swift-mirror/lib/AST/RequirementMachine/RequirementMachine.cpp

//===--- RequirementMachine.cpp - Generics with term rewriting ------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2021 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
//
//===----------------------------------------------------------------------===//

#include "swift/AST/RequirementMachine.h"
#include "swift/AST/ASTContext.h"
#include "swift/AST/Decl.h"
#include "swift/AST/GenericSignature.h"
#include "swift/AST/Module.h"
#include "swift/AST/PrettyStackTrace.h"
#include "swift/AST/Requirement.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/TinyPtrVector.h"
#include <vector>

#include "EquivalenceClassMap.h"
#include "ProtocolGraph.h"
#include "RewriteContext.h"
#include "RewriteSystem.h"

using namespace swift;
using namespace rewriting;

namespace {

/// A utility class for bulding a rewrite system from the top-level requirements
/// of a generic signature, and all protocol requirement signatures from all
/// transitively-referenced protocols.
struct RewriteSystemBuilder {
  RewriteContext &Context;
  bool Debug;

  ProtocolGraph Protocols;
  std::vector<std::pair<MutableTerm, MutableTerm>> Rules;

  CanType getConcreteSubstitutionSchema(CanType concreteType,
                                        const ProtocolDecl *proto,
                                        SmallVectorImpl<Term> &result);

  RewriteSystemBuilder(RewriteContext &ctx, bool debug)
    : Context(ctx), Debug(debug) {}
  void addGenericSignature(CanGenericSignature sig);
  void addAssociatedType(const AssociatedTypeDecl *type,
                         const ProtocolDecl *proto);
  void addRequirement(const Requirement &req,
                      const ProtocolDecl *proto);
};

} // end namespace

/// Given a concrete type that may contain type parameters in structural positions,
/// collect all the structural type parameter components, and replace them all with
/// fresh generic parameters. The fresh generic parameters all have a depth of 0,
/// and the index is an index into the 'result' array.
///
/// For example, given the concrete type Foo<X.Y, Array<Z>>, this produces the
/// result type Foo<τ_0_0, Array<τ_0_1>>, with result array {X.Y, Z}.
CanType
RewriteSystemBuilder::getConcreteSubstitutionSchema(CanType concreteType,
                                                    const ProtocolDecl *proto,
                                                    SmallVectorImpl<Term> &result) {
  assert(!concreteType->isTypeParameter() && "Must have a concrete type here");

  if (!concreteType->hasTypeParameter())
    return concreteType;

  return CanType(concreteType.transformRec(
    [&](Type t) -> Optional<Type> {
      if (!t->isTypeParameter())
        return None;

      unsigned index = result.size();
      result.push_back(Context.getTermForType(CanType(t), proto));

      return CanGenericTypeParamType::get(/*depth=*/0, index, Context.getASTContext());
    }));
}

void RewriteSystemBuilder::addGenericSignature(CanGenericSignature sig) {
  // Collect all protocols transitively referenced from the generic signature's
  // requirements.
  Protocols.visitRequirements(sig->getRequirements());
  Protocols.compute();

  // Add rewrite rules for each protocol.
  for (auto *proto : Protocols.getProtocols()) {
    if (Debug) {
      llvm::dbgs() << "protocol " << proto->getName() << " {\n";
    }

    const auto &info = Protocols.getProtocolInfo(proto);

    for (auto *assocType : info.AssociatedTypes)
      addAssociatedType(assocType, proto);

    for (auto *assocType : info.InheritedAssociatedTypes)
      addAssociatedType(assocType, proto);

    for (auto req : info.Requirements)
      addRequirement(req.getCanonical(), proto);

    if (Debug) {
      llvm::dbgs() << "}\n";
    }
  }

  // Add rewrite rules for all requirements in the top-level signature.
  for (const auto &req : sig->getRequirements())
    addRequirement(req, /*proto=*/nullptr);
}

/// For an associated type T in a protocol P, we add a rewrite rule:
///
///   [P].T => [P:T]
///
/// Intuitively, this means "if a type conforms to P, it has a nested type
/// named T".
void RewriteSystemBuilder::addAssociatedType(const AssociatedTypeDecl *type,
                                             const ProtocolDecl *proto) {
  MutableTerm lhs;
  lhs.add(Atom::forProtocol(proto, Context));
  lhs.add(Atom::forName(type->getName(), Context));

  MutableTerm rhs;
  rhs.add(Atom::forAssociatedType(proto, type->getName(), Context));

