[Sema][ObjC] Fix spurious -Wcast-qual warnings.

We do not meaningfully track object const-ness of Objective-C object
types. Silence the -Wcast-qual warning that is issued when casting to or
from Objective-C object types results in losing const qualification.

rdar://problem/33807915

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@310672 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Sema/SemaCast.cpp

@@ -552,7 +552,14 @@ CastsAwayConstness(Sema &Self, QualType SrcType, QualType DestType,
     Qualifiers SrcQuals, DestQuals;
     Self.Context.getUnqualifiedArrayType(UnwrappedSrcType, SrcQuals);
     Self.Context.getUnqualifiedArrayType(UnwrappedDestType, DestQuals);
-
+
+    // We do not meaningfully track object const-ness of Objective-C object
+    // types. Remove const from the source type if either the source or
+    // the destination is an Objective-C object type.
+    if (UnwrappedSrcType->isObjCObjectType() ||
+        UnwrappedDestType->isObjCObjectType())
+      SrcQuals.removeConst();
+
     Qualifiers RetainedSrcQuals, RetainedDestQuals;
     if (CheckCVR) {
       RetainedSrcQuals.setCVRQualifiers(SrcQuals.getCVRQualifiers());

test/SemaObjC/illegal-nonarc-bridged-cast.m

@@ -1,8 +1,9 @@
-// RUN: %clang_cc1 -triple x86_64-apple-darwin11 -fsyntax-only -fblocks -verify %s
+// RUN: %clang_cc1 -triple x86_64-apple-darwin11 -fsyntax-only -fblocks -Wcast-qual -verify %s
 // rdar://10597832
 
 typedef const void *CFTypeRef;
 typedef const struct __CFString *CFStringRef;
+@class NSString;
 
 @interface NSString
 @end
@@ -18,7 +19,7 @@ @interface NSString
 void from_cf() {
   id obj1 = (__bridge_transfer id)CFCreateSomething(); // expected-warning {{'__bridge_transfer' casts have no effect when not using ARC}}
   id obj2 = (__bridge_transfer NSString*)CFCreateString(); // expected-warning {{'__bridge_transfer' casts have no effect when not using ARC}}
-  (__bridge int*)CFCreateSomething(); // expected-warning {{expression result unused}}
+  (__bridge int*)CFCreateSomething(); // expected-warning {{expression result unused}} expected-warning {{cast from 'const void *' to 'int *' drops const qualifier}}
   id obj3 = (__bridge id)CFGetSomething();
   id obj4 = (__bridge NSString*)CFGetString();
 }
@@ -41,3 +42,15 @@ void to_cf_ignored(id obj) {
   CFTypeRef cf1 = (__bridge_retained CFTypeRef)CreateSomething(); // no-warning
   CFTypeRef cf3 = (__bridge CFTypeRef)CreateSomething(); // no-warning
 }
+
+// Check that clang doesn't warn about dropping const from Objective-C object
+// types.
+void test_wcast_qual() {
+  CFStringRef c;
+  NSString *n0 = (NSString *)c;
+  NSString **n1 = (NSString **)&c;
+  const NSString *n2;
+  const NSString **n3;
+  void *p0 = (void *)n2;
+  void **p1 = (void **)n3;
+}