[OBJC] Allow __attribute__((NSObject)) types be used as lightweight g…

…eneric specifiers (#84593)

As per
https://clang.llvm.org/docs/AutomaticReferenceCounting.html#retainable-object-pointers,
types with `__attribute__((NSObject))` are retainable, and thus should
be eligible to be used as lightweight generic specifiers.

Fix for #84592 84592

clang/lib/Sema/SemaType.cpp

@@ -1018,6 +1018,11 @@ static QualType applyObjCTypeArgs(Sema &S, SourceLocation loc, QualType type,
       return type;
     }
 
+    // Types that have __attribute__((NSObject)) are permitted.
+    if (typeArg->isObjCNSObjectType()) {
+      continue;
+    }
+
     // Dependent types will be checked at instantiation time.
     if (typeArg->isDependentType()) {
       continue;

clang/test/SemaObjC/attr-objc-NSObject.m

@@ -0,0 +1,23 @@
+// RUN: %clang_cc1 -verify -Wno-objc-root-class -fsyntax-only %s
+
+@interface NSArray<__covariant ObjectType>
+- (void)containsObject:(ObjectType)anObject; // expected-note {{passing argument to parameter 'anObject' here}}
+- (void)description;
+@end
+
+typedef __attribute__((NSObject)) struct Foo *FooRef;
+typedef struct Bar *BarRef;
+
+void good() {
+  FooRef object;
+  NSArray<FooRef> *array;
+  [array containsObject:object];
+  [object description];
+}
+
+void bad() {
+  BarRef object;
+  NSArray<BarRef> *array; // expected-error {{type argument 'BarRef' (aka 'struct Bar *') is neither an Objective-C object nor a block type}}
+  [array containsObject:object]; // expected-warning {{incompatible pointer types sending 'BarRef' (aka 'struct Bar *') to parameter of type 'id'}}
+  [object description]; // expected-warning {{receiver type 'BarRef' (aka 'struct Bar *') is not 'id' or interface pointer, consider casting it to 'id'}}
+}