  Rules.emplace_back(lhs, rhs);
}

/// Lowers a generic requirement to a rewrite rule.
///
/// If \p proto is null, this is a generic requirement from the top-level
/// generic signature. The added rewrite rule will be rooted in a generic
/// parameter atom.
///
/// If \p proto is non-null, this is a generic requirement in the protocol's
/// requirement signature. The added rewrite rule will be rooted in a
/// protocol atom.
void RewriteSystemBuilder::addRequirement(const Requirement &req,
                                          const ProtocolDecl *proto) {
  if (Debug) {
    llvm::dbgs() << "+ ";
    req.dump(llvm::dbgs());
    llvm::dbgs() << "\n";
  }

  // Compute the left hand side.
  auto subjectType = CanType(req.getFirstType());
  auto subjectTerm = Context.getMutableTermForType(subjectType, proto);

  // Compute the right hand side.
  MutableTerm constraintTerm;

  switch (req.getKind()) {
  case RequirementKind::Conformance: {
    // A conformance requirement T : P becomes a rewrite rule
    //
    //   T.[P] == T
    //
    // Intuitively, this means "any type ending with T conforms to P".
    auto *proto = req.getProtocolDecl();

    constraintTerm = subjectTerm;
    constraintTerm.add(Atom::forProtocol(proto, Context));
    break;
  }

  case RequirementKind::Superclass: {
    // A superclass requirement T : C<X, Y> becomes a rewrite rule
    //
    //   T.[superclass: C<X, Y>] => T
    auto otherType = CanType(req.getSecondType());

    SmallVector<Term, 1> substitutions;
    otherType = getConcreteSubstitutionSchema(otherType, proto,
                                              substitutions);

    constraintTerm = subjectTerm;
    constraintTerm.add(Atom::forSuperclass(otherType, substitutions,
                                           Context));
    break;
  }

  case RequirementKind::Layout: {
    // A layout requirement T : L becomes a rewrite rule
    //
    //   T.[layout: L] == T
    constraintTerm = subjectTerm;
    constraintTerm.add(Atom::forLayout(req.getLayoutConstraint(),
                                       Context));
    break;
  }

  case RequirementKind::SameType: {
    auto otherType = CanType(req.getSecondType());

    if (!otherType->isTypeParameter()) {
      // A concrete same-type requirement T == C<X, Y> becomes a
      // rewrite rule
      //
      //   T.[concrete: C<X, Y>] => T
      SmallVector<Term, 1> substitutions;
      otherType = getConcreteSubstitutionSchema(otherType, proto,
                                                substitutions);

      constraintTerm = subjectTerm;
      constraintTerm.add(Atom::forConcreteType(otherType, substitutions,
                                               Context));
      break;
    }

    constraintTerm = Context.getMutableTermForType(otherType, proto);
    break;
  }
  }

  Rules.emplace_back(subjectTerm, constraintTerm);
}


/// We use the PIMPL pattern to avoid creeping header dependencies.
struct RequirementMachine::Implementation {
  RewriteContext Context;
  RewriteSystem System;
  EquivalenceClassMap Map;
  CanGenericSignature Sig;
  bool Complete = false;

  /// All conformance access paths computed so far.
  llvm::DenseMap<std::pair<CanType, ProtocolDecl *>,
                 ConformanceAccessPath> ConformanceAccessPaths;

  /// Conformance access paths computed during the last round. All elements
  /// have the same length. If a conformance access path of greater length
  /// is requested, we refill CurrentConformanceAccessPaths with all paths of
  /// length N+1, and add them to the ConformanceAccessPaths map.
  std::vector<std::pair<CanType, ConformanceAccessPath>>
      CurrentConformanceAccessPaths;

  explicit Implementation(ASTContext &ctx)
      : Context(ctx),
        System(Context),
        Map(Context, System.getProtocols()) {}
  void verify(const MutableTerm &term);
  void dump(llvm::raw_ostream &out);

  MutableTerm getLongestValidPrefix(const MutableTerm &term);
};

void RequirementMachine::Implementation::verify(const MutableTerm &term) {
#ifndef NDEBUG
  MutableTerm erased;

