AST: Ensure isPlatformActiveForTarget() returns the correct result for application extensions on every platform

lib/AST/PlatformKind.cpp

@@ -58,18 +58,37 @@ Optional<PlatformKind> swift::platformFromString(StringRef Name) {
       .Default(Optional<PlatformKind>());
 }
 
+static bool isApplicationExtensionPlatform(PlatformKind Platform) {
+  switch (Platform) {
+  case PlatformKind::macOSApplicationExtension:
+  case PlatformKind::iOSApplicationExtension:
+  case PlatformKind::macCatalystApplicationExtension:
+  case PlatformKind::tvOSApplicationExtension:
+  case PlatformKind::watchOSApplicationExtension:
+    return true;
+  case PlatformKind::macOS:
+  case PlatformKind::iOS:
+  case PlatformKind::macCatalyst:
+  case PlatformKind::tvOS:
+  case PlatformKind::watchOS:
+  case PlatformKind::OpenBSD:
+  case PlatformKind::Windows:
+  case PlatformKind::none:
+    return false;
+  }
+  llvm_unreachable("bad PlatformKind");
+}
+
 static bool isPlatformActiveForTarget(PlatformKind Platform,
                                       const llvm::Triple &Target,
                                       bool EnableAppExtensionRestrictions) {
   if (Platform == PlatformKind::none)
     return true;
-
-  if (Platform == PlatformKind::macOSApplicationExtension ||
-      Platform == PlatformKind::iOSApplicationExtension ||
-      Platform == PlatformKind::macCatalystApplicationExtension)
-    if (!EnableAppExtensionRestrictions)
-      return false;
-
+
+  if (!EnableAppExtensionRestrictions &&
+      isApplicationExtensionPlatform(Platform))
+    return false;
+
   // FIXME: This is an awful way to get the current OS.
   switch (Platform) {
     case PlatformKind::macOS:

test/attr/attr_availability_appext_unavailable.swift

@@ -0,0 +1,19 @@
+// Verify that declarations unavailable to application extensions are diagnosed
+// as unavailable when compiling with `-application-extension`
+// RUN: %target-typecheck-verify-swift -application-extension
+
+// Remove `-application-extension` and verify no errors are emitted.
+// RUN: %target-swift-frontend -typecheck %s
+
+// REQUIRES: OS=macosx || OS=ios || OS=tvos || OS=watchos
+
+@available(macOSApplicationExtension, unavailable)
+@available(macCatalystApplicationExtension, unavailable)
+@available(iOSApplicationExtension, unavailable)
+@available(tvOSApplicationExtension, unavailable)
+@available(watchOSApplicationExtension, unavailable)
+func unavailableToExtensions() {} // expected-note {{'unavailableToExtensions()' has been explicitly marked unavailable here}}
+
+func alwaysAvailable() {
+  unavailableToExtensions() // expected-error {{'unavailableToExtensions()' is unavailable in application extensions for}}
+}