[ObjC] Boxed strings should use the nullability from stringWithUTF8String's return type

[arphaman]

Summary

Objective-C NSString has a class method stringWithUTF8String that creates a new NSString from a C string. Objective-C box expression @(...) can be used to create an NSString instead of invoking the stringWithUTF8String method directly (The compiler lowers it down to the invocation though). This patch ensures that the type of @(string-value) gets the same nullability attributes as the return type of stringWithUTF8String to ensure that the diagnostics are consistent between the two.

rdar://33847186

[ahatanak]

LGTM

lib/Sema/SemaExprObjC.cpp

if (Context.hasSameUnqualifiedType(PointeeType, Context.CharTy)) {
                                   stringWithUTF8String, BoxingMethod))
            return ExprError();
 
         StringWithUTF8StringMethod = BoxingMethod;
       }
       
       BoxingMethod = StringWithUTF8StringMethod;
       BoxedType = NSStringPointer;
+      // Transfer the nullability from method's return type.
+      Optional<NullabilityKind> Nullability =
+          BoxingMethod->getReturnType()->getNullability(Context);
+      if (Nullability)
+        BoxedType = Context.getAttributedType(
+            AttributedType::getNullabilityAttrKind(*Nullability), BoxedType,
+            BoxedType);
     }
   } else if (ValueType->isBuiltinType()) {
     // The other types we support are numeric, char and BOOL/bool. We could also
     // provide limited support for structure types, such as NSRange, NSRect, and
     // NSSize. See NSValue (NSValueGeometryExtensions) in <Foundation/NSGeometry.h>
     // for more details.
 
     // Check for a top-level character literal.

test/SemaObjC/transfer-boxed-string-nullability.m

+// RUN: %clang_cc1 -fblocks -fobjc-arc -Wnullable-to-nonnull-conversion -fsyntax-only -verify -Wno-objc-root-class %s
+// RUN: %clang_cc1 -fblocks -fobjc-arc -Wnullable-to-nonnull-conversion -fsyntax-only -verify -Wno-objc-root-class -DNOWARN %s
+
+@interface NSString
+
++ (NSString*
+#ifndef NOWARN
+  _Nullable
+#else
+  _Nonnull
+#endif
+) stringWithUTF8String:(const char*)x;
+
+@end
+
+void takesNonNull(NSString * _Nonnull ptr);
+
+void testBoxedString() {
+  const char *str = "hey";
+  takesNonNull([NSString stringWithUTF8String:str]);
+  takesNonNull(@(str));
+#ifndef NOWARN
+  // expected-warning@-3 {{implicit conversion from nullable pointer 'NSString * _Nullable' to non-nullable pointer type 'NSString * _Nonnull'}}
+  // expected-warning@-3 {{implicit conversion from nullable pointer 'NSString * _Nullable' to non-nullable pointer type 'NSString * _Nonnull'}}
+#else
+  // expected-no-diagnostics
+#endif
+}