
    return type;

  // Find the nominal type declaration and its parent type.
  if (type->is<ProtocolCompositionType>())
    type = type->getCanonicalType();

  Type parentType = type->getNominalParent();
  GenericTypeDecl *genericDecl = type->getAnyGeneric();

  // Reconstruct the parent, if there is one.
  if (parentType) {
    // Build the nested extension type.
    auto parentGenericParams = genericDecl->getGenericParams()
                                 ? genericParams->getOuterParameters()
                                 : genericParams;
    parentType =
      formExtensionInterfaceType(ext, parentType, parentGenericParams,
                                 sameTypeReqs, mustInferRequirements);
  }

  // Find the nominal type.
  auto nominal = dyn_cast<NominalTypeDecl>(genericDecl);
  auto typealias = dyn_cast<TypeAliasDecl>(genericDecl);
  if (!nominal) {
    Type underlyingType = typealias->getUnderlyingType();
    nominal = underlyingType->getNominalOrBoundGenericNominal();
  }

  // Form the result.
  Type resultType;
  SmallVector<Type, 2> genericArgs;
  if (!nominal->isGeneric() || isa<ProtocolDecl>(nominal)) {
    resultType = NominalType::get(nominal, parentType,
                                  nominal->getASTContext());
  } else {
    auto currentBoundType = type->getAs<BoundGenericType>();

    // Form the bound generic type with the type parameters provided.
    unsigned gpIndex = 0;
    for (auto gp : *genericParams) {
      SWIFT_DEFER { ++gpIndex; };

      auto gpType = gp->getDeclaredInterfaceType();
      genericArgs.push_back(gpType);

      if (currentBoundType) {
        sameTypeReqs.emplace_back(RequirementKind::SameType, gpType,
                                  currentBoundType->getGenericArgs()[gpIndex]);
      }
    }

    resultType = BoundGenericType::get(nominal, parentType, genericArgs);
  }

  // If we have a typealias, try to form type sugar.
  if (typealias && TypeChecker::isPassThroughTypealias(
                       typealias, typealias->getUnderlyingType(), nominal)) {
    auto typealiasSig = typealias->getGenericSignature();
    SubstitutionMap subMap;
    if (typealiasSig) {
      subMap = typealiasSig->getIdentitySubstitutionMap();

      mustInferRequirements = true;
    }

    resultType = TypeAliasType::get(typealias, parentType, subMap, resultType);
  }


  return resultType;
}

/// Retrieve the generic parameter depth of the extended type.
static unsigned getExtendedTypeGenericDepth(ExtensionDecl *ext) {
  auto nominal = ext->getSelfNominalTypeDecl();
  if (!nominal) return static_cast<unsigned>(-1);

  auto sig = nominal->getGenericSignatureOfContext();
  if (!sig) return static_cast<unsigned>(-1);

  return sig->getGenericParams().back()->getDepth();
}

llvm::Expected<GenericSignature>
GenericSignatureRequest::evaluate(Evaluator &evaluator,
                                  GenericContext *GC) const {
  // The signature of a Protocol is trivial (Self: TheProtocol) so let's compute
  // it.
  if (auto PD = dyn_cast<ProtocolDecl>(GC)) {
    auto self = PD->getSelfInterfaceType()->castTo<GenericTypeParamType>();
    auto req =
        Requirement(RequirementKind::Conformance, self, PD->getDeclaredType());
    auto sig = GenericSignature::get({self}, {req});

    // Debugging of the generic signature builder and generic signature
    // generation.
    if (GC->getASTContext().LangOpts.DebugGenericSignatures) {
      PD->printContext(llvm::errs());
      llvm::errs() << "\n";
      llvm::errs() << "Generic signature: ";
      sig->print(llvm::errs());
      llvm::errs() << "\n";
      llvm::errs() << "Canonical generic signature: ";
      sig->getCanonicalSignature()->print(llvm::errs());
      llvm::errs() << "\n";
    }
    return sig;
  }

  // We can fast-path computing the generic signature of non-generic
  // declarations by re-using the parent context's signature.
  auto *gp = GC->getGenericParams();
  if (!gp) {
    return GC->getParent()->getGenericSignatureOfContext();
  }

  // Setup the depth of the generic parameters.
  gp->setDepth(GC->getGenericContextDepth());

