Sema: Handle protocol compositions containing type variables in matchTypes()

We would previously fail to match something like (C<$T1> & P) against
(C<Int> & P) when the constraint kind was <= Subtype, because we would
fall back to a type equality test in that case.

However, this was only valid for protocol compositions without superclass
constraints since the superclass could contain a type variable on one
side of the constraint.

Fix this by adding support for protocol composition types to
matchDeepEqualityTypes().

lib/Sema/CSSimplify.cpp

@@ -1640,6 +1640,43 @@ ConstraintSystem::matchDeepEqualityTypes(Type type1, Type type2,
     return matchDeepTypeArguments(*this, subflags, args1, args2, locator);
   }
 
+  // Handle protocol compositions.
+  if (auto existential1 = type1->getAs<ProtocolCompositionType>()) {
+    if (auto existential2 = type2->getAs<ProtocolCompositionType>()) {
+      auto layout1 = existential1->getExistentialLayout();
+      auto layout2 = existential2->getExistentialLayout();
+
+      // Explicit AnyObject and protocols must match exactly.
+      if (layout1.hasExplicitAnyObject != layout2.hasExplicitAnyObject)
+        return getTypeMatchFailure(locator);
+
+      if (layout1.getProtocols().size() != layout2.getProtocols().size())
+        return getTypeMatchFailure(locator);
+
+      for (unsigned i: indices(layout1.getProtocols())) {
+        if (!layout1.getProtocols()[i]->isEqual(layout2.getProtocols()[i]))
+          return getTypeMatchFailure(locator);
+      }
+
+      // This is the only interesting case. We might have type variables
+      // on either side of the superclass constraint, so make sure we
+      // recursively call matchTypes() here.
+      if (layout1.explicitSuperclass || layout2.explicitSuperclass) {
+        if (!layout1.explicitSuperclass || !layout2.explicitSuperclass)
+          return getTypeMatchFailure(locator);
+
+        auto result = matchTypes(layout1.explicitSuperclass,
+                                 layout2.explicitSuperclass,
+                                 ConstraintKind::Bind, subflags,
+                                 locator.withPathElement(
+                                   ConstraintLocator::ExistentialSuperclassType));
+        if (result.isFailure())
+          return result;
+      }
+
+      return getTypeMatchSuccess();
+    }
+  }
   // Handle nominal types that are not directly generic.
   if (auto nominal1 = type1->getAs<NominalType>()) {
     auto nominal2 = type2->castTo<NominalType>();
@@ -2631,7 +2668,22 @@ ConstraintSystem::matchTypes(Type type1, Type type2, ConstraintKind kind,
       llvm_unreachable("Polymorphic function type should have been opened");
 
     case TypeKind::ProtocolComposition:
-      // Existential types handled below.
+      switch (kind) {
+      case ConstraintKind::Equal:
+      case ConstraintKind::Bind:
+      case ConstraintKind::BindParam:
+        // If we are matching types for equality, we might still have
+        // type variables inside the protocol composition's superclass
+        // constraint.
+        conversionsOrFixes.push_back(ConversionRestrictionKind::DeepEquality);
+        break;
+
+      default:
+        // Subtype constraints where the RHS is an existential type are
+        // handled below.
+        break;
+      }
+
       break;
 
     case TypeKind::LValue:

lib/Sema/ConstraintLocator.cpp

@@ -63,6 +63,7 @@ void ConstraintLocator::Profile(llvm::FoldingSetNodeID &id, Expr *anchor,
     case RValueAdjustment:
     case ClosureResult:
     case ParentType:
+    case ExistentialSuperclassType:
     case InstanceType:
     case SequenceElementType:
     case AutoclosureResult:
@@ -309,6 +310,10 @@ void ConstraintLocator::dump(SourceManager *sm, raw_ostream &out) {
       out << "parent type";
       break;
 
+    case ExistentialSuperclassType:
+      out << "existential superclass type";
+      break;
+
     case LValueConversion:
       out << "@lvalue-to-inout conversion";
       break;

lib/Sema/ConstraintLocator.h

@@ -96,6 +96,8 @@ public:
     ClosureResult,
     /// The parent of a nested type.
     ParentType,
+    /// The superclass of a protocol existential type.
+    ExistentialSuperclassType,
     /// The instance of a metatype type.
     InstanceType,
     /// The element type of a sequence in a for ... in ... loop.
@@ -157,6 +159,7 @@ public:
     case ClosureResult:
     case ParentType:
     case InstanceType:
+    case ExistentialSuperclassType:
     case SequenceElementType:
     case AutoclosureResult:
     case Requirement:
@@ -215,6 +218,7 @@ public:
     case MemberRefBase:
     case UnresolvedMember:
     case ParentType:
+    case ExistentialSuperclassType:
     case LValueConversion:
     case RValueAdjustment:
     case SubscriptMember:

test/type/subclass_composition.swift

@@ -530,3 +530,21 @@ struct DerivedBox<T : Derived> {}
 
 func takesBoxWithP3(_: DerivedBox<Derived & P3>) {}
 // expected-error@-1 {{'DerivedBox' requires that 'Derived & P3' inherit from 'Derived'}}
+
+// A bit of a tricky setup -- the real problem is that matchTypes() did the
+// wrong thing when solving a Bind constraint where both sides were protocol
+// compositions, but one of them had a superclass constraint containing type
+// variables. We were checking type equality in this case, which is not
+// correct; we have to do a 'deep equality' check, recursively matching the
+// superclass types.
+struct Generic<T> {
+  var _x: (Base<T> & P2)!
+
+  var x: (Base<T> & P2)? {
+    get { return _x }
+    set { _x = newValue }
+    _modify {
+      yield &_x
+    }
+  }
+}