GenericSignature.cpp

//===--- GenericSignature.cpp - Generic Signature AST ---------------------===//
//
// 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 the GenericSignature class.
//
//===----------------------------------------------------------------------===//

#include "swift/AST/GenericSignature.h"
#include "swift/AST/ASTContext.h"
#include "swift/AST/GenericSignatureBuilder.h"
#include "swift/AST/Decl.h"
#include "swift/AST/GenericEnvironment.h"
#include "swift/AST/Module.h"
#include "swift/AST/Types.h"
#include "swift/Basic/STLExtras.h"
#include <functional>

using namespace swift;

void ConformanceAccessPath::print(raw_ostream &out) const {
  interleave(begin(), end(),
             [&](const Entry &entry) {
               entry.first.print(out);
               out << ": " << entry.second->getName();
             }, [&] {
               out << " -> ";
             });
}

void ConformanceAccessPath::dump() const {
  print(llvm::errs());
  llvm::errs() << "\n";
}

GenericSignature::GenericSignature(ArrayRef<GenericTypeParamType *> params,
                                   ArrayRef<Requirement> requirements,
                                   bool isKnownCanonical)
  : NumGenericParams(params.size()), NumRequirements(requirements.size()),
    CanonicalSignatureOrASTContext()
{
  auto paramsBuffer = getGenericParamsBuffer();
  for (unsigned i = 0; i < NumGenericParams; ++i) {
    paramsBuffer[i] = params[i];
  }

  auto reqtsBuffer = getRequirementsBuffer();
  for (unsigned i = 0; i < NumRequirements; ++i) {
    reqtsBuffer[i] = requirements[i];
  }

#ifndef NDEBUG
  // Make sure generic parameters are in the right order, and
  // none are missing.
  unsigned depth = 0;
  unsigned count = 0;
  for (auto param : params) {
    if (param->getDepth() != depth) {
      assert(param->getDepth() > depth &&
             "Generic parameter depth mismatch");
      depth = param->getDepth();
      count = 0;
    }
    assert(param->getIndex() == count &&
           "Generic parameter index mismatch");
    count++;
  }
#endif

  if (isKnownCanonical)
    CanonicalSignatureOrASTContext = &getASTContext(params, requirements);
}

ArrayRef<GenericTypeParamType *> 
GenericSignature::getInnermostGenericParams() const {
  auto params = getGenericParams();

  // Find the point at which the depth changes.
  unsigned depth = params.back()->getDepth();
  for (unsigned n = params.size(); n > 0; --n) {
    if (params[n-1]->getDepth() != depth) {
      return params.slice(n);
    }
  }

  // All parameters are at the same depth.
  return params;
}


SmallVector<GenericTypeParamType *, 2>
GenericSignature::getSubstitutableParams() const {
  SmallVector<GenericTypeParamType *, 2> result;

  enumeratePairedRequirements([&](Type depTy, ArrayRef<Requirement>) -> bool {
    if (auto *paramTy = depTy->getAs<GenericTypeParamType>())
      result.push_back(paramTy);
    return false;
  });

  return result;
}

std::string GenericSignature::gatherGenericParamBindingsText(
    ArrayRef<Type> types, TypeSubstitutionFn substitutions) const {
  llvm::SmallPtrSet<GenericTypeParamType *, 2> knownGenericParams;
  for (auto type : types) {
    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 : this->getGenericParams()) {
    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();
}

ASTContext &GenericSignature::getASTContext(
                                ArrayRef<swift::GenericTypeParamType *> params,
                                ArrayRef<swift::Requirement> requirements) {
  // The params and requirements cannot both be empty.
  if (!params.empty())
    return params.front()->getASTContext();
  else
    return requirements.front().getFirstType()->getASTContext();
}

GenericSignatureBuilder *GenericSignature::getGenericSignatureBuilder(ModuleDecl &mod) {
  // The generic signature builder is associated with the canonical signature.
  if (!isCanonical())
    return getCanonicalSignature()->getGenericSignatureBuilder(mod);

