TypeJoinMeet.cpp

//===--- TypeJoinMeet.cpp - Swift Type "join" and "meet"  -----------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2018 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
//
//  This file implements the "join" operation for types (and, eventually,
//  "meet").
//
//===----------------------------------------------------------------------===//
#include "swift/AST/ASTContext.h"
#include "swift/AST/CanTypeVisitor.h"
#include "swift/AST/Decl.h"
#include "swift/AST/Type.h"
#include "swift/AST/Types.h"
#include "llvm/ADT/Optional.h"
#include "llvm/ADT/SmallPtrSet.h"
using namespace swift;

namespace {

// FIXME: This is currently woefully incomplete, and is only currently
// used for optimizing away extra exploratory work in the constraint
// solver. It should eventually encompass all of the subtyping rules
// of the language.
struct TypeJoin : CanTypeVisitor<TypeJoin, CanType> {
  // The type we're joining with another type (the latter of which is
  // passed as an argument in the visitor.
  CanType First;

  // Always null. Used as a marker for places where we can improve the
  // implementation.
  CanType Unimplemented;

  // Always null. Used as a marker for places where there is no join
  // of two types in our type system.
  CanType Nonexistent;

  // For convenience, TheAnyType from ASTContext;
  CanType TheAnyType;

  TypeJoin(CanType First) : First(First), Unimplemented(CanType()) {
    assert(First && "Unexpected null type!");
    TheAnyType = First->getASTContext().TheAnyType;
  }

  static CanType getSuperclassJoin(CanType first, CanType second);
  CanType computeProtocolCompositionJoin(ArrayRef<Type> firstMembers,
                                         ArrayRef<Type> secondMembers);


  CanType visitErrorType(CanType second);
  CanType visitTupleType(CanType second);
  CanType visitEnumType(CanType second);
  CanType visitStructType(CanType second);
  CanType visitClassType(CanType second);
  CanType visitProtocolType(CanType second);
  CanType visitBoundGenericClassType(CanType second);
  CanType visitBoundGenericEnumType(CanType second);
  CanType visitBoundGenericStructType(CanType second);
  CanType visitMetatypeType(CanType second);
  CanType visitExistentialMetatypeType(CanType second);
  CanType visitModuleType(CanType second);
  CanType visitDynamicSelfType(CanType second);
  CanType visitArchetypeType(CanType second);
  CanType visitGenericTypeParamType(CanType second);
  CanType visitDependentMemberType(CanType second);
  CanType visitFunctionType(CanType second);
  CanType visitGenericFunctionType(CanType second);
  CanType visitProtocolCompositionType(CanType second);
  CanType visitLValueType(CanType second);
  CanType visitInOutType(CanType second);
  CanType visitBuiltinType(CanType second);

  CanType visitType(CanType second) {
    return Unimplemented;
  }

public:
  static CanType join(CanType first, CanType second) {
    assert(!first->hasTypeVariable() && !second->hasTypeVariable() &&
           "Cannot compute join of types involving type variables");

    assert(first->getWithoutSpecifierType()->isEqual(first) &&
           "Expected simple type!");
    assert(second->getWithoutSpecifierType()->isEqual(second) &&
           "Expected simple type!");

    // If the types are equivalent, the join is obvious.
    if (first == second)
      return first;

    // Optionals broadly interact with all the other types since
    //   T <: T? for any T (including Any)
    // So we'll always attempt to dispatch Optional here rather than
    // make every visitor check for it explicitly.
    if (first->getOptionalObjectType())
      return TypeJoin(second).visit(first);

    if (second->getOptionalObjectType())
      return TypeJoin(first).visit(second);

    // Likewise, rather than making every visitor deal with Any,
    // always dispatch to the protocol composition side of the join.
    if (first->is<ProtocolCompositionType>())
      return TypeJoin(second).visit(first);

    if (second->is<ProtocolCompositionType>())
      return TypeJoin(first).visit(second);

    // Otherwise the first type might be an optional (or not), so
    // dispatch there.
    return TypeJoin(second).visit(first);
  }
};

CanType TypeJoin::getSuperclassJoin(CanType first, CanType second) {
  assert(first != second);

  // FIXME: Handle joins of classes and a single protocol?
  if (!first->mayHaveSuperclass() || !second->mayHaveSuperclass())
    return CanType();

  /// Walk the superclasses of `first` looking for `second`. Record them
  /// for our second step.
  llvm::SmallPtrSet<CanType, 8> superclassesOfFirst;
  for (Type super = first; super; super = super->getSuperclass()) {
    auto canSuper = super->getCanonicalType();

    // If we have found the second type, we're done.
    if (canSuper == second)
      return canSuper;

    superclassesOfFirst.insert(canSuper);
  }

  // Look through the superclasses of second to determine if any were also
  // superclasses of first.
  for (Type super = second; super; super = super->getSuperclass()) {
    auto canSuper = super->getCanonicalType();

