Allow same-selector async/completion requirements

An @objc protocol can now explicitly declare both the `async` and completion handler signatures of a method as long as the completion handler one is marked with `@available(*, renamed:)`.

This is not yet handled correctly in PrintAsClang.

Author: beccadax

lib/AST/NameLookup.cpp

@@ -1699,6 +1699,16 @@ NominalTypeDecl::lookupDirect(ObjCSelector selector, bool isInstance) {
   return stored.Methods;
 }
 
+/// Is the new method an async alternative of any existing method, or vice
+/// versa?
+static bool isAnAsyncAlternative(AbstractFunctionDecl *newDecl,
+                                 llvm::TinyPtrVector<AbstractFunctionDecl *> &vec) {
+  return llvm::any_of(vec, [&](AbstractFunctionDecl *oldDecl) {
+    return newDecl->getAsyncAlternative(/*isKnownObjC=*/true) == oldDecl
+              || oldDecl->getAsyncAlternative(/*isKnownObjC=*/true) == newDecl;
+  });
+}
+
 void NominalTypeDecl::recordObjCMethod(AbstractFunctionDecl *method,
                                        ObjCSelector selector) {
   if (!ObjCMethodLookup && !createObjCMethodLookup())
@@ -1716,7 +1726,7 @@ void NominalTypeDecl::recordObjCMethod(AbstractFunctionDecl *method,
   if (auto *sf = method->getParentSourceFile()) {
     if (vec.empty()) {
       sf->ObjCMethodList.push_back(method);
-    } else {
+    } else if (!isa<ProtocolDecl>(this) || !isAnAsyncAlternative(method, vec)) {
       // We have a conflict.
       sf->ObjCMethodConflicts.insert({ this, selector, isInstanceMethod });
     }

test/ClangImporter/objc_async_conformance.swift

@@ -63,6 +63,33 @@ class SelectorOK2 : NSObject, RequiredObserverOnlyCompletion {
   @nonobjc func hello(_ completion : @escaping (Bool) -> Void) -> Void { completion(true) }
 }
 
+// can declare an @objc protocol with both selectors...
+@objc protocol SelectorBothAsyncProto {
+  @objc(helloWithCompletion:)
+  func hello() async -> Bool
+
+  @available(*, renamed: "hello()")
+  @objc(helloWithCompletion:)
+  func hello(completion: @escaping (Bool) -> Void)
+}
+
+// and conform by implementing either one...
+class SelectorBothAsync1: NSObject, SelectorBothAsyncProto {
+  func hello() async -> Bool { true }
+}
+class SelectorBothAsync2: NSObject, SelectorBothAsyncProto {
+  func hello(completion: @escaping (Bool) -> Void) { completion(true) }
+}
+
+// but not without declaring the async alternative.
+@objc protocol BadSelectorBothAsyncProto {
+  @objc(helloWithCompletion:)
+  func hello() async -> Bool // expected-note {{method 'hello()' declared here}}
+
+  @objc(helloWithCompletion:)
+  func hello(completion: @escaping (Bool) -> Void) // expected-warning {{method 'hello(completion:)' with Objective-C selector 'helloWithCompletion:' conflicts with method 'hello()' with the same Objective-C selector; this is an error in Swift 6}}
+}
+
 // additional coverage for situation like C4, where the method names don't
 // clash on the ObjC side, but they do on Swift side, BUT their ObjC selectors
 // differ, so it's OK.