  // First, "erase" resolved associated types from the term, and try
  // to simplify it again.
  for (auto atom : term) {
    if (erased.empty()) {
      switch (atom.getKind()) {
      case Atom::Kind::Protocol:
      case Atom::Kind::GenericParam:
        erased.add(atom);
        continue;

      case Atom::Kind::AssociatedType:
        erased.add(Atom::forProtocol(atom.getProtocols()[0], Context));
        break;

      case Atom::Kind::Name:
      case Atom::Kind::Layout:
      case Atom::Kind::Superclass:
      case Atom::Kind::ConcreteType:
        llvm::errs() << "Bad initial atom in " << term << "\n";
        abort();
        break;
      }
    }

    switch (atom.getKind()) {
    case Atom::Kind::Name:
      assert(!erased.empty());
      erased.add(atom);
      break;

    case Atom::Kind::AssociatedType:
      erased.add(Atom::forName(atom.getName(), Context));
      break;

    case Atom::Kind::Protocol:
    case Atom::Kind::GenericParam:
    case Atom::Kind::Layout:
    case Atom::Kind::Superclass:
    case Atom::Kind::ConcreteType:
      llvm::errs() << "Bad interior atom " << atom << " in " << term << "\n";
      abort();
      break;
    }
  }

  MutableTerm simplified = erased;
  System.simplify(simplified);

  // We should end up with the same term.
  if (simplified != term) {
    llvm::errs() << "Term verification failed\n";
    llvm::errs() << "Initial term:    " << term << "\n";
    llvm::errs() << "Erased term:     " << erased << "\n";
    llvm::errs() << "Simplified term: " << simplified << "\n";
    llvm::errs() << "\n";
    dump(llvm::errs());
    abort();
  }
#endif
}

void RequirementMachine::Implementation::dump(llvm::raw_ostream &out) {
  out << "Requirement machine for " << Sig << "\n";
  System.dump(out);
  Map.dump(out);
}

RequirementMachine::RequirementMachine(ASTContext &ctx) : Context(ctx) {
  Impl = new Implementation(ctx);
}

RequirementMachine::~RequirementMachine() {
  delete Impl;
}

void RequirementMachine::addGenericSignature(CanGenericSignature sig) {
  Impl->Sig = sig;

  PrettyStackTraceGenericSignature debugStack("building rewrite system for", sig);

  auto *Stats = Context.Stats;

  if (Stats)
    ++Stats->getFrontendCounters().NumRequirementMachines;

  FrontendStatsTracer tracer(Stats, "build-rewrite-system");

  if (Context.LangOpts.DebugRequirementMachine) {
    llvm::dbgs() << "Adding generic signature " << sig << " {\n";
  }

  // Collect the top-level requirements, and all transtively-referenced
  // protocol requirement signatures.
  RewriteSystemBuilder builder(Impl->Context,
                               Context.LangOpts.DebugRequirementMachine);
  builder.addGenericSignature(sig);

  // Add the initial set of rewrite rules to the rewrite system, also
  // providing the protocol graph to use for the linear order on terms.
  Impl->System.initialize(std::move(builder.Rules),
                          std::move(builder.Protocols));

  computeCompletion();

  if (Context.LangOpts.DebugRequirementMachine) {
    llvm::dbgs() << "}\n";
  }
}

/// Attempt to obtain a confluent rewrite system using the completion
/// procedure.
void RequirementMachine::computeCompletion() {
  while (true) {
    // First, run the Knuth-Bendix algorithm to resolve overlapping rules.
    auto result = Impl->System.computeConfluentCompletion(
        Context.LangOpts.RequirementMachineStepLimit,
        Context.LangOpts.RequirementMachineDepthLimit);

    if (Context.Stats) {
      Context.Stats->getFrontendCounters()
          .NumRequirementMachineCompletionSteps += result.second;
    }

    // Check for failure.
    auto checkCompletionResult = [&]() {
      switch (result.first) {
      case RewriteSystem::CompletionResult::Success:
        break;

      case RewriteSystem::CompletionResult::MaxIterations:
        llvm::errs() << "Generic signature " << Impl->Sig
                     << " exceeds maximum completion step count\n";
        Impl->System.dump(llvm::errs());
        abort();

      case RewriteSystem::CompletionResult::MaxDepth:
        llvm::errs() << "Generic signature " << Impl->Sig
                     << " exceeds maximum completion depth\n";
        Impl->System.dump(llvm::errs());
        abort();
      }
    };

    checkCompletionResult();

    // Simplify right hand sides in preparation for building the
    // equivalence class map.
    Impl->System.simplifyRightHandSides();

    // Build the equivalence class map, which performs concrete term
    // unification; if this added any new rules, run the completion
    // procedure again.
    result = Impl->System.buildEquivalenceClassMap(
        Impl->Map,
        Context.LangOpts.RequirementMachineStepLimit,
        Context.LangOpts.RequirementMachineDepthLimit);

    if (Context.Stats) {
      Context.Stats->getFrontendCounters()
        .NumRequirementMachineUnifiedConcreteTerms += result.second;
    }

    checkCompletionResult();

