ProtocolCompositionType: Don't build compositions with a single member and no layout constraint

Author: AnthonyLatsis

lib/AST/ASTContext.cpp

@@ -3210,6 +3210,8 @@ ClassType *ClassType::get(ClassDecl *D, Type Parent, const ASTContext &C) {
 ProtocolCompositionType *
 ProtocolCompositionType::build(const ASTContext &C, ArrayRef<Type> Members,
                                bool HasExplicitAnyObject) {
+  assert(Members.size() != 1 || HasExplicitAnyObject);
+
   // Check to see if we've already seen this protocol composition before.
   void *InsertPos = nullptr;
   llvm::FoldingSetNodeID ID;

lib/AST/Type.cpp

@@ -3712,6 +3712,16 @@ bool ProtocolCompositionType::requiresClass() {
 Type ProtocolCompositionType::get(const ASTContext &C,
                                   ArrayRef<Type> Members,
                                   bool HasExplicitAnyObject) {
+  // Fast path for 'AnyObject' and 'Any'.
+  if (Members.empty()) {
+    return build(C, Members, HasExplicitAnyObject);
+  }
+
+  // If there's a single member and no layout constraint, return that type.
+  if (Members.size() == 1 && !HasExplicitAnyObject) {
+    return Members.front();
+  }
+
   for (Type t : Members) {
     if (!t->isCanonical())
       return build(C, Members, HasExplicitAnyObject);
@@ -3730,11 +3740,6 @@ Type ProtocolCompositionType::get(const ASTContext &C,
   if (Superclass)
     HasExplicitAnyObject = false;
 
-  // If one protocol remains with no further constraints, its nominal
-  // type is the canonical type.
-  if (Protocols.size() == 1 && !Superclass && !HasExplicitAnyObject)
-    return Protocols.front()->getDeclaredInterfaceType();
-
   // Form the set of canonical protocol types from the protocol
   // declarations, and use that to build the canonical composition type.
   SmallVector<Type, 4> CanTypes;
@@ -3744,6 +3749,10 @@ Type ProtocolCompositionType::get(const ASTContext &C,
       Protocols, std::back_inserter(CanTypes),
       [](ProtocolDecl *Proto) { return Proto->getDeclaredInterfaceType(); });
 
+  // If one member remains and no layout constraint, return that type.
+  if (CanTypes.size() == 1 && !HasExplicitAnyObject)
+    return CanTypes.front();
+
   // TODO: Canonicalize away HasExplicitAnyObject if it is implied
   // by one of our member protocols.
   return build(C, CanTypes, HasExplicitAnyObject);

test/Constraints/generics.swift

@@ -358,8 +358,8 @@ func testFixItClassBound() {
   let y1: String = y // expected-error {{cannot convert value of type 'ClassBound2<X>' to specified type 'String'}}
 
   // ...but not in types.
-  let z1: ClassBound // expected-error {{reference to generic type 'ClassBound' requires arguments in <...>}} {{21-21=<X>}}
-  let z2: ClassBound2 // expected-error {{reference to generic type 'ClassBound2' requires arguments in <...>}} {{22-22=<X>}}
+  let z1: ClassBound // expected-error {{reference to generic type 'ClassBound' requires arguments in <...>}} {{21-21=<<#Foo: X#>>}}
+  let z2: ClassBound2 // expected-error {{reference to generic type 'ClassBound2' requires arguments in <...>}} {{22-22=<<#Foo: X#>>}}
 }
 
 func testFixItCasting(x: Any) {