Tweak missing impl diagnostic for async members

The diagnostic mentions both possible names, but only provides a stub for the `async` variant.

Author: beccadax

include/swift/AST/DiagnosticsSema.def

@@ -1990,6 +1990,9 @@ NOTE(objc_implementation_missing_impls_fixit,none,
 NOTE(objc_implementation_missing_impl,none,
      "missing %kind0",
      (ValueDecl *))
+NOTE(objc_implementation_missing_impl_either,none,
+     "missing %kind0 or %1",
+     (ValueDecl *, ValueDecl *))
 
 ERROR(objc_implementation_class_or_instance_mismatch,none,
       "%kind0 does not match %kindonly1 declared in header",

lib/Sema/TypeCheckDeclObjC.cpp

@@ -4060,7 +4060,15 @@ class ObjCImplementationChecker {
       }
 
       numEmitted += 1;
-      diagnose(ext, diag::objc_implementation_missing_impl, req);
+
+      // Emit different diagnostic if there's an async alternative.
+      if (auto asyncAlternative = getAsyncAlternative(req)) {
+        diagnose(ext, diag::objc_implementation_missing_impl_either,
+                 asyncAlternative, req);
+        req = asyncAlternative;
+      } else {
+        diagnose(ext, diag::objc_implementation_missing_impl, req);
+      }
 
       // Append stub for this requirement into eventual fix-it.
       swift::printRequirementStub(req, ext, ext->getSelfInterfaceType(),

test/decl/ext/Inputs/objc_implementation.h

@@ -123,6 +123,7 @@
 - (void)doSomethingAsynchronousWithCompletionHandler:(void (^ _Nonnull)(id _Nullable result, NSError * _Nullable error))completionHandler;
 - (void)doSomethingElseAsynchronousWithCompletionHandler:(void (^ _Nullable)(id _Nonnull result))completionHandler;
 - (void)doSomethingFunAndAsynchronousWithCompletionHandler:(void (^ _Nonnull)(id _Nullable result, NSError * _Nullable error))completionHandler;
+- (void)doSomethingMissingAndAsynchronousWithCompletionHandler:(void (^ _Nonnull)(id _Nullable result, NSError * _Nullable error))completionHandler;
 
 - (void)doSomethingOverloadedWithCompletionHandler:(void (^ _Nonnull)())completionHandler;
 - (void)doSomethingOverloaded __attribute__((__swift_attr__("@_unavailableFromAsync(message: \"Use async doSomethingOverloaded instead.\")")));

test/decl/ext/objc_implementation.swift

@@ -372,6 +372,10 @@ protocol EmptySwiftProto {}
 }
 
 @objc(Effects) @implementation extension ObjCClass {
+  // expected-error@-1 {{extension for category 'Effects' does not provide all required implementations}}
+  // expected-note@-2 {{missing instance method 'doSomethingMissingAndAsynchronous()' or 'doSomethingMissingAndAsynchronous(completionHandler:)'}}
+  // expected-note@-3 {{add stub for missing '@implementation' requirement}} {{53-53=\n    @objc(doSomethingMissingAndAsynchronousWithCompletionHandler:)\n    open func doSomethingMissingAndAsynchronous() async throws -> Any {\n        <#code#>\n    \}\n}}
+
   @available(SwiftStdlib 5.1, *)
   public func doSomethingAsynchronous() async throws -> Any {
     return self

test/decl/ext/objc_implementation_early_adopter.swift

@@ -372,6 +372,10 @@ protocol EmptySwiftProto {}
 }
 
 @_objcImplementation(Effects) extension ObjCClass {
+  // expected-warning@-1 {{extension for category 'Effects' does not provide all required implementations; this will become an error after adopting '@implementation'}}
+  // expected-note@-2 {{missing instance method 'doSomethingMissingAndAsynchronous()' or 'doSomethingMissingAndAsynchronous(completionHandler:)'}}
+  // expected-note@-3 {{add stub for missing '@implementation' requirement}} {{52-52=\n    @objc(doSomethingMissingAndAsynchronousWithCompletionHandler:)\n    open func doSomethingMissingAndAsynchronous() async throws -> Any {\n        <#code#>\n    \}\n}}
+
   @available(SwiftStdlib 5.1, *)
   public func doSomethingAsynchronous() async throws -> Any {
     return self