[SemaObjC] Don't infer the availabilty of messages to +new from -init if the receiver has Class type

[erik.pilkington]

Summary

Fixes rdar://47713266

Thanks for taking a look!
Erik

[arphaman]

LGTM, Thanks!

clang/lib/Sema/SemaExprObjC.cpp

               << Method->getSelector() << Sel;
             Diag(Method->getLocation(), diag::note_method_declared_at)
               << Method->getDeclName();
           }
         }
       } else {
         if (ObjCMethodDecl *CurMeth = getCurMethodDecl()) {
           if (ObjCInterfaceDecl *ClassDecl = CurMeth->getClassInterface()) {
-            // FIXME: Is this correct? Why are we assuming that a message to
-            // Class will call a method in the current interface?
+            // As a guess, try looking for the method in the current interface.
+            // This very well may not produce the "right" method.
 
             // First check the public methods in the class interface.
             Method = ClassDecl->lookupClassMethod(Sel);
 
             if (!Method)
               Method = ClassDecl->lookupPrivateClassMethod(Sel);
 
-            if (Method && DiagnoseUseOfDecl(Method, SelectorSlotLocs, nullptr,
-                                            false, false, ClassDecl))
+            if (Method && DiagnoseUseOfDecl(Method, SelectorSlotLocs))
               return ExprError();
           }
         }
         if (!Method) {
           // If not messaging 'self', look for any factory method named 'Sel'.
           if (!Receiver || !isSelfExpr(Receiver)) {
             // If no class (factory) method was found, check if an _instance_
             // method of the same name exists in the root class only.

clang/test/SemaObjC/infer-availability-from-init.m

 +(FromSelf*)another_one;
 @end
 
 @implementation FromSelf
 +(FromSelf*)another_one {
   [self new];
 }
 @end
+
+@interface NoInit : NSObject
+-(instancetype)init __attribute__((unavailable)); // expected-note {{'init' has been explicitly marked unavailable here}}
+@end
+
+@interface NoInitSub : NoInit @end
+
+@implementation NoInitSub
+-(void)meth:(Class)c {
+  [c new]; // No error; unknown interface.
+  [NoInitSub new]; // expected-error {{'new' is unavailable}}
+}
+@end