    // If buildEquivalenceClassMap() added new rules, we run another
    // round of Knuth-Bendix, and build the equivalence class map again.
    if (result.second == 0)
      break;
  }

  if (Context.LangOpts.DebugRequirementMachine) {
    dump(llvm::dbgs());
  }

  assert(!Impl->Complete);
  Impl->Complete = true;
}

bool RequirementMachine::isComplete() const {
  return Impl->Complete;
}

void RequirementMachine::dump(llvm::raw_ostream &out) const {
  Impl->dump(out);
}

/// Collects all requirements on a type parameter that are used to construct
/// its ArchetypeType in a GenericEnvironment.
GenericSignature::LocalRequirements
RequirementMachine::getLocalRequirements(
    Type depType,
    TypeArrayView<GenericTypeParamType> genericParams) const {
  auto term = Impl->Context.getMutableTermForType(depType->getCanonicalType(),
                                                  /*proto=*/nullptr);
  Impl->System.simplify(term);
  Impl->verify(term);

  auto &protos = Impl->System.getProtocols();

  GenericSignature::LocalRequirements result;
  result.anchor = Impl->Context.getTypeForTerm(term, genericParams, protos);

  auto *equivClass = Impl->Map.lookUpEquivalenceClass(term);
  if (!equivClass)
    return result;

  if (equivClass->isConcreteType()) {
    result.concreteType = equivClass->getConcreteType({}, protos,
                                                      Impl->Context);
    return result;
  }

  if (equivClass->hasSuperclassBound()) {
    result.superclass = equivClass->getSuperclassBound({}, protos,
                                                       Impl->Context);
  }

  for (const auto *proto : equivClass->getConformsToExcludingSuperclassConformances())
    result.protos.push_back(const_cast<ProtocolDecl *>(proto));

  result.layout = equivClass->getLayoutConstraint();

  return result;
}

bool RequirementMachine::requiresClass(Type depType) const {
  auto term = Impl->Context.getMutableTermForType(depType->getCanonicalType(),
                                                  /*proto=*/nullptr);
  Impl->System.simplify(term);
  Impl->verify(term);

  auto *equivClass = Impl->Map.lookUpEquivalenceClass(term);
  if (!equivClass)
    return false;

  if (equivClass->isConcreteType())
    return false;

  auto layout = equivClass->getLayoutConstraint();
  return (layout && layout->isClass());
}

LayoutConstraint RequirementMachine::getLayoutConstraint(Type depType) const {
  auto term = Impl->Context.getMutableTermForType(depType->getCanonicalType(),
                                                  /*proto=*/nullptr);
  Impl->System.simplify(term);
  Impl->verify(term);

  auto *equivClass = Impl->Map.lookUpEquivalenceClass(term);
  if (!equivClass)
    return LayoutConstraint();

  return equivClass->getLayoutConstraint();
}

bool RequirementMachine::requiresProtocol(Type depType,
                                          const ProtocolDecl *proto) const {
  auto term = Impl->Context.getMutableTermForType(depType->getCanonicalType(),
                                                  /*proto=*/nullptr);
  Impl->System.simplify(term);
  Impl->verify(term);

  auto *equivClass = Impl->Map.lookUpEquivalenceClass(term);
  if (!equivClass)
    return false;

  if (equivClass->isConcreteType())
    return false;

  for (auto *otherProto : equivClass->getConformsTo()) {
    if (otherProto == proto)
      return true;
  }

  return false;
}

GenericSignature::RequiredProtocols
RequirementMachine::getRequiredProtocols(Type depType) const {
  auto term = Impl->Context.getMutableTermForType(depType->getCanonicalType(),
                                                  /*proto=*/nullptr);
  Impl->System.simplify(term);
  Impl->verify(term);

  auto *equivClass = Impl->Map.lookUpEquivalenceClass(term);
  if (!equivClass)
    return { };

  if (equivClass->isConcreteType())
    return { };

  GenericSignature::RequiredProtocols result;
  for (auto *otherProto : equivClass->getConformsTo()) {
    result.push_back(const_cast<ProtocolDecl *>(otherProto));
  }

