Undo behavior change for final overrides

A previous change made it so that an objcImpl extension could declare a `final` or `static` member which overrode an `@objc` superclass method, or witnessed an `@objc` protocol requirement, without itself being `@objc`. Prevent that from happening by allowing processing of `final` members to continue to later codepaths, rather than returning early.

Fixes rdar://136113393.

Author: beccadax

lib/Sema/TypeCheckDeclObjC.cpp

@@ -1519,11 +1519,9 @@ static std::optional<ObjCReason> shouldMarkAsObjC(const ValueDecl *VD,
   ObjCImplementationAttr *attrToWarnOfObjCImplBehaviorChange = nullptr;
 
   if (isMemberOfObjCImplementationExtension(VD)) {
-    // A `final` member of an @objc @implementation extension is not @objc.
-    if (VD->isFinal())
-      return std::nullopt;
-    // Other members get a special reason.
-    if (canInferImplicitObjC(/*allowAnyAccess*/true)) {
+    // A non-`final` member of an @objc @implementation extension is @objc
+    // with a special reason.
+    if (!VD->isFinal() && canInferImplicitObjC(/*allowAnyAccess*/true)) {
       auto ext = VD->getDeclContext()->getAsDecl();
       auto attr = ext->getAttrs()
                    .getAttribute<ObjCImplementationAttr>(/*AllowInvalid=*/true);

test/decl/ext/objc_implementation.swift

@@ -446,6 +446,11 @@ protocol EmptySwiftProto {}
     // expected-error@-1 {{method cannot be in an @objc @implementation extension of a class (without final or @nonobjc) because the type of the parameter cannot be represented in Objective-C}}
     // expected-note@-2 {{protocol-constrained type containing protocol 'EmptySwiftProto' cannot be represented in Objective-C}}
   }
+
+  override public static func superclassClassMethod() {
+    // rdar://136113393: `static override` could make a non-`@objc` override
+    // of an `@objc` member when using new syntax.
+  }
 }
 
 // Intentionally using `@_objcImplementation` for this test; do not upgrade!

test/decl/ext/objc_implementation_early_adopter.swift

@@ -446,6 +446,12 @@ protocol EmptySwiftProto {}
     // expected-warning@-1 {{instance method 'privateNonObjCMethod' will become implicitly '@objc' after adopting '@objc @implementation'; add 'final' to keep the current behavior}} {{3-3=final }}
     // expected-note@-2 {{add '@objc' to expose this instance method to Objective-C}} {{3-3=@objc }}
   }
+
+  override public static func superclassClassMethod() {
+    // rdar://136113393: `static override` could make a non-`@objc` override
+    // of an `@objc` member when using new syntax. (We would get a warning here
+    // if that bug were still in place.)
+  }
 }
 
 // Intentionally using `@_objcImplementation` for this test; do not upgrade!