  // Accessors can always use the generic context of their storage
  // declarations. This is a compile-time optimization since it lets us
  // avoid the requirements-gathering phase, but it also simplifies that
  // work for accessors which don't mention the value type in their formal
  // signatures (like the read and modify coroutines, since yield types
  // aren't tracked in the AST type yet).
  if (auto accessor = dyn_cast<AccessorDecl>(GC->getAsDecl())) {
    return cast<SubscriptDecl>(accessor->getStorage())->getGenericSignature();
  }

  auto parentSig = GC->getParent()->getGenericSignatureOfContext();
  bool allowConcreteGenericParams = false;
  SmallVector<TypeLoc, 2> inferenceSources;
  SmallVector<Requirement, 2> sameTypeReqs;
  if (auto VD = dyn_cast_or_null<ValueDecl>(GC->getAsDecl())) {
    auto func = dyn_cast<AbstractFunctionDecl>(VD);
    auto subscr = dyn_cast<SubscriptDecl>(VD);

    // For functions and subscripts, resolve the parameter and result types and
    // note them as inference sources.
    if (subscr || func) {
      // Gather requirements from the parameter list.
      auto resolution = TypeResolution::forStructural(GC);

      TypeResolutionOptions options =
          (func ? TypeResolverContext::AbstractFunctionDecl
                : TypeResolverContext::SubscriptDecl);

      auto params = func ? func->getParameters() : subscr->getIndices();
      for (auto param : *params) {
        auto *typeRepr = param->getTypeRepr();
        if (typeRepr == nullptr)
          continue;

        auto paramOptions = options;
        paramOptions.setContext(param->isVariadic()
                                    ? TypeResolverContext::VariadicFunctionInput
                                    : TypeResolverContext::FunctionInput);
        paramOptions |= TypeResolutionFlags::Direct;

        auto type = resolution.resolveType(typeRepr, paramOptions);

        if (auto *specifier = dyn_cast<SpecifierTypeRepr>(typeRepr))
          typeRepr = specifier->getBase();

        inferenceSources.emplace_back(typeRepr, type);
      }

      // Gather requirements from the result type.
      auto *resultTypeRepr = [&subscr, &func]() -> TypeRepr * {
        if (subscr) {
          return subscr->getElementTypeLoc().getTypeRepr();
        } else if (auto *FD = dyn_cast<FuncDecl>(func)) {
          return FD->getBodyResultTypeLoc().getTypeRepr();
        } else {
          return nullptr;
        }
      }();
      if (resultTypeRepr && !isa<OpaqueReturnTypeRepr>(resultTypeRepr)) {
        auto resultType = resolution.resolveType(
            resultTypeRepr, TypeResolverContext::FunctionResult);

        inferenceSources.emplace_back(resultTypeRepr, resultType);
      }
    }
  } else if (auto *ext = dyn_cast<ExtensionDecl>(GC)) {
    // Form the interface type of the extension so we can use it as an inference
    // source.
    //
    // FIXME: Push this into the "get interface type" request.
    bool mustInferRequirements = false;
    Type extInterfaceType =
      formExtensionInterfaceType(ext, ext->getExtendedType(),
                                 gp, sameTypeReqs,
                                 mustInferRequirements);
    
    auto cannotReuseNominalSignature = [&]() -> bool {
      const auto finalDepth = gp->getParams().back()->getDepth();
      return mustInferRequirements
          || !sameTypeReqs.empty()
          || ext->getTrailingWhereClause()
          || (getExtendedTypeGenericDepth(ext) != finalDepth);
    };
    
    // Re-use the signature of the type being extended by default.
    if (!cannotReuseNominalSignature()) {
      return ext->getSelfNominalTypeDecl()->getGenericSignatureOfContext();
    }

    // Allow parameters to be equated with concrete types.
    allowConcreteGenericParams = true;
    // Extensions must occur at the top level, they have no
    // (valid) parent signature.
    parentSig = nullptr;
    inferenceSources.emplace_back(nullptr, extInterfaceType);
  }

  // EGREGIOUS HACK: The GSB cannot handle the addition of parent signatures
  // from malformed decls in many cases.  Check the invalid bit and null out the
  // parent signature.
  if (auto *DD = GC->getParent()->getAsDecl()) {
    parentSig = DD->isInvalid() ? nullptr : parentSig;
  }
  
  return TypeChecker::checkGenericSignature(
      gp, GC, parentSig,
      allowConcreteGenericParams,
      sameTypeReqs, inferenceSources);
}