  ProtocolType::canonicalizeProtocols(result);

  return result;
}

Type RequirementMachine::getSuperclassBound(Type depType) const {
  auto term = Impl->Context.getMutableTermForType(depType->getCanonicalType(),
                                                  /*proto=*/nullptr);
  Impl->System.simplify(term);
  Impl->verify(term);

  auto *equivClass = Impl->Map.lookUpEquivalenceClass(term);
  if (!equivClass)
    return Type();

  if (!equivClass->hasSuperclassBound())
    return Type();

  auto &protos = Impl->System.getProtocols();
  return equivClass->getSuperclassBound({ }, protos, Impl->Context);
}

bool RequirementMachine::isConcreteType(Type depType) const {
  auto term = Impl->Context.getMutableTermForType(depType->getCanonicalType(),
                                                  /*proto=*/nullptr);
  Impl->System.simplify(term);
  Impl->verify(term);

  auto *equivClass = Impl->Map.lookUpEquivalenceClass(term);
  if (!equivClass)
    return false;

  return equivClass->isConcreteType();
}

Type RequirementMachine::getConcreteType(Type depType) const {
  auto term = Impl->Context.getMutableTermForType(depType->getCanonicalType(),
                                                  /*proto=*/nullptr);
  Impl->System.simplify(term);
  Impl->verify(term);

  auto *equivClass = Impl->Map.lookUpEquivalenceClass(term);
  if (!equivClass)
    return Type();

  if (!equivClass->isConcreteType())
    return Type();

  auto &protos = Impl->System.getProtocols();
  return equivClass->getConcreteType({ }, protos, Impl->Context);
}

bool RequirementMachine::areSameTypeParameterInContext(Type depType1,
                                                       Type depType2) const {
  auto term1 = Impl->Context.getMutableTermForType(depType1->getCanonicalType(),
                                                   /*proto=*/nullptr);
  Impl->System.simplify(term1);
  Impl->verify(term1);

  auto term2 = Impl->Context.getMutableTermForType(depType2->getCanonicalType(),
                                                   /*proto=*/nullptr);
  Impl->System.simplify(term2);
  Impl->verify(term2);

  return (term1 == term2);
}

MutableTerm
RequirementMachine::Implementation::getLongestValidPrefix(const MutableTerm &term) {
  MutableTerm prefix;

  for (auto atom : term) {
    switch (atom.getKind()) {
    case Atom::Kind::Name:
      return prefix;

    case Atom::Kind::Protocol:
      assert(prefix.empty() &&
             "Protocol atom can only appear at the start of a type term");
      if (!System.getProtocols().isKnownProtocol(atom.getProtocol()))
        return prefix;

      break;

    case Atom::Kind::GenericParam:
      assert(prefix.empty() &&
             "Generic parameter atom can only appear at the start of a type term");
      break;

    case Atom::Kind::AssociatedType: {
      const auto *equivClass = Map.lookUpEquivalenceClass(prefix);
      if (!equivClass)
        return prefix;

      auto conformsTo = equivClass->getConformsTo();

      for (const auto *proto : atom.getProtocols()) {
        if (!System.getProtocols().isKnownProtocol(proto))
          return prefix;

        // T.[P:A] is valid iff T conforms to P.
        if (std::find(conformsTo.begin(), conformsTo.end(), proto)
              == conformsTo.end())
          return prefix;
      }

      break;
    }

    case Atom::Kind::Layout:
    case Atom::Kind::Superclass:
    case Atom::Kind::ConcreteType:
      llvm_unreachable("Property atom cannot appear in a type term");
    }

    // This atom is valid, add it to the longest prefix.
    prefix.add(atom);
  }

  return prefix;
}

/// Unlike most other queries, the input type can be any type, not just a
/// type parameter.
///
/// Returns true if all structural components that are type parameters are
/// in their canonical form, and are not concrete (in which case they're
/// not considered canonical, since they can be replaced with their
/// concrete type).
bool RequirementMachine::isCanonicalTypeInContext(Type type) const {
  auto &protos = Impl->System.getProtocols();

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

    auto term = Impl->Context.getMutableTermForType(component->getCanonicalType(),
                                                    /*proto=*/nullptr);

    Impl->System.simplify(term);
    Impl->verify(term);

    auto *equivClass = Impl->Map.lookUpEquivalenceClass(term);
    if (!equivClass)
      return false;

    if (equivClass->isConcreteType())
      return true;

    auto anchor = Impl->Context.getTypeForTerm(term, {}, protos);
    return CanType(anchor) != CanType(component);
  });
}

/// Unlike most other queries, the input type can be any type, not just a
/// type parameter.
///
/// Replaces all structural components that are type parameters with their
/// most canonical form, which is either a (possibly different)
/// type parameter, or a concrete type, in which case we recursively
/// simplify any type parameters appearing in structural positions of
/// that concrete type as well, and so on.
Type RequirementMachine::getCanonicalTypeInContext(
    Type type,
    TypeArrayView<GenericTypeParamType> genericParams) const {
  const auto &protos = Impl->System.getProtocols();

  return type.transformRec([&](Type t) -> Optional<Type> {
    if (!t->isTypeParameter())
      return None;

