[Property delegates] Use $$foo for the backing storage and make it private.

When the property delegate type overrides the delegate value by providing
a delegateValue property, name the backing storage $$foo and make it private.

Author: DougGregor

lib/Sema/CodeSynthesis.cpp

@@ -1535,7 +1535,7 @@ PropertyDelegateBackingPropertyInfoRequest::evaluate(Evaluator &evaluator,
   ASTContext &ctx = var->getASTContext();
   SmallString<64> nameBuf;
   if (delegateInfo.delegateValueVar)
-    nameBuf = "$__delegate_storage_$_";
+    nameBuf = "$$";
   else
     nameBuf = "$";
   nameBuf += var->getName().str();
@@ -1596,14 +1596,19 @@ PropertyDelegateBackingPropertyInfoRequest::evaluate(Evaluator &evaluator,
   if (dc->getSelfClassDecl())
     makeFinal(ctx, backingVar);
 
+  // When there is a `delegateValue`, lower the access of the
+  AccessLevel defaultAccess =
+      delegateInfo.delegateValueVar ? AccessLevel::Private
+                                    : AccessLevel::Internal;
+
   // Determine the access level for the backing property.
   AccessLevel access =
-      std::min(AccessLevel::Internal, var->getFormalAccess());
+      std::min(defaultAccess, var->getFormalAccess());
   backingVar->overwriteAccess(access);
 
   // Determine setter access.
   AccessLevel setterAccess =
-      std::min(AccessLevel::Internal, var->getSetterFormalAccess());
+      std::min(defaultAccess, var->getSetterFormalAccess());
   backingVar->overwriteSetterAccess(setterAccess);
 
   // If there is a storage delegate property (delegateVar) in the delegate,

test/decl/var/property_delegates.swift

@@ -627,7 +627,11 @@ extension Wrapper {
 }
 
 struct TestStorageRef {
-  @WrapperWithStorageRef var x: Int
+  @WrapperWithStorageRef var x: Int // expected-note{{'$$x' declared here}}
+
+  init(x: Int) {
+    self.$$x = WrapperWithStorageRef(value: x)
+  }
 
   mutating func test() {
     let _: Wrapper = $x
@@ -642,6 +646,7 @@ struct TestStorageRef {
 
 func testStorageRef(tsr: TestStorageRef) {
   let _: Wrapper = tsr.$x
+  _ = tsr.$$x // expected-error{{'$$x' is inaccessible due to 'private' protection level}}
 }
 
 // ---------------------------------------------------------------------------