  // generic signature builders are stored on the ASTContext.
  return getASTContext().getOrCreateGenericSignatureBuilder(CanGenericSignature(this),
                                                     &mod);
}

bool GenericSignature::isCanonical() const {
  if (CanonicalSignatureOrASTContext.is<ASTContext*>()) return true;

  return getCanonicalSignature() == this;
}

CanGenericSignature GenericSignature::getCanonical(
                                        ArrayRef<GenericTypeParamType *> params,
                                        ArrayRef<Requirement> requirements) {
  // Canonicalize the parameters and requirements.
  SmallVector<GenericTypeParamType*, 8> canonicalParams;
  canonicalParams.reserve(params.size());
  for (auto param : params) {
    canonicalParams.push_back(cast<GenericTypeParamType>(param->getCanonicalType()));
  }

  SmallVector<Requirement, 8> canonicalRequirements;
  canonicalRequirements.reserve(requirements.size());
  for (auto &reqt : requirements) {
    if (reqt.getKind() != RequirementKind::Layout) {
      auto secondTy = reqt.getSecondType();
      canonicalRequirements.push_back(
          Requirement(reqt.getKind(), reqt.getFirstType()->getCanonicalType(),
                      secondTy ? secondTy->getCanonicalType() : CanType()));
    } else
      canonicalRequirements.push_back(
          Requirement(reqt.getKind(), reqt.getFirstType()->getCanonicalType(),
                      reqt.getLayoutConstraint()));
  }
  auto canSig = get(canonicalParams, canonicalRequirements,
                    /*isKnownCanonical=*/true);
  return CanGenericSignature(canSig);
}

CanGenericSignature
GenericSignature::getCanonicalSignature() const {
  // If we haven't computed the canonical signature yet, do so now.
  if (CanonicalSignatureOrASTContext.isNull()) {
    // Compute the canonical signature.
    CanGenericSignature canSig = getCanonical(getGenericParams(),
                                              getRequirements());

    // Record either the canonical signature or an indication that
    // this is the canonical signature.
    if (canSig != this)
      CanonicalSignatureOrASTContext = canSig;
    else
      CanonicalSignatureOrASTContext = &getGenericParams()[0]->getASTContext();

    // Return the canonical signature.
    return canSig;
  }

  // A stored ASTContext indicates that this is the canonical
  // signature.
  if (CanonicalSignatureOrASTContext.is<ASTContext*>())
    // TODO: CanGenericSignature should be const-correct.
    return CanGenericSignature(const_cast<GenericSignature*>(this));
  
  // Otherwise, return the stored canonical signature.
  return CanGenericSignature(
           CanonicalSignatureOrASTContext.get<GenericSignature*>());
}

GenericEnvironment *GenericSignature::createGenericEnvironment(
                                                             ModuleDecl &mod) {
  auto *builder = getGenericSignatureBuilder(mod);
  return GenericEnvironment::getIncomplete(this, builder);
}


ASTContext &GenericSignature::getASTContext() const {
  // Canonical signatures store the ASTContext directly.
  if (auto ctx = CanonicalSignatureOrASTContext.dyn_cast<ASTContext *>())
    return *ctx;

  // For everything else, just get it from the generic parameter.
  return getASTContext(getGenericParams(), getRequirements());
}

Optional<ProtocolConformanceRef>
GenericSignature::lookupConformance(CanType type, ProtocolDecl *proto) const {
  // FIXME: Actually implement this properly.
  auto *M = proto->getParentModule();

  if (type->isTypeParameter())
    return ProtocolConformanceRef(proto);

  return M->lookupConformance(type, proto,
                              M->getASTContext().getLazyResolver());
}

bool GenericSignature::enumeratePairedRequirements(
               llvm::function_ref<bool(Type, ArrayRef<Requirement>)> fn) const {
  // We'll be walking through the list of requirements.
  ArrayRef<Requirement> reqs = getRequirements();
  unsigned curReqIdx = 0, numReqs = reqs.size();