    // If we found the first type, we're done.
    if (superclassesOfFirst.count(canSuper))
      return canSuper;
  }

  // FIXME: Unimplemented.
  return CanType();
}

CanType TypeJoin::visitErrorType(CanType second) {
  llvm_unreachable("join with ErrorType not supported");
  return second;
}

CanType TypeJoin::visitTupleType(CanType second) {
  assert(First != second);

  return TheAnyType;
}

CanType TypeJoin::visitEnumType(CanType second) {
  assert(First != second);

  return Unimplemented;
}

CanType TypeJoin::visitStructType(CanType second) {
  assert(First != second);

  // Deal with inout cases in visitInOutType.
  if (First->is<InOutType>())
    return TypeJoin(second).visit(First);

  // FIXME: When possible we should return a protocol or protocol
  // composition.
  return TheAnyType;
}

CanType TypeJoin::visitClassType(CanType second) {
  return getSuperclassJoin(First, second);
}

CanType TypeJoin::visitBoundGenericClassType(CanType second) {
  return getSuperclassJoin(First, second);
}

/// The subtype relationship of Optionals is as follows:
///   S  <: S?
///   S? <: T? if S <: T (covariant)
static Optional<CanType> joinOptional(CanType first, CanType second) {
  auto firstObject = first.getOptionalObjectType();
  auto secondObject = second.getOptionalObjectType();

  // If neither is any kind of Optional, we're done.
  if (!firstObject && !secondObject)
    return None;

  first = (firstObject ? firstObject : first);
  second = (secondObject ? secondObject : second);

  auto join = TypeJoin::join(first, second);
  if (!join)
    return None;

  return OptionalType::get(join)->getCanonicalType();
}

CanType TypeJoin::visitBoundGenericEnumType(CanType second) {
  // Deal with either First or second (or both) being optionals.
  if (auto joined = joinOptional(First, second))
    return joined.getValue();

  assert(First != second);

  return Unimplemented;
}

CanType TypeJoin::visitBoundGenericStructType(CanType second) {
  assert(First != second);

  // Deal with inout cases in visitInOutType.
  if (First->is<InOutType>())
    return TypeJoin(second).visit(First);

  return Unimplemented;
}

CanType TypeJoin::visitMetatypeType(CanType second) {
  assert(First != second);

  if (First->getKind() != second->getKind())
    return TheAnyType;

  auto firstInstance =
      First->castTo<AnyMetatypeType>()->getInstanceType()->getCanonicalType();
  auto secondInstance =
      second->castTo<AnyMetatypeType>()->getInstanceType()->getCanonicalType();

  auto joinInstance = join(firstInstance, secondInstance);
  if (!joinInstance)
    return CanType();

  return MetatypeType::get(joinInstance)->getCanonicalType();
}

CanType TypeJoin::visitExistentialMetatypeType(CanType second) {
  assert(First != second);

  if (First->getKind() != second->getKind())
    return TheAnyType;

  auto firstInstance =
      First->castTo<AnyMetatypeType>()->getInstanceType()->getCanonicalType();
  auto secondInstance =
      second->castTo<AnyMetatypeType>()->getInstanceType()->getCanonicalType();

  auto joinInstance = join(firstInstance, secondInstance);
  if (!joinInstance)
    return CanType();

  return ExistentialMetatypeType::get(joinInstance)->getCanonicalType();
}

CanType TypeJoin::visitModuleType(CanType second) {
  assert(First != second);

  return TheAnyType;
}

CanType TypeJoin::visitDynamicSelfType(CanType second) {
  return getSuperclassJoin(First, second);
}

CanType TypeJoin::visitArchetypeType(CanType second) {
  return getSuperclassJoin(First, second);
}

CanType TypeJoin::visitGenericTypeParamType(CanType second) {
  llvm_unreachable("Saw GenericTypeParamType in TypeJoin::join");
}

CanType TypeJoin::visitDependentMemberType(CanType second) {
  assert(First != second);

  if (First->getKind() != second->getKind())
    return TheAnyType;

  return Unimplemented;
}

CanType TypeJoin::visitFunctionType(CanType second) {
  assert(First != second);

  auto secondFnTy = second->castTo<FunctionType>();

  if (First->getKind() != second->getKind()) {
    if (secondFnTy->getExtInfo().isNoEscape()) {
      return Nonexistent;
    } else {
      return TheAnyType;
    }
  }

  auto firstFnTy = First->castTo<FunctionType>();

  auto firstExtInfo = firstFnTy->getExtInfo();
  auto secondExtInfo = secondFnTy->getExtInfo();

