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 "TypeCheckProtocol.h"
#include "TypeCheckType.h"
#include "TypeChecker.h"
#include "swift/AST/ASTWalker.h"
#include "swift/AST/DiagnosticsSema.h"
#include "swift/AST/ExistentialLayout.h"
#include "swift/AST/GenericEnvironment.h"
#include "swift/AST/ParameterList.h"
#include "swift/AST/ProtocolConformance.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();
}

// An alias to avoid repeating the `SmallVector`'s size parameter.
using CollectedOpaqueReprs = SmallVector<OpaqueReturnTypeRepr *, 2>;

/// Walk `repr` recursively, collecting any `OpaqueReturnTypeRepr`s.
static CollectedOpaqueReprs collectOpaqueReturnTypeReprs(TypeRepr *repr) {
  class Walker : public ASTWalker {
    CollectedOpaqueReprs &Reprs;

  public:
    explicit Walker(CollectedOpaqueReprs &reprs) : Reprs(reprs) {}

    bool walkToTypeReprPre(TypeRepr *repr) override {
      if (auto opaqueRepr = dyn_cast<OpaqueReturnTypeRepr>(repr))
        Reprs.push_back(opaqueRepr);
      return true;
    }
  };

  CollectedOpaqueReprs reprs;
  repr->walk(Walker(reprs));
  return reprs;
}

//
// Generic functions
//

/// Get the opaque type representing the return type of a declaration, or
/// create it if it does not yet exist.
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();
    }

    std::string result;
    const char *const placeholder = "<#AssocType#>";
    {
      llvm::raw_string_ostream out(result);
      out << "associatedtype " << placeholder << ": ";
      // FIXME [OPAQUE SUPPORT]: to produce the right associate type for the
      // replacement in general, we would need to recurse into the type repr and
      // replace every `OpaqueReturnType` with its 'constraint'. Things get
      // trickier when we allow named opaque return types.
      if (isa<OpaqueReturnTypeRepr>(repr)) {
        cast<OpaqueReturnTypeRepr>(repr)->getConstraint()->print(out);
      } else {
        out << "<#type#>";
      }
      out << "\n";
    }

    ctx.Diags
        .diagnose(repr->getLoc(), diag::opaque_type_in_protocol_requirement)
        .fixItInsert(fixitLoc, result)
        .fixItReplace(repr->getSourceRange(), placeholder);

    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()));
    }
  }

  // Create a generic signature for the opaque environment. This is the outer
  // generic signature with an added generic parameters representing the opaque
  // types and their interface constraints.
  auto originatingDC = originatingDecl->getInnermostDeclContext();
  auto outerGenericSignature = originatingDC->getGenericSignatureOfContext();
  unsigned opaqueSignatureDepth =
      outerGenericSignature
          ? outerGenericSignature.getGenericParams().back()->getDepth() + 1
          : 0;

  // Determine the context of the opaque type declaration we'll be creating.
  auto parentDC = originatingDecl->getDeclContext();
  auto originatingGenericContext = originatingDecl->getAsGenericContext();
  GenericParamList *genericParams;
  GenericSignature interfaceSignature;
  CollectedOpaqueReprs opaqueReprs;
  if (auto namedOpaque = dyn_cast<NamedOpaqueReturnTypeRepr>(repr)) {
    // Produce the generic signature for the opaque type.
    genericParams = namedOpaque->getGenericParams();
    genericParams->setDepth(opaqueSignatureDepth);

    InferredGenericSignatureRequest request{
        originatingDC->getParentModule(),
        outerGenericSignature.getPointer(),
        genericParams,
        WhereClauseOwner(originatingDC, genericParams),
        /*addedRequirements=*/{},
        /*inferenceSources=*/{},
        /*allowConcreteGenericParams=*/false};

    interfaceSignature = evaluateOrDefault(
        ctx.evaluator, request, GenericSignatureWithError())
          .getPointer();
    if (!interfaceSignature) {
      // Already produced an error.
      return nullptr;
    }
  } else {
    opaqueReprs = collectOpaqueReturnTypeReprs(repr);
    SmallVector<GenericTypeParamType *, 2> genericParamTypes;
    SmallVector<Requirement, 2> requirements;
    for (unsigned i = 0; i < opaqueReprs.size(); ++i) {
      auto *currentRepr = opaqueReprs[i];