  // ... and walking through the list of generic parameters.
  ArrayRef<GenericTypeParamType *> genericParams = getGenericParams();
  unsigned curGenericParamIdx = 0, numGenericParams = genericParams.size();

  // Figure out which generic parameters are complete.
  SmallVector<bool, 4> genericParamsAreConcrete(genericParams.size(), false);
  for (auto req : reqs) {
    if (req.getKind() != RequirementKind::SameType) continue;
    if (req.getSecondType()->isTypeParameter()) continue;

    auto gp = req.getFirstType()->getAs<GenericTypeParamType>();
    if (!gp) continue;

    unsigned index = GenericParamKey(gp).findIndexIn(genericParams);
    genericParamsAreConcrete[index] = true;
  }

  /// Local function to 'catch up' to the next dependent type we're going to
  /// visit, calling the function for each of the generic parameters in the
  /// generic parameter list prior to this parameter.
  auto enumerateGenericParamsUpToDependentType = [&](CanType depTy) -> bool {
    // Figure out where we should stop when enumerating generic parameters.
    unsigned stopDepth, stopIndex;
    if (auto gp = dyn_cast_or_null<GenericTypeParamType>(depTy)) {
      stopDepth = gp->getDepth();
      stopIndex = gp->getIndex();
    } else {
      stopDepth = genericParams.back()->getDepth() + 1;
      stopIndex = 0;
    }

    // Enumerate generic parameters up to the stopping point, calling the
    // callback function for each one
    while (curGenericParamIdx != numGenericParams) {
      auto curGenericParam = genericParams[curGenericParamIdx];

      // If the current generic parameter is before our stopping point, call
      // the function.
      if (curGenericParam->getDepth() < stopDepth ||
          (curGenericParam->getDepth() == stopDepth &&
           curGenericParam->getIndex() < stopIndex)) {
        if (!genericParamsAreConcrete[curGenericParamIdx] &&
            fn(curGenericParam, { }))
          return true;

        ++curGenericParamIdx;
        continue;
      }

      // If the current generic parameter is at our stopping point, we're
      // done.
      if (curGenericParam->getDepth() == stopDepth &&
          curGenericParam->getIndex() == stopIndex) {
        ++curGenericParamIdx;
        return false;
      }

      // Otherwise, there's nothing to do.
      break;
    }

    return false;
  };

  // Walk over all of the requirements.
  while (curReqIdx != numReqs) {
    // "Catch up" by enumerating generic parameters up to this dependent type.
    CanType depTy = reqs[curReqIdx].getFirstType()->getCanonicalType();
    if (enumerateGenericParamsUpToDependentType(depTy)) return true;

    // Utility to skip over non-conformance constraints that apply to this
    // type.
    auto skipNonConformanceConstraints = [&] {
      while (curReqIdx != numReqs &&
             reqs[curReqIdx].getKind() != RequirementKind::Conformance &&
             reqs[curReqIdx].getFirstType()->getCanonicalType() == depTy) {
        ++curReqIdx;
      }
    };

    // First, skip past any non-conformance constraints on this type.
    skipNonConformanceConstraints();

    // Collect all of the conformance constraints for this dependent type.
    unsigned startIdx = curReqIdx;
    unsigned endIdx = curReqIdx;
    while (curReqIdx != numReqs &&
           reqs[curReqIdx].getKind() == RequirementKind::Conformance &&
           reqs[curReqIdx].getFirstType()->getCanonicalType() == depTy) {
      ++curReqIdx;
      endIdx = curReqIdx;
    }

    // Skip any trailing non-conformance constraints.
    skipNonConformanceConstraints();

    // If there were any conformance constraints, or we have a generic
    // parameter we can't skip, invoke the callback.
    if ((startIdx != endIdx ||
         (isa<GenericTypeParamType>(depTy) &&
          !genericParamsAreConcrete[
            GenericParamKey(cast<GenericTypeParamType>(depTy))
              .findIndexIn(genericParams)])) &&
        fn(depTy, reqs.slice(startIdx, endIdx-startIdx)))
      return true;
  }

