[objc_direct] Allow for direct messages be sent to `self` when it is a Class

[MadCoder]

Summary

Sending a message to self when it is const and within a class method is safe because we know that self is the Class itself.

We can only relax this error in ARC.

Radar-Id: rdar://problem/58581965

[MadCoder]

Added some more tests to make sure it works along inheritance chains as expected

[arphaman]

Please add a test that exercises path without ARC enabled.

clang/lib/Sema/SemaExprObjC.cpp

3019

NIT: It feels easier to read if the condition is && !(Receiver->isObjCSelfExpr() && getLangOpts().ObjCAutoRefCount).

[MadCoder]

fixed @ahatanak feedback

[MadCoder]

adding the non ARC test I forgot to add

[MadCoder]

with the test for real

[arphaman]

LGTM

clang/lib/Sema/SemaExprObjC.cpp

ExprResult Sema::BuildInstanceMessage(Expr *Receiver,
   if (Method && Method->isDirectMethod()) {
     if (ReceiverType->isObjCIdType() && !isImplicit) {
       Diag(Receiver->getExprLoc(),
            diag::err_messaging_unqualified_id_with_direct_method);
       Diag(Method->getLocation(), diag::note_direct_method_declared_at)
           << Method->getDeclName();
     }
 
-    if (ReceiverType->isObjCClassType() && !isImplicit) {
+    // Under ARC, self can't be assigned, and doing a direct call to `self`
+    // when it's a Class is hence safe.  For other cases, we can't trust `self`
+    // is what we think it is, so we reject it.
+    if (ReceiverType->isObjCClassType() && !isImplicit &&
+        !(Receiver->isObjCSelfExpr() && getLangOpts().ObjCAutoRefCount)) {
       Diag(Receiver->getExprLoc(),
            diag::err_messaging_class_with_direct_method);
       Diag(Method->getLocation(), diag::note_direct_method_declared_at)
           << Method->getDeclName();
     }
 
     if (SuperLoc.isValid()) {
       Diag(SuperLoc, diag::err_messaging_super_with_direct_method);

clang/test/SemaObjC/method-direct-arc.m

+// RUN: %clang_cc1 -fobjc-arc -fsyntax-only -verify -Wselector-type-mismatch %s
+
+extern Class object_getClass(id);
+
+__attribute__((objc_root_class))
+@interface Root
+- (Class)class;
++ (void)directMethod __attribute__((objc_direct)); // expected-note {{direct method 'directMethod' declared here}}
++ (void)anotherDirectMethod __attribute__((objc_direct));
+@end
+
+@implementation Root
+- (Class)class
+{
+  return object_getClass(self);
+}
++ (void)directMethod {
+}
++ (void)anotherDirectMethod {
+  [self directMethod]; // this should not warn
+}
++ (void)regularMethod {
+  [self directMethod];        // this should not warn
+  [self anotherDirectMethod]; // this should not warn
+}
+- (void)regularInstanceMethod {
+  [[self class] directMethod]; // expected-error {{messaging a Class with a method that is possibly direct}}
+}
+@end
+
+@interface Sub : Root
+@end
+
+@implementation Sub
++ (void)foo {
+  [self directMethod]; // this should not warn
+}
+@end
+
+__attribute__((objc_root_class))
+@interface Other
+@end
+
+@implementation Other
++ (void)bar {
+  [self directMethod]; // expected-error {{no known class method for selector 'directMethod'}}
+}
+@end

clang/test/SemaObjC/method-direct.m

 + (void)classRootRegular {
 }
 - (void)rootDirect {
 }
 + (void)classRootDirect {
 }
 - (void)otherRootDirect {
 }
++ (void)someRootDirectMethod { // expected-note {{direct method 'someRootDirectMethod' declared here}}
+}
 + (void)otherClassRootDirect {
+  [self someRootDirectMethod]; // expected-error {{messaging a Class with a method that is possibly direct}}
 }
 - (void)rootExtensionDirect {
 }
 + (void)classRootExtensionDirect {
 }
 - (void)rootExtensionRegular {
 }
 + (void)classRootExtensionRegular {