Allow @available(swift, ...) nested within @available(macOS, ...)

Previously, we treated this as an attempt to widen the availability
of a member beyond its context, but it's really a different axis of
availability.

Also, warn about using @available on extensions without an OS, which
we just completely ignore right now.

rdar://problem/32632327

include/swift/AST/DiagnosticsSema.def

@@ -3647,6 +3647,10 @@ WARNING(availability_query_useless_enclosing_scope, none,
 NOTE(availability_query_useless_enclosing_scope_here, none,
      "enclosing scope here", ())
 
+WARNING(availability_extension_platform_agnostic, none,
+        "'@available' without an OS is ignored on extensions; "
+        "apply the attribute to each member instead", ())
+
 ERROR(availability_global_script_no_potential,
       none, "global variable cannot be marked potentially "
       "unavailable with '@available' in script mode", ())

lib/Sema/TypeCheckAttr.cpp

@@ -70,7 +70,6 @@ public:
   bool visitDeclAttribute(DeclAttribute *A) = delete;
 
 #define IGNORED_ATTR(X) void visit##X##Attr(X##Attr *) {}
-  IGNORED_ATTR(Available)
   IGNORED_ATTR(CDecl)
   IGNORED_ATTR(ClangImporterSynthesizedType)
   IGNORED_ATTR(Convenience)
@@ -115,6 +114,14 @@ public:
   IGNORED_ATTR(WeakLinked)
 #undef IGNORED_ATTR
 
+  void visitAvailableAttr(AvailableAttr *attr) {
+    if (!isa<ExtensionDecl>(D))
+      return;
+    if (attr->hasPlatform())
+      return;
+    diagnoseAndRemoveAttr(attr, diag::availability_extension_platform_agnostic);
+  }
+
   // @noreturn has been replaced with a 'Never' return type.
   void visitNoReturnAttr(NoReturnAttr *attr) {
     if (auto FD = dyn_cast<FuncDecl>(D)) {
@@ -1073,8 +1080,10 @@ void AttributeChecker::visitAvailableAttr(AvailableAttr *attr) {
   if (TC.getLangOpts().DisableAvailabilityChecking)
     return;
 
-  if (!attr->isActivePlatform(TC.Context) || !attr->Introduced.hasValue())
+  if (!attr->hasPlatform() || !attr->isActivePlatform(TC.Context) ||
+      !attr->Introduced.hasValue()) {
     return;
+  }
 
   SourceLoc attrLoc = attr->getLocation();
 

test/attr/attr_availability.swift

@@ -807,3 +807,9 @@ rdar32526620_2(a: 42, b: .bar)
 func rdar32526620_3(a: Int, b: E_32526620, c: String) {} // expected-note {{here}}
 rdar32526620_3(a: 42, b: .bar, c: "question")
 // expected-error@-1 {{'rdar32526620_3(a:b:c:)' has been replaced by instance method 'E_32526620.set(a:c:)'}} {{1-15=E_32526620.bar.set}} {{23-32=}}
+
+@available(*, unavailable) // expected-warning {{'@available' without an OS is ignored on extensions; apply the attribute to each member instead}} {{1-28=}}
+extension DummyType {}
+
+@available(*, deprecated) // expected-warning {{'@available' without an OS is ignored on extensions; apply the attribute to each member instead}} {{1-27=}}
+extension DummyType {}

test/attr/attr_availability_osx.swift

@@ -71,3 +71,36 @@ doSomethingDeprecatedOnOSX() // expected-warning{{'doSomethingDeprecatedOnOSX()'
 func doSomethingDeprecatedOniOS() { }
 
 doSomethingDeprecatedOniOS() // okay
+
+
+struct TestStruct {}
+
+@available(macOS 10.10, *)
+extension TestStruct { // expected-note {{enclosing scope here}}
+  @available(swift 400)
+  func doTheThing() {} // expected-note {{'doTheThing()' was introduced in Swift 400}}
+
+  @available(macOS 10.9, *) // expected-error {{declaration cannot be more available than enclosing scope}}
+  @available(swift 400)
+  func doAnotherThing() {} // expected-note {{'doAnotherThing()' was introduced in Swift 400}}
+
+  @available(macOS 10.12, *)
+  @available(swift 400)
+  func doThirdThing() {} // expected-note {{'doThirdThing()' was introduced in Swift 400}}
+
+  @available(macOS 10.12, *)
+  @available(swift 1)
+  func doFourthThing() {}
+
+  @available(*, deprecated)
+  func doDeprecatedThing() {}
+}
+
+@available(macOS 10.11, *)
+func testMemberAvailability() {
+  TestStruct().doTheThing() // expected-error {{'doTheThing()' is unavailable}}
+  TestStruct().doAnotherThing() // expected-error {{'doAnotherThing()' is unavailable}}
+  TestStruct().doThirdThing() // expected-error {{'doThirdThing()' is unavailable}}
+  TestStruct().doFourthThing() // expected-error {{'doFourthThing()' is only available on OS X 10.12 or newer}} expected-note {{'if #available'}}
+  TestStruct().doDeprecatedThing() // expected-warning {{'doDeprecatedThing()' is deprecated}}
+}

test/attr/attr_availability_swift.swift

@@ -20,3 +20,30 @@ func baz() {
 @available(swift, introduced: 3.0.1, obsoleted: 3.0.2, message: "tiny bug")
 func bug() {
 }
+
+struct TestStruct {}
+
+@available(macOS 10.11, *)
+extension TestStruct {
+  @available(swift 400)
+  func doTheThing() {} // expected-note {{'doTheThing()' was introduced in Swift 400}}
+}
+
+@available(swift 400) // expected-warning {{'@available' without an OS is ignored on extensions; apply the attribute to each member instead}} {{1-23=}}
+extension TestStruct {
+  func doAnotherThing() {}
+}
+
+@available(macOS 10.11, *)
+func testMemberAvailability() {
+  TestStruct().doTheThing() // expected-error {{'doTheThing()' is unavailable}}
+  TestStruct().doAnotherThing() // okay (for now)
+}
+
+@available(swift 400) // expected-warning {{'@available' without an OS is ignored on extensions; apply the attribute to each member instead}} {{1-23=}}
+@available(macOS 10.11, *)
+extension TestStruct {}
+
+@available(macOS 10.11, *)
+@available(swift 400) // expected-warning {{'@available' without an OS is ignored on extensions; apply the attribute to each member instead}} {{1-23=}}
+extension TestStruct {}