    // Get a simplified term T.
    auto term = Impl->Context.getMutableTermForType(t->getCanonicalType(),
                                                    /*proto=*/nullptr);
    Impl->System.simplify(term);

    // We need to handle "purely concrete" member types, eg if I have a
    // signature <T where T == Foo>, and we're asked to canonicalize the
    // type T.[P:A] where Foo : A.
    //
    // This comes up because we can derive the signature <T where T == Foo>
    // from a generic signature like <T where T : P>; adding the
    // concrete requirement 'T == Foo' renders 'T : P' redundant. We then
    // want to take interface types written against the original signature
    // and canonicalize them with respect to the derived signature.
    //
    // The problem is that T.[P:A] is not a valid term in the rewrite system
    // for <T where T == Foo>, since we do not have the requirement T : P.
    //
    // A more principled solution would build a substitution map when
    // building a derived generic signature that adds new requirements;
    // interface types would first be substituted before being canonicalized
    // in the new signature.
    //
    // For now, we handle this with a two-step process; we split a term up
    // into a longest valid prefix, which must resolve to a concrete type,
    // and the remaining suffix, which we use to perform a concrete
    // substitution using subst().

    // In the below, let T be a type term, with T == UV, where U is the
    // longest valid prefix.
    //
    // Note that V can be empty if T is fully valid; we expect this to be
    // true most of the time.
    auto prefix = Impl->getLongestValidPrefix(term);

    // Get a type (concrete or dependent) for U.
    auto prefixType = [&]() -> Type {
      Impl->verify(prefix);

      auto *equivClass = Impl->Map.lookUpEquivalenceClass(prefix);
      if (equivClass && equivClass->isConcreteType()) {
        auto concreteType = equivClass->getConcreteType(genericParams,
                                                        protos, Impl->Context);
        if (!concreteType->hasTypeParameter())
          return concreteType;

        // FIXME: Recursion guard is needed here
        return getCanonicalTypeInContext(concreteType, genericParams);
      }

      return Impl->Context.getTypeForTerm(prefix, genericParams, protos);
    }();

    // If T is already valid, the longest valid prefix U of T is T itself, and
    // V is empty. Just return the type we computed above.
    //
    // This is the only case where U is allowed to be dependent.
    if (prefix.size() == term.size())
      return prefixType;

    // If U is not concrete, we have an invalid member type of a dependent
    // type, which is not valid in this generic signature. Give up.
    if (prefixType->isTypeParameter()) {
      llvm::errs() << "Invalid type parameter in getCanonicalTypeInContext()\n";
      llvm::errs() << "Original type: " << type << "\n";
      llvm::errs() << "Simplified term: " << term << "\n";
      llvm::errs() << "Longest valid prefix: " << prefix << "\n";
      llvm::errs() << "Prefix type: " << prefixType << "\n";
      llvm::errs() << "\n";
      dump(llvm::errs());
      abort();
    }

    // Compute the type of the unresolved suffix term V, rooted in the
    // generic parameter τ_0_0.
    auto origType = Impl->Context.getRelativeTypeForTerm(
        term, prefix, Impl->System.getProtocols());

    // Substitute τ_0_0 in the above relative type with the concrete type
    // for U.
    //
    // Example: if T == A.B.C and the longest valid prefix is A.B which
    // maps to a concrete type Foo<Int>, then we have:
    //
    // U == A.B
    // V == C
    //
    // prefixType == Foo<Int>
    // origType   == τ_0_0.C
    // substType  == Foo<Int>.C
    //
    auto substType = origType.subst(
      [&](SubstitutableType *type) -> Type {
        assert(cast<GenericTypeParamType>(type)->getDepth() == 0);
        assert(cast<GenericTypeParamType>(type)->getIndex() == 0);

        return prefixType;
      },
      LookUpConformanceInSignature(Impl->Sig.getPointer()));

    // FIXME: Recursion guard is needed here
    return getCanonicalTypeInContext(substType, genericParams);
  });
}

/// Replace 'Self' in the given dependent type (\c depTy) with the given
/// dependent type, producing a type that refers to
/// the nested type. This limited operation makes sure that it does not
/// create any new potential archetypes along the way, so it should only be
/// used in cases where we're reconstructing something that we know exists.
static Type replaceSelfWithType(Type selfType, Type depTy) {
  if (auto depMemTy = depTy->getAs<DependentMemberType>()) {
    Type baseType = replaceSelfWithType(selfType, depMemTy->getBase());
    assert(depMemTy->getAssocType() && "Missing associated type");
    return DependentMemberType::get(baseType, depMemTy->getAssocType());
  }

  assert(depTy->is<GenericTypeParamType>() && "missing Self?");
  return selfType;
}