      // Usually, we resolve the opaque constraint and bail if it isn't a class
      // or existential type (see below). However, in this case we know we will
      // fail, so we can bail early and provide a better diagnostic.
      if (auto *optionalRepr =
              dyn_cast<OptionalTypeRepr>(currentRepr->getConstraint())) {
        std::string buf;
        llvm::raw_string_ostream stream(buf);
        stream << "(some " << optionalRepr->getBase() << ")?";

        ctx.Diags.diagnose(currentRepr->getLoc(),
                           diag::opaque_type_invalid_constraint);
        ctx.Diags
            .diagnose(currentRepr->getLoc(), diag::opaque_of_optional_rewrite)
            .fixItReplaceChars(currentRepr->getStartLoc(),
                               currentRepr->getEndLoc(), stream.str());
        return nullptr;
      }

      auto *paramType = GenericTypeParamType::get(/*type sequence*/ false,
                                                  opaqueSignatureDepth, i, ctx);
      genericParamTypes.push_back(paramType);

      // Try to resolve the constraint repr in the parent decl context. It
      // should be some kind of existential type. Pass along the error type if
      // resolving the repr failed.
      auto constraintType = TypeResolution::forInterface(
                                dc, TypeResolverContext::GenericRequirement,
                                // Unbound generics and placeholders are
                                // meaningless in opaque types.
                                /*unboundTyOpener*/ nullptr,
                                /*placeholderHandler*/ nullptr)
                                .resolveType(currentRepr->getConstraint());

      if (constraintType->hasError())
        return nullptr;

      // Error out if the constraint type isn't a class or existential type.
      if (!constraintType->getClassOrBoundGenericClass() &&
          !constraintType->isExistentialType()) {
        ctx.Diags.diagnose(currentRepr->getLoc(),
                           diag::opaque_type_invalid_constraint);
        return nullptr;
      }

      if (constraintType->hasArchetype())
        constraintType = constraintType->mapTypeOutOfContext();

      if (constraintType->getClassOrBoundGenericClass()) {
        requirements.push_back(
            Requirement(RequirementKind::Superclass, paramType,
                        constraintType));
      } else {
        // In this case, the constraint type is an existential
        requirements.push_back(
            Requirement(RequirementKind::Conformance, paramType,
                        constraintType));
      }
    }

    interfaceSignature = buildGenericSignature(ctx, outerGenericSignature,
                                               genericParamTypes,
                                               std::move(requirements));
    genericParams = originatingGenericContext
        ? originatingGenericContext->getGenericParams()
        : nullptr;
  }

  // Create the OpaqueTypeDecl for the result type.
  auto opaqueDecl = OpaqueTypeDecl::get(
      originatingDecl, genericParams, parentDC, interfaceSignature,
      opaqueReprs);
  opaqueDecl->copyFormalAccessFrom(originatingDecl);
  if (auto originatingSig = originatingDC->getGenericSignatureOfContext()) {
    opaqueDecl->setGenericSignature(originatingSig);
  }

  // Resolving in the context of `opaqueDecl` allows type resolution to create
  // opaque archetypes where needed
  auto interfaceType =
      TypeResolution::forInterface(opaqueDecl, TypeResolverContext::None,
                                   /*unboundTyOpener*/ nullptr,
                                   /*placeholderHandler*/ nullptr)
          .resolveType(repr);

  // Opaque types cannot be used in parameter position.
  Type desugared = interfaceType->getDesugaredType();
  bool hasError = desugared.findIf([&](Type type) -> bool {
    if (auto *fnType = type->getAs<FunctionType>()) {
      for (auto param : fnType->getParams()) {
        if (!param.getPlainType()->hasOpaqueArchetype())
          continue;

        ctx.Diags.diagnose(repr->getLoc(),
                           diag::opaque_type_in_parameter,
                           interfaceType);
        return true;
      }
    }

    return false;
  });

  if (hasError)
    return nullptr;

  auto metatype = MetatypeType::get(interfaceType);
  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 &ctx = proto->getASTContext();
    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.getProtocolDecl() == proto &&
          req.getFirstType()->is<GenericTypeParamType>())
        continue;

      ctx.Diags.diagnose(decl,
                         diag::requirement_restricts_self,
                         decl->getDescriptiveKind(), decl->getName(),
                         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.
      paramDecl->diagnose(diag::unreferenced_generic_parameter,
                          paramDecl->getNameStr());
      decl->setInvalid();
    }
  }
}

///
/// Generic types
///

/// 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,
                         const 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 if (genericParams) {
    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();
}

GenericSignature
GenericSignatureRequest::evaluate(Evaluator &evaluator,
                                  GenericContext *GC) const {
  auto &ctx = GC->getASTContext();

  // 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->getDeclaredInterfaceType());
    auto sig = GenericSignature::get({self}, {req});