  // Catch up on any remaining generic parameters.
  return enumerateGenericParamsUpToDependentType(CanType());
}

SubstitutionMap
GenericSignature::getSubstitutionMap(SubstitutionList subs) const {
  SubstitutionMap result(const_cast<GenericSignature *>(this));

  enumeratePairedRequirements(
    [&](Type depTy, ArrayRef<Requirement> reqts) -> bool {
      auto sub = subs.front();
      subs = subs.slice(1);

      auto canTy = depTy->getCanonicalType();
      if (isa<SubstitutableType>(canTy))
        result.addSubstitution(cast<SubstitutableType>(canTy),
                               sub.getReplacement());
      assert(reqts.size() == sub.getConformances().size());
      for (auto conformance : sub.getConformances())
        result.addConformance(canTy, conformance);

      return false;
    });

  assert(subs.empty() && "did not use all substitutions?!");
  result.verify();
  return result;
}

SubstitutionMap
GenericSignature::
getSubstitutionMap(TypeSubstitutionFn subs,
                   GenericSignature::LookupConformanceFn lookupConformance) const {
  SubstitutionMap subMap(const_cast<GenericSignature *>(this));

  // Enumerate all of the requirements that require substitution.
  enumeratePairedRequirements([&](Type depTy, ArrayRef<Requirement> reqs) {
    auto canTy = depTy->getCanonicalType();

    // Compute the replacement type.
    Type currentReplacement = depTy.subst(subs, lookupConformance,
                                          SubstFlags::UseErrorType);
    if (auto substTy = dyn_cast<SubstitutableType>(canTy))
      subMap.addSubstitution(substTy, currentReplacement);

    // Collect the conformances.
    for (auto req: reqs) {
      assert(req.getKind() == RequirementKind::Conformance);
      auto protoType = req.getSecondType()->castTo<ProtocolType>();
      if (auto conformance = lookupConformance(canTy,
                                               currentReplacement,
                                               protoType)) {
        subMap.addConformance(canTy, *conformance);
      }
    }

    return false;
  });

  subMap.verify();
  return subMap;
}

void GenericSignature::
getSubstitutions(TypeSubstitutionFn subs,
                 GenericSignature::LookupConformanceFn lookupConformance,
                 SmallVectorImpl<Substitution> &result) const {

  // Enumerate all of the requirements that require substitution.
  enumeratePairedRequirements([&](Type depTy, ArrayRef<Requirement> reqs) {
    auto &ctx = getASTContext();

    // Compute the replacement type.
    Type currentReplacement = depTy.subst(subs, lookupConformance);
    if (!currentReplacement)
      currentReplacement = ErrorType::get(depTy);

    // Collect the conformances.
    SmallVector<ProtocolConformanceRef, 4> currentConformances;
    for (auto req: reqs) {
      assert(req.getKind() == RequirementKind::Conformance);
      auto protoType = req.getSecondType()->castTo<ProtocolType>();
      if (auto conformance = lookupConformance(depTy->getCanonicalType(),
                                               currentReplacement,
                                               protoType)) {
        currentConformances.push_back(*conformance);
      } else {
        if (!currentReplacement->hasError())
          currentReplacement = ErrorType::get(currentReplacement);
        currentConformances.push_back(
                                  ProtocolConformanceRef(protoType->getDecl()));
      }
    }

    // Add it to the final substitution list.
    result.push_back({
      currentReplacement,
      ctx.AllocateCopy(currentConformances)
    });

    return false;
  });
}

void GenericSignature::
getSubstitutions(const SubstitutionMap &subMap,
                 SmallVectorImpl<Substitution> &result) const {
  getSubstitutions(QuerySubstitutionMap{subMap},
                   LookUpConformanceInSubstitutionMap(subMap),
                   result);
}

bool GenericSignature::requiresClass(Type type, ModuleDecl &mod) {
  if (!type->isTypeParameter()) return false;

  auto &builder = *getGenericSignatureBuilder(mod);
  auto pa = builder.resolveArchetype(type);
  if (!pa) return false;

  pa = pa->getRepresentative();

