Allow the use of unavailable types from unavailable code

This change permits the use of unavailable types in unavailable signatures.
It affects only usage of `available(*, unavailable)`.

This fixes a compilation error in swift-corelibs-foundation/Foundation/
URLSession/URLSessionConfiguration.swift where an unavailable property
was typed by an unavailable type.

lib/Sema/TypeCheckAvailability.cpp

@@ -1503,13 +1503,17 @@ static bool isInsideUnavailableDeclaration(SourceRange ReferenceRange,
 /// Returns true if the reference or any of its parents is an
 /// unconditional unavailable declaration for the same platform.
 static bool isInsideCompatibleUnavailableDeclaration(
-    SourceRange ReferenceRange, const DeclContext *ReferenceDC,
-    const AvailableAttr *attr) {
+    const ValueDecl *referencedD, SourceRange ReferenceRange,
+    const DeclContext *ReferenceDC, const AvailableAttr *attr) {
   if (!attr->isUnconditionallyUnavailable()) {
     return false;
   }
+
+  // Refuse calling unavailable functions from unavailable code,
+  // but allow the use of types.
   PlatformKind platform = attr->Platform;
-  if (platform == PlatformKind::none) {
+  if (platform == PlatformKind::none &&
+      !isa<TypeDecl>(referencedD)) {
     return false;
   }
 
@@ -2129,7 +2133,7 @@ bool swift::diagnoseExplicitUnavailability(
   // unavailability is OK -- the eventual caller can't call the
   // enclosing code in the same situations it wouldn't be able to
   // call this code.
-  if (isInsideCompatibleUnavailableDeclaration(R, DC, Attr)) {
+  if (isInsideCompatibleUnavailableDeclaration(D, R, DC, Attr)) {
     return false;
   }
 

test/attr/attr_availability.swift

@@ -1087,3 +1087,16 @@ func rdar46348825_deprecated() {}
 @available(swift, obsoleted: 4.0, obsoleted: 4.0)
 // expected-warning@-1 {{'obsoleted' argument has already been specified}}
 func rdar46348825_obsoleted() {}
+
+// Referencing unavailable types in signatures of unavailable functions should be accepted
+@available(*, unavailable)
+protocol UnavailableProto {
+}
+
+@available(*, unavailable)
+func unavailableFunc(_ arg: UnavailableProto) -> UnavailableProto {}
+
+@available(*, unavailable)
+struct S {
+  var a: UnavailableProto
+}