    // Debugging of the generic signature builder and generic signature
    // generation.
    if (ctx.TypeCheckerOpts.DebugGenericSignatures) {
      llvm::errs() << "\n";
      PD->printContext(llvm::errs());
      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;
  }

  bool allowConcreteGenericParams = false;
  const auto *genericParams = GC->getGenericParams();
  const auto *where = GC->getTrailingWhereClause();

  if (genericParams) {
    // Setup the depth of the generic parameters.
    const_cast<GenericParamList *>(genericParams)
        ->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();
    }
  }

  // ...or we may only have a contextual where clause.
  if (where) {
    // If there is no generic context for the where clause to
    // rely on, diagnose that now and bail out.
    if (!GC->isGenericContext()) {
      ctx.Diags.diagnose(where->getWhereLoc(),
                         GC->getParent()->isModuleScopeContext()
                             ? diag::where_nongeneric_toplevel
                             : diag::where_nongeneric_ctx);
      return nullptr;
    }
  }

  if (!genericParams && where)
    allowConcreteGenericParams = true;

  if (!genericParams && !where) {
    // We can fast-path computing the generic signature of non-generic
    // declarations by re-using the parent context's signature.
    if (auto accessor = dyn_cast<AccessorDecl>(GC->getAsDecl()))
      if (auto subscript = dyn_cast<SubscriptDecl>(accessor->getStorage()))
         return subscript->getGenericSignature();

    return GC->getParentForLookup()->getGenericSignatureOfContext();
  }

  auto parentSig = GC->getParentForLookup()->getGenericSignatureOfContext();
  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) {
      const auto baseOptions =
          TypeResolutionOptions(func ? TypeResolverContext::AbstractFunctionDecl
                                     : TypeResolverContext::SubscriptDecl);

      const auto resolution =
          TypeResolution::forStructural(GC, baseOptions,
                                        /*unboundTyOpener*/ nullptr,
                                        /*placeholderHandler*/ nullptr);
      auto params = func ? func->getParameters() : subscr->getIndices();
      for (auto param : *params) {
        auto *typeRepr = param->getTypeRepr();
        if (typeRepr == nullptr)
          continue;

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

        const auto type =
            resolution.withOptions(paramOptions).resolveType(typeRepr);

        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->getElementTypeRepr();
        } else if (auto *FD = dyn_cast<FuncDecl>(func)) {
          return FD->getResultTypeRepr();
        } else {
          return nullptr;
        }
      }();
      if (resultTypeRepr && !resultTypeRepr->hasOpaque()) {
        const auto resultType =
            resolution.withOptions(TypeResolverContext::FunctionResult)
                .resolveType(resultTypeRepr);

        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(),
                                 genericParams, sameTypeReqs,
                                 mustInferRequirements);
    
    auto cannotReuseNominalSignature = [&]() -> bool {
      const auto finalDepth = genericParams->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;

    inferenceSources.emplace_back(nullptr, extInterfaceType);
  }

  auto request = InferredGenericSignatureRequest{
      GC->getParentModule(), parentSig.getPointer(),
      GC->getGenericParams(), WhereClauseOwner(GC),
      sameTypeReqs, inferenceSources,
      allowConcreteGenericParams};
  auto sig = evaluateOrDefault(ctx.evaluator, request,
                               GenericSignatureWithError()).getPointer();

  // Debugging of the generic signature builder and generic signature
  // generation.
  if (ctx.TypeCheckerOpts.DebugGenericSignatures) {
    llvm::errs() << "\n";
    if (auto *VD = dyn_cast_or_null<ValueDecl>(GC->getAsDecl())) {
      VD->dumpRef(llvm::errs());
      llvm::errs() << "\n";
    } else {
      GC->printContext(llvm::errs());
    }
    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;
}

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

RequirementCheckResult TypeChecker::checkGenericArguments(
    ModuleDecl *module, SourceLoc loc, SourceLoc noteLoc, Type owner,
    TypeArrayView<GenericTypeParamType> genericParams,
    ArrayRef<Requirement> requirements,
    TypeSubstitutionFn substitutions,
    SubstOptions options) {
  bool valid = true;