  // If this type was mapped to a concrete type, then there is no
  // requirement.
  if (pa->isConcreteType()) return false;

  // If there is a layout constraint, it might be a class.
  if (auto layout = pa->getLayout())
    if (layout->isClass())
      return true;

  // If there is a superclass bound, then obviously it must be a class.
  if (pa->getSuperclass()) return true;

  // If any of the protocols are class-bound, then it must be a class.
  for (auto proto : pa->getConformsTo()) {
    if (proto->requiresClass()) return true;
  }

  return false;
}

/// Determine the superclass bound on the given dependent type.
Type GenericSignature::getSuperclassBound(Type type, ModuleDecl &mod) {
  if (!type->isTypeParameter()) return nullptr;

  auto &builder = *getGenericSignatureBuilder(mod);
  auto pa = builder.resolveArchetype(type);
  if (!pa) return nullptr;

  pa = pa->getRepresentative();

  // If this type was mapped to a concrete type, then there is no
  // requirement.
  if (pa->isConcreteType()) return nullptr;

  // Retrieve the superclass bound.
  return pa->getSuperclass();
}

/// Determine the set of protocols to which the given dependent type
/// must conform.
SmallVector<ProtocolDecl *, 2> GenericSignature::getConformsTo(Type type,
                                                               ModuleDecl &mod) {
  if (!type->isTypeParameter()) return { };

  auto &builder = *getGenericSignatureBuilder(mod);
  auto pa = builder.resolveArchetype(type);
  if (!pa) return { };

  pa = pa->getRepresentative();

  // If this type was mapped to a concrete type, then there are no
  // requirements.
  if (pa->isConcreteType()) return { };

  // Retrieve the protocols to which this type conforms.
  SmallVector<ProtocolDecl *, 2> result;
  for (auto proto : pa->getConformsTo())
    result.push_back(proto);

  // Canonicalize the resulting set of protocols.
  ProtocolType::canonicalizeProtocols(result);

  return result;
}

bool GenericSignature::conformsToProtocol(Type type, ProtocolDecl *proto,
                                          ModuleDecl &mod) {
  // FIXME: Deal with concrete conformances here?
  if (!type->isTypeParameter()) return false;

  auto &builder = *getGenericSignatureBuilder(mod);
  auto pa = builder.resolveArchetype(type);
  if (!pa) return false;

  pa = pa->getRepresentative();

  // FIXME: Deal with concrete conformances here?
  if (pa->isConcreteType()) return false;

  // Check whether the representative conforms to this protocol.
  if (auto equivClass = pa->getEquivalenceClassIfPresent())
    if (equivClass->conformsTo.count(proto) > 0)
      return true;

  return false;
}

/// Determine whether the given dependent type is equal to a concrete type.
bool GenericSignature::isConcreteType(Type type, ModuleDecl &mod) {
  return bool(getConcreteType(type, mod));
}

/// Return the concrete type that the given dependent type is constrained to,
/// or the null Type if it is not the subject of a concrete same-type
/// constraint.
Type GenericSignature::getConcreteType(Type type, ModuleDecl &mod) {
  if (!type->isTypeParameter()) return Type();

  auto &builder = *getGenericSignatureBuilder(mod);
  auto pa = builder.resolveArchetype(type);
  if (!pa) return Type();

  pa = pa->getRepresentative();
  if (!pa->isConcreteType()) return Type();

  return pa->getConcreteType();
}

LayoutConstraint GenericSignature::getLayoutConstraint(Type type,
                                                       ModuleDecl &mod) {
  if (!type->isTypeParameter()) return LayoutConstraint();

  auto &builder = *getGenericSignatureBuilder(mod);
  auto pa = builder.resolveArchetype(type);
  if (!pa) return LayoutConstraint();

