Support overrides of unavailable inits in objcImpl

Suppose a superclass declares an initializer unavailable and then a subclass wants to redeclare and use it. Formally, the subclass declaration overrides the superclass one; however, Swift will not actually require the subclass to use the `override` keyword. As currently implemented, this means that the requirement will be skipped as an override, but the candidate will be included as a member implementation. Result: a “candidate does not match any requirement” diagnostic.

Fix this by skipping requirements that are overrides *only* if the declaration they override is not unavailable.

Fixes rdar://109541045.

Author: beccadax

lib/Sema/TypeCheckDeclObjC.cpp

@@ -2948,14 +2948,21 @@ class ObjCImplementationChecker {
   void addRequirements(IterableDeclContext *idc) {
     assert(idc->getDecl()->hasClangNode());
     for (Decl *_member : idc->getMembers()) {
-      // Skip accessors; we'll match their storage instead. Also skip overrides;
-      // the override checker handles those.
+      // Skip accessors; we'll match their storage instead.
       auto member = dyn_cast<ValueDecl>(_member);
-      if (!member || isa<AccessorDecl>(member) || member->getOverriddenDecl())
+      if (!member || isa<AccessorDecl>(member))
+        continue;
+
+      ASTContext &ctx = member->getASTContext();
+
+      // Also skip overrides, unless they override an unavailable decl, which
+      // makes them not formally overrides anymore.
+      if (member->getOverriddenDecl() &&
+          !member->getOverriddenDecl()->getAttrs().isUnavailable(ctx))
         continue;
 
       // Skip alternate Swift names for other language modes.
-      if (member->getAttrs().isUnavailable(member->getASTContext()))
+      if (member->getAttrs().isUnavailable(ctx))
         continue;
 
       // Skip async versions of members. We'll match against the completion

test/decl/ext/Inputs/objc_implementation.h

@@ -2,6 +2,7 @@
 
 @interface ObjCBaseClass
 
+- (instancetype)init __attribute__((unavailable));
 
 // Need two initializers to reproduce certain conflict bugs.
 - (instancetype)initFromSuperclass:(int)param  __attribute__((objc_designated_initializer));
@@ -161,6 +162,12 @@
 
 @end
 
+@interface ObjCBasicInitClass : ObjCBaseClass
+
+- (nonnull instancetype)init __attribute__((objc_designated_initializer));
+
+@end
+
 
 
 struct ObjCStruct {

test/decl/ext/objc_implementation.swift

@@ -393,6 +393,12 @@ protocol EmptySwiftProto {}
     }
 }
 
+@_objcImplementation extension ObjCBasicInitClass {
+  init() {
+    // OK
+  }
+}
+
 @_objcImplementation extension ObjCClass {}
 // expected-error@-1 {{duplicate implementation of Objective-C class 'ObjCClass'}}