///
/// Checking bound generic type arguments
///

RequirementCheckResult TypeChecker::checkGenericArguments(
    DeclContext *dc, SourceLoc loc, SourceLoc noteLoc, Type owner,
    TypeArrayView<GenericTypeParamType> genericParams,
    ArrayRef<Requirement> requirements,
    TypeSubstitutionFn substitutions,
    LookupConformanceFn conformances,
    ConformanceCheckOptions conformanceOptions,
    GenericRequirementsCheckListener *listener,
    SubstOptions options) {
  bool valid = true;

  // We handle any conditional requirements ourselves.
  conformanceOptions |= ConformanceCheckFlags::SkipConditionalRequirements;

  struct RequirementSet {
    ArrayRef<Requirement> Requirements;
    SmallVector<ParentConditionalConformance, 4> Parents;
  };

  SmallVector<RequirementSet, 8> pendingReqs;
  pendingReqs.push_back({requirements, {}});

  ASTContext &ctx = dc->getASTContext();
  while (!pendingReqs.empty()) {
    auto current = pendingReqs.pop_back_val();

    for (const auto &rawReq : current.Requirements) {
      auto req = rawReq;
      if (current.Parents.empty()) {
        auto substed = rawReq.subst(substitutions, conformances, options);
        if (!substed) {
          // Another requirement will fail later; just continue.
          valid = false;
          continue;
        }

        req = *substed;
      }

      auto kind = req.getKind();
      Type rawFirstType = rawReq.getFirstType();
      Type firstType = req.getFirstType();
      if (firstType->hasTypeParameter())
        firstType = dc->mapTypeIntoContext(firstType);

      Type rawSecondType, secondType;
      if (kind != RequirementKind::Layout) {
        rawSecondType = rawReq.getSecondType();
        secondType = req.getSecondType();
        if (secondType->hasTypeParameter())
          secondType = dc->mapTypeIntoContext(secondType);
      }

      // Don't do further checking on error types.
      if (firstType->hasError() || (secondType && secondType->hasError())) {
        // Another requirement will fail later; just continue.
        valid = false;
        continue;
      }

      bool requirementFailure = false;
      if (listener && !listener->shouldCheck(kind, firstType, secondType))
        continue;

      Diag<Type, Type, Type> diagnostic;
      Diag<Type, Type, StringRef> diagnosticNote;

      switch (kind) {
      case RequirementKind::Conformance: {
        // Protocol conformance requirements.
        auto proto = secondType->castTo<ProtocolType>();
        // FIXME: This should track whether this should result in a private
        // or non-private dependency.
        // FIXME: Do we really need "used" at this point?
        // FIXME: Poor location information. How much better can we do here?
        // FIXME: This call should support listener to be able to properly
        //        diagnose problems with conformances.
        auto conformance = conformsToProtocol(firstType, proto->getDecl(), dc,
                                              conformanceOptions, loc);

        if (conformance) {
          // Report the conformance.
          if (listener && valid && current.Parents.empty()) {
            listener->satisfiedConformance(rawFirstType, firstType,
                                           conformance);
          }

          auto conditionalReqs = conformance.getConditionalRequirements();
          if (!conditionalReqs.empty()) {
            auto history = current.Parents;
            history.push_back({firstType, proto});
            pendingReqs.push_back({conditionalReqs, std::move(history)});
          }
          continue;
        }

        // A failure at the top level is diagnosed elsewhere.
        if (current.Parents.empty())
          return RequirementCheckResult::Failure;

        // Failure needs to emit a diagnostic.
        diagnostic = diag::type_does_not_conform_owner;
        diagnosticNote = diag::type_does_not_inherit_or_conform_requirement;
        requirementFailure = true;
        break;
      }

      case RequirementKind::Layout:
        // TODO: Statically check other layout constraints, once they can
        // be spelled in Swift.
        if (req.getLayoutConstraint()->isClass() &&
            !firstType->satisfiesClassConstraint()) {
          diagnostic = diag::type_is_not_a_class;
          diagnosticNote = diag::anyobject_requirement;
          requirementFailure = true;
        }
        break;

      case RequirementKind::Superclass: {
        // Superclass requirements.
        if (!secondType->isExactSuperclassOf(firstType)) {
          diagnostic = diag::type_does_not_inherit;
          diagnosticNote = diag::type_does_not_inherit_or_conform_requirement;
          requirementFailure = true;
        }
        break;
      }

      case RequirementKind::SameType:
        if (!firstType->isEqual(secondType)) {
          diagnostic = diag::types_not_equal;
          diagnosticNote = diag::types_not_equal_requirement;
          requirementFailure = true;
        }
        break;
      }

      if (!requirementFailure)
        continue;