  pa = pa->getRepresentative();
  return pa->getLayout();
}

bool GenericSignature::areSameTypeParameterInContext(Type type1, Type type2,
                                                     ModuleDecl &mod) {
  assert(type1->isTypeParameter());
  assert(type2->isTypeParameter());

  if (type1.getPointer() == type2.getPointer())
    return true;

  auto &builder = *getGenericSignatureBuilder(mod);
  auto pa1 = builder.resolveArchetype(type1);
  assert(pa1 && "not a valid dependent type of this signature?");
  pa1 = pa1->getRepresentative();
  assert(!pa1->isConcreteType());

  auto pa2 = builder.resolveArchetype(type2);
  assert(pa2 && "not a valid dependent type of this signature?");
  pa2 = pa2->getRepresentative();
  assert(!pa2->isConcreteType());

  return pa1 == pa2;
}

bool GenericSignature::isCanonicalTypeInContext(Type type, ModuleDecl &mod) {
  // If the type isn't independently canonical, it's certainly not canonical
  // in this context.
  if (!type->isCanonical())
    return false;

  // All the contextual canonicality rules apply to type parameters, so if the
  // type doesn't involve any type parameters, it's already canonical.
  if (!type->hasTypeParameter())
    return true;

  auto &builder = *getGenericSignatureBuilder(mod);
  return isCanonicalTypeInContext(type, builder);
}

bool GenericSignature::isCanonicalTypeInContext(Type type,
                                                GenericSignatureBuilder &builder) {
  // If the type isn't independently canonical, it's certainly not canonical
  // in this context.
  if (!type->isCanonical())
    return false;

  // All the contextual canonicality rules apply to type parameters, so if the
  // type doesn't involve any type parameters, it's already canonical.
  if (!type->hasTypeParameter())
    return true;

  // Look for non-canonical type parameters.
  return !type.findIf([&](Type component) -> bool {
    if (!component->isTypeParameter()) return false;

    auto pa = builder.resolveArchetype(component);
    if (!pa) return false;

    auto rep = pa->getArchetypeAnchor(builder);
    return (rep->isConcreteType() || pa != rep);
  });
}

CanType GenericSignature::getCanonicalTypeInContext(Type type,
                                                    GenericSignatureBuilder &builder) {
  type = type->getCanonicalType();

  // All the contextual canonicality rules apply to type parameters, so if the
  // type doesn't involve any type parameters, it's already canonical.
  if (!type->hasTypeParameter())
    return CanType(type);

  // Replace non-canonical type parameters.
  type = type.transformRec([&](TypeBase *component) -> Optional<Type> {
    if (!isa<GenericTypeParamType>(component) &&
        !isa<DependentMemberType>(component))
      return None;

    // Resolve the potential archetype.  This can be null in nested generic
    // types, which we can't immediately canonicalize.
    auto pa = builder.resolveArchetype(Type(component));
    if (!pa) return None;

    auto rep = pa->getArchetypeAnchor(builder);
    if (rep->isConcreteType()) {
      return getCanonicalTypeInContext(rep->getConcreteType(), builder);
    }

    return rep->getDependentType(getGenericParams(), /*allowUnresolved*/ false);
  });
  
  auto result = type->getCanonicalType();

  assert(isCanonicalTypeInContext(result, builder));
  return result;
}

CanType GenericSignature::getCanonicalTypeInContext(Type type,
                                                    ModuleDecl &mod) {
  type = type->getCanonicalType();

  // All the contextual canonicality rules apply to type parameters, so if the
  // type doesn't involve any type parameters, it's already canonical.
  if (!type->hasTypeParameter())
    return CanType(type);

  auto &builder = *getGenericSignatureBuilder(mod);
  return getCanonicalTypeInContext(type, builder);
}

