AST: App extension platforms always inherit availability from their parents.

include/swift/AST/PlatformKind.h

@@ -51,6 +51,12 @@ llvm::Optional<StringRef> closestCorrectedPlatformString(StringRef candidate);
 /// for emission in diagnostics (e.g., "macOS").
 StringRef prettyPlatformString(PlatformKind platform);
 
+/// Returns the base platform for an application-extension platform. For example
+/// `iOS` would be returned for `iOSApplicationExtension`. Returns `None` for
+/// platforms which are not application extension platforms.
+llvm::Optional<PlatformKind>
+basePlatformForExtensionPlatform(PlatformKind Platform);
+
 /// Returns whether the passed-in platform is active, given the language
 /// options. A platform is active if either it is the target platform or its
 /// AppExtension variant is the target platform. For example, OS X is

lib/AST/PlatformKind.cpp

@@ -83,14 +83,19 @@ swift::closestCorrectedPlatformString(StringRef candidate) {
   return (minDistance < distanceThreshold) ? result : llvm::None;
 }
 
-static bool isApplicationExtensionPlatform(PlatformKind Platform) {
+llvm::Optional<PlatformKind>
+swift::basePlatformForExtensionPlatform(PlatformKind Platform) {
   switch (Platform) {
   case PlatformKind::macOSApplicationExtension:
+    return PlatformKind::macOS;
   case PlatformKind::iOSApplicationExtension:
+    return PlatformKind::iOS;
   case PlatformKind::macCatalystApplicationExtension:
+    return PlatformKind::macCatalyst;
   case PlatformKind::tvOSApplicationExtension:
+    return PlatformKind::tvOS;
   case PlatformKind::watchOSApplicationExtension:
-    return true;
+    return PlatformKind::watchOS;
   case PlatformKind::macOS:
   case PlatformKind::iOS:
   case PlatformKind::macCatalyst:
@@ -99,11 +104,15 @@ static bool isApplicationExtensionPlatform(PlatformKind Platform) {
   case PlatformKind::OpenBSD:
   case PlatformKind::Windows:
   case PlatformKind::none:
-    return false;
+    return llvm::None;
   }
   llvm_unreachable("bad PlatformKind");
 }
 
+static bool isApplicationExtensionPlatform(PlatformKind Platform) {
+  return basePlatformForExtensionPlatform(Platform).has_value();
+}
+
 static bool isPlatformActiveForTarget(PlatformKind Platform,
                                       const llvm::Triple &Target,
                                       bool EnableAppExtensionRestrictions) {
@@ -187,20 +196,21 @@ PlatformKind swift::targetPlatform(const LangOptions &LangOpts) {
 
 bool swift::inheritsAvailabilityFromPlatform(PlatformKind Child,
                                              PlatformKind Parent) {
+  if (auto ChildPlatformBase = basePlatformForExtensionPlatform(Child)) {
+    if (Parent == ChildPlatformBase)
+      return true;
+  }
+
   if (Child == PlatformKind::macCatalyst && Parent == PlatformKind::iOS)
     return true;
 
   if (Child == PlatformKind::macCatalystApplicationExtension) {
     if (Parent == PlatformKind::iOS ||
-        Parent == PlatformKind::iOSApplicationExtension ||
-        Parent == PlatformKind::macCatalyst) {
+        Parent == PlatformKind::iOSApplicationExtension) {
       return true;
     }
   }
 
-  // Ideally we would have all ApplicationExtension platforms
-  // inherit from their non-extension platform.
-
   return false;
 }
 

test/attr/attr_availability_transitive_ios_appext.swift

@@ -8,7 +8,7 @@
 func ios() {} // expected-note 2{{'ios()' has been explicitly marked unavailable here}}
 
 @available(iOSApplicationExtension, unavailable)
-func ios_extension() {} // expected-note 2{{'ios_extension()' has been explicitly marked unavailable here}}
+func ios_extension() {} // expected-note {{'ios_extension()' has been explicitly marked unavailable here}}
 
 func call_ios_extension() {
     ios_extension() // expected-error {{'ios_extension()' is unavailable}}
@@ -19,7 +19,7 @@ func call_ios() {
 
 @available(iOS, unavailable)
 func ios_call_ios_extension() {
-    ios_extension() // expected-error {{'ios_extension()' is unavailable}}
+    ios_extension()
 }
 
 @available(iOS, unavailable)

test/attr/attr_availability_transitive_osx_appext.swift

@@ -7,7 +7,7 @@
 func osx() {} // expected-note 3{{'osx()' has been explicitly marked unavailable here}}
 
 @available(OSXApplicationExtension, unavailable)
-func osx_extension() {} // expected-note 3{{'osx_extension()' has been explicitly marked unavailable here}}
+func osx_extension() {} // expected-note {{'osx_extension()' has been explicitly marked unavailable here}}
 
 func call_osx_extension() {
     osx_extension() // expected-error {{'osx_extension()' is unavailable}}
@@ -18,7 +18,7 @@ func call_osx() {
 
 @available(OSX, unavailable)
 func osx_call_osx_extension() {
-    osx_extension() // expected-error {{'osx_extension()' is unavailable}}
+    osx_extension()
 }
 
 @available(OSX, unavailable)
@@ -46,7 +46,7 @@ extension NotOnOSX {
   }
 
   func osx_call_osx_extension() {
-    osx_extension() // expected-error {{'osx_extension()' is unavailable in application extensions for macOS}}
+    osx_extension()
   }
 }