Revert "'@available' without an OS is ignored on extensions" warning

This is still useful for the feature where deprecated declarations are
allowed to be used within a context that's marked deprecated. There's
probably a better balance to be found here, because marking the
extension deprecated still does not mark all the members deprecated,
but for now it still has an effect, and we shouldn't produce a warning
for legitimately making use of that effect.

The warning was added in f21d9f3, mostly as an opportunity "fix".

https://bugs.swift.org/browse/SR-7577

Author: jrose-apple

include/swift/AST/DiagnosticsSema.def

@@ -3758,10 +3758,6 @@ 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

@@ -70,6 +70,7 @@ class AttributeEarlyChecker : public AttributeVisitor<AttributeEarlyChecker> {
   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)
@@ -115,14 +116,6 @@ class AttributeEarlyChecker : public AttributeVisitor<AttributeEarlyChecker> {
   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)) {

test/attr/attr_availability.swift

@@ -809,13 +809,6 @@ 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 // expected-error@-1 {{'rdar32526620_3(a:b:c:)' has been replaced by instance method 'E_32526620.set(a:c:)'}} {{1-15=E_32526620.bar.set}} {{23-32=}}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 {}
-
-
 var deprecatedGetter: Int {
   @available(*, deprecated) get { return 0 }
   set {}

test/attr/attr_availability_swift.swift

@@ -29,7 +29,7 @@ extension TestStruct {
   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=}}
+@available(swift 400) // FIXME: This has no effect and should be complained about.
 extension TestStruct {
   func doAnotherThing() {}
 }
@@ -40,10 +40,10 @@ func testMemberAvailability() {
   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(swift 400) // FIXME: This has no effect and should be complained about.
 @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=}}
+@available(swift 400) // FIXME: This has no effect and should be complained about.
 extension TestStruct {}