GenericEnvironment *CanGenericSignature::getGenericEnvironment(
                                                     ModuleDecl &module) const {
  // generic signature builders are stored on the ASTContext.
  return module.getASTContext().getOrCreateCanonicalGenericEnvironment(
           module.getASTContext().getOrCreateGenericSignatureBuilder(*this, &module),
           module);
}

/// Remove all of the associated type declarations from the given type
/// parameter, producing \c DependentMemberTypes with names alone.
static Type eraseAssociatedTypes(Type type) {
  if (auto depMemTy = type->getAs<DependentMemberType>())
    return DependentMemberType::get(eraseAssociatedTypes(depMemTy->getBase()),
                                    depMemTy->getName());

  return type;
}

namespace {
  typedef GenericSignatureBuilder::RequirementSource RequirementSource;

  template<typename T>
  using GSBConstraint = GenericSignatureBuilder::Constraint<T>;
}

/// Retrieve the best requirement source from the list
static const RequirementSource *
getBestRequirementSource(ArrayRef<GSBConstraint<ProtocolDecl *>> constraints) {
  const RequirementSource *bestSource = nullptr;
  for (const auto &constraint : constraints) {
    auto source = constraint.source;
    if (!bestSource || source->compare(bestSource) < 0)
      bestSource = source;
  }

  return bestSource;
}

ConformanceAccessPath GenericSignature::getConformanceAccessPath(
                                                       Type type,
                                                       ProtocolDecl *protocol,
                                                       ModuleDecl &mod) {
  assert(type->isTypeParameter() && "not a type parameter");

  // Resolve this type to a potential archetype.
  auto &builder = *getGenericSignatureBuilder(mod);
  auto pa = builder.resolveArchetype(type);
  auto equivClass = pa->getOrCreateEquivalenceClass();

  // Dig out the conformance of this type to the given protocol, because we
  // want its requirement source.
  auto conforms = equivClass->conformsTo.find(protocol);
  assert(conforms != equivClass->conformsTo.end());

  // Follow the requirement source to form the conformance access path.
  typedef GenericSignatureBuilder::RequirementSource RequirementSource;
  ConformanceAccessPath path;

#ifndef NDEBUG
  // Local function to determine whether there is a conformance of the given
  // subject type to the given protocol within the given generic signature's
  // explicit requirements.
  auto hasConformanceInSignature = [&](const GenericSignature *genericSig,
                                       Type subjectType,
                                       ProtocolDecl *proto) -> bool {
    // Make sure this requirement exists in the requirement signature.
    for (const auto& req: genericSig->getRequirements()) {
      if (req.getKind() == RequirementKind::Conformance &&
          req.getFirstType()->isEqual(subjectType) &&
          req.getSecondType()->castTo<ProtocolType>()->getDecl()
            == proto) {
        return true;
      }
    }

    return false;
  };
#endif

  // Local function to construct the conformance access path from the
  // requirement.
  std::function<void(GenericSignature *, const RequirementSource *,
                     ProtocolDecl *, Type)> buildPath;
  buildPath = [&](GenericSignature *sig, const RequirementSource *source,
                  ProtocolDecl *conformingProto, Type rootType) {
    // Each protocol requirement is a step along the path.
    if (source->kind == RequirementSource::ProtocolRequirement ||
        source->kind == RequirementSource::InferredProtocolRequirement) {
      // Follow the rest of the path to derive the conformance into which
      // this particular protocol requirement step would look.
      auto inProtocol = source->getProtocolDecl();
      buildPath(sig, source->parent, inProtocol, rootType);
      assert(path.path.back().second == inProtocol &&
             "path produces incorrect conformance");

      // If this step was computed via the requirement signature, add it
      // directly.
      if (source->usesRequirementSignature) {
        // Add this step along the path, which involves looking for the
        // conformance we want (\c conformingProto) within the protocol
        // described by this source.