/// Retrieve the conformance access path used to extract the conformance of
/// interface \c type to the given \c protocol.
///
/// \param type The interface type whose conformance access path is to be
/// queried.
/// \param protocol A protocol to which \c type conforms.
///
/// \returns the conformance access path that starts at a requirement of
/// this generic signature and ends at the conformance that makes \c type
/// conform to \c protocol.
///
/// \seealso ConformanceAccessPath
ConformanceAccessPath
RequirementMachine::getConformanceAccessPath(Type type,
                                             ProtocolDecl *protocol) {
  auto canType = getCanonicalTypeInContext(type, { })->getCanonicalType();
  assert(canType->isTypeParameter());

  // Check if we've already cached the result before doing anything else.
  auto found = Impl->ConformanceAccessPaths.find(
      std::make_pair(canType, protocol));
  if (found != Impl->ConformanceAccessPaths.end()) {
    return found->second;
  }

  auto *Stats = Context.Stats;

  FrontendStatsTracer tracer(Stats, "get-conformance-access-path");

  auto recordPath = [&](CanType type, ProtocolDecl *proto,
                        ConformanceAccessPath path) {
    // Add the path to the buffer.
    Impl->CurrentConformanceAccessPaths.emplace_back(type, path);

    // Add the path to the map.
    auto key = std::make_pair(type, proto);
    auto inserted = Impl->ConformanceAccessPaths.insert(
        std::make_pair(key, path));
    assert(inserted.second);
    (void) inserted;

    if (Stats)
      ++Stats->getFrontendCounters().NumConformanceAccessPathsRecorded;
  };

  // If this is the first time we're asked to look up a conformance access path,
  // visit all of the root conformance requirements in our generic signature and
  // add them to the buffer.
  if (Impl->ConformanceAccessPaths.empty()) {
    for (const auto &req : Impl->Sig->getRequirements()) {
      // We only care about conformance requirements.
      if (req.getKind() != RequirementKind::Conformance)
        continue;

      auto rootType = CanType(req.getFirstType());
      auto *rootProto = req.getProtocolDecl();

      ConformanceAccessPath::Entry root(rootType, rootProto);
      ArrayRef<ConformanceAccessPath::Entry> path(root);
      ConformanceAccessPath result(Context.AllocateCopy(path));

      recordPath(rootType, rootProto, result);
    }
  }

  // We enumerate conformance access paths in lexshort order until we find the
  // path whose corresponding type canonicalizes to the one we are looking for.
  while (true) {
    auto found = Impl->ConformanceAccessPaths.find(
        std::make_pair(canType, protocol));
    if (found != Impl->ConformanceAccessPaths.end()) {
      return found->second;
    }

    assert(Impl->CurrentConformanceAccessPaths.size() > 0);

    // The buffer consists of all conformance access paths of length N.
    // Swap it out with an empty buffer, and fill it with all paths of
    // length N+1.
    std::vector<std::pair<CanType, ConformanceAccessPath>> oldPaths;
    std::swap(Impl->CurrentConformanceAccessPaths, oldPaths);

    for (const auto &pair : oldPaths) {
      const auto &lastElt = pair.second.back();
      auto *lastProto = lastElt.second;

      // A copy of the current path, populated as needed.
      SmallVector<ConformanceAccessPath::Entry, 4> entries;

      for (const auto &req : lastProto->getRequirementSignature()) {
        // We only care about conformance requirements.
        if (req.getKind() != RequirementKind::Conformance)
          continue;

        auto nextSubjectType = req.getFirstType()->getCanonicalType();
        auto *nextProto = req.getProtocolDecl();

        // Compute the canonical anchor for this conformance requirement.
        auto nextType = replaceSelfWithType(pair.first, nextSubjectType);
        auto nextCanType = getCanonicalTypeInContext(nextType, { })
            ->getCanonicalType();

        // Skip "derived via concrete" sources.
        if (!nextCanType->isTypeParameter())
          continue;

        // If we've already seen a path for this conformance, skip it and
        // don't add it to the buffer. Note that because we iterate over
        // conformance access paths in lexshort order, the existing
        // conformance access path is shorter than the one we found just now.
        if (Impl->ConformanceAccessPaths.count(
                std::make_pair(nextCanType, nextProto)))
          continue;

        if (entries.empty()) {
          // Fill our temporary vector.
          entries.insert(entries.begin(),
                         pair.second.begin(),
                         pair.second.end());
        }

