I expanded the testcase and fixed the grammar in the actual warning.

New patch attached.

On Tue, Nov 11, 2014 at 05:03:33PM -0800, David Blaikie wrote:
> (it's a bit easier if you include the test in the same patch file - also
> you can use Phabricator if you like - some reviewers perefer it)
> 
> Since you've got the loop there for seeing through multiple levels of
> pointer, should you have a test case that exercises that on a > 1 level of
> depth? Demonstrate that we warn on both levels (if that's the right thing
> to do?)?
> 
> Optionally (probably in a separate follow-up patch) you could add a note
> with a fixit to include the missing consts.
> 
> On Tue, Nov 11, 2014 at 10:58 AM, Roman Divacky <[email protected]> wrote:
> 
> > Hi,
> >
> > I implemented -Wcast-qual. The patch is actually quite short (attached + a
> > test
> > case).
> >
> > This fixes #13772 and also note that -Wcast-qual is used in llvm build
> > itself.
> >
> > Is this ok to be commited? Thanks
> >
> > Roman
> >
> >
> > _______________________________________________
> > cfe-commits mailing list
> > [email protected]
> > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
> >
> >
Index: include/clang/Basic/DiagnosticGroups.td
===================================================================
--- include/clang/Basic/DiagnosticGroups.td	(revision 221694)
+++ include/clang/Basic/DiagnosticGroups.td	(working copy)
@@ -60,7 +60,7 @@
 def KeywordCompat : DiagGroup<"keyword-compat">;
 def GNUCaseRange : DiagGroup<"gnu-case-range">;
 def CastAlign : DiagGroup<"cast-align">;
-def : DiagGroup<"cast-qual">;
+def CastQual : DiagGroup<"cast-qual">;
 def : DiagGroup<"char-align">;
 def Comment : DiagGroup<"comment">;
 def GNUComplexInteger : DiagGroup<"gnu-complex-integer">;
Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td	(revision 221694)
+++ include/clang/Basic/DiagnosticSemaKinds.td	(working copy)
@@ -6097,6 +6097,8 @@
 def warn_zero_size_struct_union_in_extern_c : Warning<"%select{|empty }0"
   "%select{struct|union}1 has size 0 in C, %select{size 1|non-zero size}2 in C++">,
   InGroup<ExternCCompat>;
+def warn_cast_qual : Warning<"cast from %0 to %1 drops const qualifier">,
+  InGroup<CastQual>, DefaultIgnore;
 } // End of general sema category.
 
 // inline asm.
Index: lib/Sema/SemaCast.cpp
===================================================================
--- lib/Sema/SemaCast.cpp	(revision 221694)
+++ lib/Sema/SemaCast.cpp	(working copy)
@@ -2371,6 +2371,20 @@
 
   if (Kind == CK_BitCast)
     checkCastAlign();
+
+  // -Wcast-qual
+  const PointerType *DestPtr = nullptr;
+  const PointerType *SrcPtr = nullptr;
+
+  while ((DestPtr = DestType->getAs<PointerType>()) && (SrcPtr = SrcType->getAs<PointerType>())) {
+    QualType OrigDestType = DestType;
+    QualType OrigSrcType = SrcType;
+    DestType = DestPtr->getPointeeType();
+    SrcType = SrcPtr->getPointeeType();
+    if (SrcType.isConstQualified() && !DestType.isConstQualified()) {
+      Self.Diag(SrcExpr.get()->getLocStart(), diag::warn_cast_qual) << OrigSrcType << OrigDestType;
+    }
+  }
 }
 
 ExprResult Sema::BuildCStyleCastExpr(SourceLocation LPLoc,
Index: test/Sema/warn-cast-qual.c
===================================================================
--- test/Sema/warn-cast-qual.c	(revision 0)
+++ test/Sema/warn-cast-qual.c	(working copy)
@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-unknown -fsyntax-only -Wcast-qual -verify %s
+
+#include <stdint.h>
+
+void foo() {
+  const char * const ptr = 0;
+  const char * const *ptrptr = 0;
+  char *y = (char *)ptr;	// expected-warning {{cast from 'const char *' to 'char *' drops const qualifier}}
+  char **y1 = (char **)ptrptr;	// expected-warning {{cast from 'const char *const *' to 'char **' drops const qualifier}} expected-warning {{cast from 'const char *const' to 'char *' drops const qualifier}}
+  const char **y2 = (const char **)ptrptr;	// expected-warning {{cast from 'const char *const *' to 'const char **' drops const qualifier}}
+
+  char *z = (char *)(uintptr_t)(const void *)ptr;	// no warning
+  char *z1 = (char *)(const void *)ptr;	// expected-warning {{cast from 'const void *' to 'char *' drops const qualifier}}
+}
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