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

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

  ASTContext &ctx = module->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, LookUpConformanceInModule(module), options);
        if (!substed) {
          // Another requirement will fail later; just continue.
          valid = false;
          continue;
        }

        req = *substed;
      }

      ArrayRef<Requirement> conditionalRequirements;
      if (req.isSatisfied(conditionalRequirements, /*allowMissing=*/true)) {
        if (!conditionalRequirements.empty()) {
          assert(req.getKind() == RequirementKind::Conformance);

          auto history = current.Parents;
          history.push_back({req.getFirstType(), req.getProtocolDecl()});
          pendingReqs.push_back({conditionalRequirements, std::move(history)});
        }
        continue;
      }

      if (loc.isValid()) {
        Diag<Type, Type, Type> diagnostic;
        Diag<Type, Type, StringRef> diagnosticNote;

        switch (req.getKind()) {
        case RequirementKind::Conformance: {
          diagnoseConformanceFailure(req.getFirstType(), req.getProtocolDecl(),
                                     module, loc);

          if (current.Parents.empty())
            return RequirementCheckResult::Failure;

          diagnostic = diag::type_does_not_conform_owner;
          diagnosticNote = diag::type_does_not_inherit_or_conform_requirement;
          break;
        }

        case RequirementKind::Layout:
          diagnostic = diag::type_is_not_a_class;
          diagnosticNote = diag::anyobject_requirement;
          break;

        case RequirementKind::Superclass:
          diagnostic = diag::type_does_not_inherit;
          diagnosticNote = diag::type_does_not_inherit_or_conform_requirement;
          break;

        case RequirementKind::SameType:
          diagnostic = diag::types_not_equal;
          diagnosticNote = diag::types_not_equal_requirement;
          break;
        }

        Type rawSecondType, secondType;
        if (req.getKind() != RequirementKind::Layout) {
          rawSecondType = rawReq.getSecondType();
          secondType = req.getSecondType();
        }

        // FIXME: Poor source-location information.
        ctx.Diags.diagnose(loc, diagnostic, owner,
                           req.getFirstType(), secondType);

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

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

      return RequirementCheckResult::Failure;
    }
  }

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

RequirementCheckResult
TypeChecker::checkGenericArguments(ModuleDecl *module,
                                   ArrayRef<Requirement> requirements,
                                   TypeSubstitutionFn substitutions) {
  SmallVector<Requirement, 4> worklist;
  bool valid = true;

  for (auto req : requirements) {
    if (auto resolved = req.subst(substitutions,
                                  LookUpConformanceInModule(module))) {
      worklist.push_back(*resolved);
    } else {
      valid = false;
    }
  }

  while (!worklist.empty()) {
    auto req = worklist.pop_back_val();
    ArrayRef<Requirement> conditionalRequirements;
    if (!req.isSatisfied(conditionalRequirements, /*allowMissing=*/true))
      return RequirementCheckResult::Failure;

    worklist.append(conditionalRequirements.begin(),
                    conditionalRequirements.end());
  }

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

Requirement
RequirementRequest::evaluate(Evaluator &evaluator,
                             WhereClauseOwner owner,
                             unsigned index,
                             TypeResolutionStage stage) const {
  auto &reqRepr = getRequirement();

  // Figure out the type resolution.
  TypeResolverContext context;
  if (reqRepr.getKind() == RequirementReprKind::SameType) {
    context = TypeResolverContext::SameTypeRequirement;
  } else {
    context = TypeResolverContext::GenericRequirement;
  }
  auto options = TypeResolutionOptions(context);
  if (owner.dc->isInSpecializeExtensionContext())
    options |= TypeResolutionFlags::AllowUsableFromInline;
  Optional<TypeResolution> resolution;
  switch (stage) {
  case TypeResolutionStage::Structural:
    resolution = TypeResolution::forStructural(owner.dc, options,
                                               /*unboundTyOpener*/ nullptr,
                                               /*placeholderHandler*/ nullptr);
    break;

  case TypeResolutionStage::Interface:
    resolution = TypeResolution::forInterface(owner.dc, options,
                                              /*unboundTyOpener*/ nullptr,
                                              /*placeholderHandler*/ nullptr);
    break;
  }

  switch (reqRepr.getKind()) {
  case RequirementReprKind::TypeConstraint: {
    Type subject = resolution->resolveType(reqRepr.getSubjectRepr());
    Type constraint = resolution->resolveType(reqRepr.getConstraintRepr());
    return Requirement(constraint->getClassOrBoundGenericClass()
                         ? RequirementKind::Superclass
                         : RequirementKind::Conformance,
                       subject, constraint);
  }

  case RequirementReprKind::SameType:
    return Requirement(RequirementKind::SameType,
                       resolution->resolveType(reqRepr.getFirstTypeRepr()),
                       resolution->resolveType(reqRepr.getSecondTypeRepr()));

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

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

  // 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);
  }

  const auto type =
      TypeResolution::forStructural(typeAlias, options,
                                    /*unboundTyOpener*/ nullptr,
                                    /*placeholderHandler*/ nullptr)
          .resolveType(underlyingTypeRepr);

  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);
}