        // Add the next entry.
        entries.emplace_back(nextSubjectType, nextProto);
        ConformanceAccessPath result = Context.AllocateCopy(entries);
        entries.pop_back();

        recordPath(nextCanType, nextProto, result);
      }
    }
  }
}

/// Compare two associated types.
static int compareAssociatedTypes(AssociatedTypeDecl *assocType1,
                                  AssociatedTypeDecl *assocType2) {
  // - by name.
  if (int result = assocType1->getName().str().compare(
                                              assocType2->getName().str()))
    return result;

  // Prefer an associated type with no overrides (i.e., an anchor) to one
  // that has overrides.
  bool hasOverridden1 = !assocType1->getOverriddenDecls().empty();
  bool hasOverridden2 = !assocType2->getOverriddenDecls().empty();
  if (hasOverridden1 != hasOverridden2)
    return hasOverridden1 ? +1 : -1;

  // - by protocol, so t_n_m.`P.T` < t_n_m.`Q.T` (given P < Q)
  auto proto1 = assocType1->getProtocol();
  auto proto2 = assocType2->getProtocol();
  if (int compareProtocols = TypeDecl::compare(proto1, proto2))
    return compareProtocols;

  // Error case: if we have two associated types with the same name in the
  // same protocol, just tie-break based on address.
  if (assocType1 != assocType2)
    return assocType1 < assocType2 ? -1 : +1;

  return 0;
}

static void lookupConcreteNestedType(NominalTypeDecl *decl,
                                     Identifier name,
                                     SmallVectorImpl<TypeDecl *> &concreteDecls) {
  SmallVector<ValueDecl *, 2> foundMembers;
  decl->getParentModule()->lookupQualified(
      decl, DeclNameRef(name),
      NL_QualifiedDefault | NL_OnlyTypes | NL_ProtocolMembers,
      foundMembers);
  for (auto member : foundMembers)
    concreteDecls.push_back(cast<TypeDecl>(member));
}

static TypeDecl *
findBestConcreteNestedType(SmallVectorImpl<TypeDecl *> &concreteDecls) {
  return *std::min_element(concreteDecls.begin(), concreteDecls.end(),
                           [](TypeDecl *type1, TypeDecl *type2) {
                             return TypeDecl::compare(type1, type2) < 0;
                           });
}

TypeDecl *
RequirementMachine::lookupNestedType(Type depType, Identifier name) const {
  auto term = Impl->Context.getMutableTermForType(depType->getCanonicalType(),
                                                  /*proto=*/nullptr);
  Impl->System.simplify(term);
  Impl->verify(term);

  auto *equivClass = Impl->Map.lookUpEquivalenceClass(term);
  if (!equivClass)
    return nullptr;

  // Look for types with the given name in protocols that we know about.
  AssociatedTypeDecl *bestAssocType = nullptr;
  SmallVector<TypeDecl *, 4> concreteDecls;

  for (const auto *proto : equivClass->getConformsTo()) {
    // Look for an associated type and/or concrete type with this name.
    for (auto member : const_cast<ProtocolDecl *>(proto)->lookupDirect(name)) {
      // If this is an associated type, record whether it is the best
      // associated type we've seen thus far.
      if (auto assocType = dyn_cast<AssociatedTypeDecl>(member)) {
        // Retrieve the associated type anchor.
        assocType = assocType->getAssociatedTypeAnchor();

        if (!bestAssocType ||
             compareAssociatedTypes(assocType, bestAssocType) < 0)
          bestAssocType = assocType;

        continue;
      }

      // If this is another type declaration, record it.
      if (auto type = dyn_cast<TypeDecl>(member)) {
        concreteDecls.push_back(type);
        continue;
      }
    }
  }

  // If we haven't found anything yet but have a concrete type or a superclass,
  // look for a type in that.
  // FIXME: Shouldn't we always look here?
  if (!bestAssocType && concreteDecls.empty()) {
    Type typeToSearch;
    if (equivClass->isConcreteType())
      typeToSearch = equivClass->getConcreteType();
    else if (equivClass->hasSuperclassBound())
      typeToSearch = equivClass->getSuperclassBound();

    if (typeToSearch)
      if (auto *decl = typeToSearch->getAnyNominal())
        lookupConcreteNestedType(decl, name, concreteDecls);
  }

  if (bestAssocType) {
    assert(bestAssocType->getOverriddenDecls().empty() &&
           "Lookup should never keep a non-anchor associated type");
    return bestAssocType;

  } else if (!concreteDecls.empty()) {
    // Find the best concrete type.
    return findBestConcreteNestedType(concreteDecls);
  }

  return nullptr;
}