  // FIXME: Properly handle these attributes.
  if (firstExtInfo.withNoEscape(false) != secondExtInfo.withNoEscape(false))
    return Unimplemented;

  if (!AnyFunctionType::equalParams(firstFnTy->getParams(),
                                    secondFnTy->getParams()))
    return Unimplemented;

  auto firstResult = firstFnTy->getResult()->getCanonicalType();
  auto secondResult = secondFnTy->getResult()->getCanonicalType();

  auto result = join(firstResult, secondResult);
  if (!result)
    return Unimplemented;

  auto extInfo = firstExtInfo;
  if (secondFnTy->getExtInfo().isNoEscape())
    extInfo = extInfo.withNoEscape(true);

  return FunctionType::get(firstFnTy->getParams(), result, extInfo)
      ->getCanonicalType();
}

CanType TypeJoin::visitGenericFunctionType(CanType second) {
  assert(First != second);

  if (First->getKind() != second->getKind())
    return TheAnyType;

  return Unimplemented;
}

// Use the distributive law to compute the join of the protocol
// compositions.
//
//   (A ^ B) v (C ^ D)
// = (A v C) ^ (A v D) ^ (B v C) ^ (B v D)
//
// In general this law only applies to distributive lattices.
//
// In our case, this should be safe because our meet operation only
// produces an existing nominal type when it is one of the operands of
// the operation. So we can never arbitrarily climb down the lattice
// in ways that would break distributivity.
//
CanType TypeJoin::computeProtocolCompositionJoin(ArrayRef<Type> firstMembers,
                                                 ArrayRef<Type> secondMembers) {
  SmallVector<Type, 8> result;
  for (auto first : firstMembers) {
    for (auto second : secondMembers) {
      auto joined = Type::join(first, second);
      if (!joined)
        return Unimplemented;

      if ((*joined)->isAny())
        continue;

      result.push_back(*joined);
    }
  }

  if (result.empty())
    return TheAnyType;

  auto &ctx = result[0]->getASTContext();
  return ProtocolCompositionType::get(ctx, result, false)->getCanonicalType();
}

CanType TypeJoin::visitProtocolCompositionType(CanType second) {
  // The join of Any and a no-escape function doesn't exist; it isn't
  // Any. If it were Any, it would mean we would allow these functions
  // to escape through Any.
  if (second->isAny()) {
    auto *fnTy = First->getAs<AnyFunctionType>();
    if (fnTy && fnTy->getExtInfo().isNoEscape())
      return Nonexistent;

    return TheAnyType;
  }

  assert(First != second);

  // FIXME: Handle other types here.
  if (!First->isExistentialType())
    return Unimplemented;

  SmallVector<Type, 1> protocolType;
  ArrayRef<Type> firstMembers;
  if (First->is<ProtocolType>()) {
    protocolType.push_back(First);
    firstMembers = protocolType;
  } else {
    firstMembers = cast<ProtocolCompositionType>(First)->getMembers();
  }
  auto secondMembers = cast<ProtocolCompositionType>(second)->getMembers();

  return computeProtocolCompositionJoin(firstMembers, secondMembers);
}

CanType TypeJoin::visitProtocolType(CanType second) {
  assert(First != second);

  assert(!First->is<ProtocolCompositionType>() &&
         !second->is<ProtocolCompositionType>());

  // FIXME: Handle other types here.
  if (First->getKind() != second->getKind())
    return TheAnyType;

  auto *firstDecl =
    cast<ProtocolDecl>(First->getNominalOrBoundGenericNominal());

  auto *secondDecl =
    cast<ProtocolDecl>(second->getNominalOrBoundGenericNominal());

  if (firstDecl->getInheritedProtocols().empty() &&
      secondDecl->getInheritedProtocols().empty())
    return TheAnyType;

  if (firstDecl->inheritsFrom(secondDecl))
    return second;

  if (secondDecl->inheritsFrom(firstDecl))
    return First;

  // One isn't the supertype of the other, so instead, treat each as
  // if it's a protocol composition of its inherited members, and join
  // those.
  SmallVector<Type, 4> firstMembers;
  for (auto *decl : firstDecl->getInheritedProtocols())
    firstMembers.push_back(decl->getDeclaredInterfaceType());

  SmallVector<Type, 4> secondMembers;
  for (auto *decl : secondDecl->getInheritedProtocols())
    secondMembers.push_back(decl->getDeclaredInterfaceType());

  return computeProtocolCompositionJoin(firstMembers, secondMembers);
}

CanType TypeJoin::visitLValueType(CanType second) { return Unimplemented; }

CanType TypeJoin::visitInOutType(CanType second) { return Unimplemented; }

CanType TypeJoin::visitBuiltinType(CanType second) {
  assert(First != second);

  // BuiltinType with any non-equal type results in Any.
  return TheAnyType;
}

} // namespace

Optional<Type> Type::join(Type first, Type second) {
  assert(first && second && "Unexpected null type!");

  if (!first || !second)
    return None;

  auto join =
      TypeJoin::join(first->getCanonicalType(), second->getCanonicalType());
  if (!join)
    return None;

  return join;
}