AST: Update ProtocolCompositionType::requiresClass() for subclass existentials

Whether a protocol composition requires a class is no longer
solely a property of what protocols it contains; now, a
protocol composition consisting of a non-class constrained
protocol and a superclass should return true from requiresClass().

This is done by looking at the ExistentialLayout, instead of
walking the members directly.

Author: slavapestov

include/swift/AST/Types.h

@@ -3605,7 +3605,7 @@ class ProtocolType : public NominalType, public llvm::FoldingSetNode {
   }
 
   /// True if only classes may conform to the protocol.
-  bool requiresClass() const;
+  bool requiresClass();
 
   // Implement isa/cast/dyncast/etc.
   static bool classof(const TypeBase *T) {
@@ -3678,7 +3678,7 @@ class ProtocolCompositionType : public TypeBase, public llvm::FoldingSetNode {
   static void Profile(llvm::FoldingSetNodeID &ID, ArrayRef<Type> Protocols);
 
   /// True if one or more of the protocols is class.
-  bool requiresClass() const;
+  bool requiresClass();
 
   // Implement isa/cast/dyncast/etc.
   static bool classof(const TypeBase *T) {

lib/AST/Decl.cpp

@@ -2727,16 +2727,16 @@ ProtocolDecl::getInheritedProtocols() const {
   // FIXME: Gather inherited protocols from the "inherited" list.
   // We shouldn't need this, but it shows up in recursive invocations.
   if (!isRequirementSignatureComputed()) {
+    SmallPtrSet<ProtocolDecl *, 4> known;
     for (auto inherited : getInherited()) {
-      SmallPtrSet<ProtocolDecl *, 4> known;
       if (auto type = inherited.getType()) {
-        if (type->isExistentialType()) {
-          SmallVector<ProtocolDecl *, 4> protocols;
-          type->getExistentialTypeProtocols(protocols);
-          for (auto proto : protocols) {
-            if (known.insert(proto).second)
-              result.push_back(proto);
-          }
+        // Only protocols can appear in the inheritance clause
+        // of a protocol -- anything else should get diagnosed
+        // elsewhere.
+        if (auto *protoTy = type->getAs<ProtocolType>()) {
+          auto *protoDecl = protoTy->getDecl();
+          if (known.insert(protoDecl).second)
+            result.push_back(protoDecl);
         }
       }
     }

lib/AST/Type.cpp

@@ -2744,21 +2744,12 @@ void ProtocolCompositionType::Profile(llvm::FoldingSetNodeID &ID,
     ID.AddPointer(P.getPointer());
 }
 
-bool ProtocolType::requiresClass() const {
+bool ProtocolType::requiresClass() {
   return getDecl()->requiresClass();
 }
 
-bool ProtocolCompositionType::requiresClass() const {
-  for (Type t : getProtocols()) {
-    if (const ProtocolType *proto = t->getAs<ProtocolType>()) {
-      if (proto->requiresClass())
-        return true;
-    } else {
-      if (t->castTo<ProtocolCompositionType>()->requiresClass())
-        return true;
-    }
-  }
-  return false;
+bool ProtocolCompositionType::requiresClass() {
+  return getExistentialLayout().requiresClass;
 }
 
 Type ProtocolCompositionType::get(const ASTContext &C,