      if (listener &&
          listener->diagnoseUnsatisfiedRequirement(rawReq, firstType,
                                                   secondType, current.Parents))
        return RequirementCheckResult::Failure;

      if (loc.isValid()) {
        // FIXME: Poor source-location information.
        ctx.Diags.diagnose(loc, diagnostic, owner, firstType, secondType);

        std::string genericParamBindingsText;
        if (!genericParams.empty()) {
          genericParamBindingsText =
            gatherGenericParamBindingsText(
              {rawFirstType, rawSecondType}, genericParams, substitutions);
        }
        ctx.Diags.diagnose(noteLoc, diagnosticNote, rawFirstType, rawSecondType,
                           genericParamBindingsText);

        ParentConditionalConformance::diagnoseConformanceStack(
            ctx.Diags, noteLoc, current.Parents);
      }

      return RequirementCheckResult::Failure;
    }
  }

  if (valid)
    return RequirementCheckResult::Success;
  return RequirementCheckResult::SubstitutionFailure;
}

llvm::Expected<Requirement>
RequirementRequest::evaluate(Evaluator &evaluator,
                             WhereClauseOwner owner,
                             unsigned index,
                             TypeResolutionStage stage) const {
  // Figure out the type resolution.
  TypeResolutionOptions options = TypeResolverContext::GenericRequirement;
  Optional<TypeResolution> resolution;
  switch (stage) {
  case TypeResolutionStage::Structural:
    resolution = TypeResolution::forStructural(owner.dc);
    break;

  case TypeResolutionStage::Interface:
    resolution = TypeResolution::forInterface(owner.dc);
    break;

  case TypeResolutionStage::Contextual:
    llvm_unreachable("No clients care about this. Use mapTypeIntoContext()");
  }

  auto resolveType = [&](TypeLoc &typeLoc) -> Type {
    Type result;
    if (auto typeRepr = typeLoc.getTypeRepr())
      result = resolution->resolveType(typeRepr, options);
    else
      result = typeLoc.getType();

    return result ? result : ErrorType::get(owner.dc->getASTContext());
  };

  auto &reqRepr = getRequirement();
  switch (reqRepr.getKind()) {
  case RequirementReprKind::TypeConstraint: {
    Type subject = resolveType(reqRepr.getSubjectLoc());
    Type constraint = resolveType(reqRepr.getConstraintLoc());
    return Requirement(constraint->getClassOrBoundGenericClass()
                         ? RequirementKind::Superclass
                         : RequirementKind::Conformance,
                       subject, constraint);
  }

  case RequirementReprKind::SameType:
    return Requirement(RequirementKind::SameType,
                       resolveType(reqRepr.getFirstTypeLoc()),
                       resolveType(reqRepr.getSecondTypeLoc()));

  case RequirementReprKind::LayoutConstraint:
    return Requirement(RequirementKind::Layout,
                       resolveType(reqRepr.getSubjectLoc()),
                       reqRepr.getLayoutConstraint());
  }
  llvm_unreachable("unhandled kind");
}

llvm::Expected<Type>
StructuralTypeRequest::evaluate(Evaluator &evaluator,
                                TypeAliasDecl *typeAlias) const {  
  TypeResolutionOptions options((typeAlias->getGenericParams()
                                     ? TypeResolverContext::GenericTypeAliasDecl
                                     : TypeResolverContext::TypeAliasDecl));

  if (!typeAlias->getDeclContext()->isCascadingContextForLookup(
          /*functionsAreNonCascading*/ true)) {
    options |= TypeResolutionFlags::KnownNonCascadingDependency;
  }
  
  // This can happen when code completion is attempted inside
  // of typealias underlying type e.g. `typealias F = () -> Int#^TOK^#`
  auto &ctx = typeAlias->getASTContext();
  auto underlyingTypeRepr = typeAlias->getUnderlyingTypeRepr();
  if (!underlyingTypeRepr) {
    typeAlias->setInvalid();
    return ErrorType::get(ctx);
  }

  auto resolution = TypeResolution::forStructural(typeAlias);
  auto type = resolution.resolveType(underlyingTypeRepr, options);
  
  auto genericSig = typeAlias->getGenericSignature();
  SubstitutionMap subs;
  if (genericSig)
    subs = genericSig->getIdentitySubstitutionMap();

  Type parent;
  auto parentDC = typeAlias->getDeclContext();
  if (parentDC->isTypeContext())
    parent = parentDC->getSelfInterfaceType();
  return TypeAliasType::get(typeAlias, parent, subs, type);
}