        // Canonicalize the subject type within the protocol's generic
        // signature.
        Type subjectType = source->getStoredType();
        subjectType = inProtocol->getGenericSignature()
          ->getCanonicalTypeInContext(subjectType,
                                      *inProtocol->getParentModule());

        assert(hasConformanceInSignature(inProtocol->getRequirementSignature(),
                                         subjectType, conformingProto) &&
               "missing explicit conformance in requirement signature");

        // Record this step.
        path.path.push_back({subjectType, conformingProto});
        return;
      }

      // Canonicalize this step with respect to the requirement signature.
      if (!inProtocol->isRequirementSignatureComputed()) {
        inProtocol->computeRequirementSignature();
        assert(inProtocol->isRequirementSignatureComputed() &&
               "missing signature");
      }

      // Get a generic signature builder for the requirement signature. This has
      // the requirement we need.
      auto reqSig = inProtocol->getRequirementSignature();
      auto &reqSigBuilder = *reqSig->getGenericSignatureBuilder(
                                               *inProtocol->getModuleContext());

      // Retrieve the stored type, but erase all of the specific associated
      // type declarations; we don't want any details of the enclosing context
      // to sneak in here.
      Type storedType = eraseAssociatedTypes(source->getStoredType());

      // Dig out the potential archetype for this stored type.
      auto pa = reqSigBuilder.resolveArchetype(storedType);
      auto equivClass = pa->getOrCreateEquivalenceClass();

      // Find the conformance of this potential archetype to the protocol in
      // question.
      auto conforms = equivClass->conformsTo.find(conformingProto);
      assert(conforms != equivClass->conformsTo.end());

      // Compute the root type, canonicalizing it w.r.t. the protocol context.
      auto inProtoSig = inProtocol->getGenericSignature();
      auto conformsSource = getBestRequirementSource(conforms->second);
      Type localRootType = conformsSource->getRootPotentialArchetype()
                             ->getDependentType(inProtoSig->getGenericParams(),
                                                /*allowUnresolved*/true);
      localRootType = inProtoSig->getCanonicalTypeInContext(
                                               localRootType,
                                               *inProtocol->getModuleContext());

      // Build the path according to the requirement signature.
      buildPath(reqSig, conformsSource, conformingProto, localRootType);

      // We're done.
      return;
    }

    // If we still have a parent, keep going.
    if (source->parent) {
      buildPath(sig, source->parent, conformingProto, rootType);
      return;
    }

    // We are at an explicit or inferred requirement.
    assert(source->kind == RequirementSource::Explicit ||
           source->kind == RequirementSource::Inferred);

    // Skip trivial path elements. These occur when querying a requirement
    // signature.
    if (!path.path.empty() && conformingProto == path.path.back().second &&
        rootType->isEqual(conformingProto->getSelfInterfaceType()))
      return;

    assert(hasConformanceInSignature(sig, rootType, conformingProto) &&
           "missing explicit conformance in signature");

    // Add the root of the path, which starts at this explicit requirement.
    path.path.push_back({rootType, conformingProto});
  };

  // Canonicalize the root type.
  auto source = getBestRequirementSource(conforms->second);
  auto subjectPA = source->getRootPotentialArchetype();
  subjectPA = subjectPA->getArchetypeAnchor(*subjectPA->getBuilder());
  Type rootType = subjectPA->getDependentType(getGenericParams(),
                                              /*allowUnresolved=*/false);

  // Build the path.
  buildPath(this, source, protocol, rootType);

  // Return the path; we're done!
  return path;
}

unsigned GenericParamKey::findIndexIn(
                  llvm::ArrayRef<GenericTypeParamType *> genericParams) const {
  // For depth 0, we have random access. We perform the extra checking so that
  // we can return
  if (Depth == 0 && Index < genericParams.size() &&
      genericParams[Index] == *this)
    return Index;

  // At other depths, perform a binary search.
  unsigned result =
      std::lower_bound(genericParams.begin(), genericParams.end(), *this,
                       Ordering())
        - genericParams.begin();
  if (result < genericParams.size() && genericParams[result] == *this)
    return result;

  // We didn't find the parameter we were looking for.
  return genericParams.size();
}