Mess with the way we do canonicalization for ProtocolCompositionTypes in a way

that doesn't violate canonical type invariants.  It's still a little messy,
but it's consistent.  Fixes <rdar://problem/11886926>.



Swift SVN r2368

include/swift/AST/Types.h

@@ -991,23 +991,18 @@ private:
 /// protocol, then the canonical type is that protocol type. Otherwise, it is
 /// a composition of the protocols in that list.
 class ProtocolCompositionType : public TypeBase, public llvm::FoldingSetNode {
-  ASTContext &Ctx; // Note: Needed for canonicalization of the empty case.
   SourceLoc FirstLoc; // FIXME: Egregious hack due to lack of TypeLocs
   ArrayRef<Type> Protocols;
   
 public:
   /// \brief Retrieve an instance of a protocol composition type with the
   /// given set of protocols.
-  static ProtocolCompositionType *
-  get(ASTContext &C, SourceLoc FirstLoc, ArrayRef<Type> Protocols);
+  static Type get(ASTContext &C, SourceLoc FirstLoc, ArrayRef<Type> Protocols);
   
   /// \brief Retrieve the source location where this type was first uttered.
   /// FIXME: This is a hackaround for the lack of TypeLocs.
   SourceLoc getFirstLoc() const { return FirstLoc; }
   
-  /// \brief Retrieve the AST context of this type.
-  ASTContext &getASTContext() const { return Ctx; }
-  
   /// \brief Retrieve the set of protocols composed to create this type.
   ArrayRef<Type> getProtocols() const { return Protocols; }
   
@@ -1025,11 +1020,14 @@ public:
   }
   
 private:
-  ProtocolCompositionType(ASTContext &Ctx, SourceLoc FirstLoc,
+  static ProtocolCompositionType *build(ASTContext &C, SourceLoc FirstLoc,
+                                        ArrayRef<Type> Protocols);
+
+  ProtocolCompositionType(ASTContext *Ctx, SourceLoc FirstLoc,
                           ArrayRef<Type> Protocols)
-    : TypeBase(TypeKind::ProtocolComposition, /*Context=*/nullptr,
+    : TypeBase(TypeKind::ProtocolComposition, /*Context=*/Ctx,
                /*Unresolved=*/false),
-      Ctx(Ctx), FirstLoc(FirstLoc), Protocols(Protocols) { }
+      FirstLoc(FirstLoc), Protocols(Protocols) { }
 };
 
 /// LValueType - An l-value is a handle to a physical object.  The