[PATCH]: Implementation of -Wcast-qual

Roman Divacky rdivacky at vlakno.cz
Thu Nov 20 04:57:22 PST 2014


Now a patch that actually survives clang tests with assertions enabled.

On Thu, Nov 20, 2014 at 10:41:28AM +0100, Roman Divacky wrote:
> Patch hopefully addressing all the concerns attached.
> 
> On Wed, Nov 19, 2014 at 10:05:37AM -0800, David Blaikie wrote:
> > On Tue, Nov 18, 2014 at 2:05 PM, Roman Divacky <rdivacky at vlakno.cz> wrote:
> > 
> > > New patch attached. I've checked the behaviour against gcc5 and we warn on
> > > the
> > > same cases.
> > >
> > 
> > Great! Thanks so much for your patience & diligence. Does GCC mention the
> > same types as Clang? (I notice in some of your examples... oh, you don't
> > have such an example. What does GCC and Clang do for "const char **" ->
> > "char **"? It looks like, judging by the "int**" -> "const int **" warning,
> > we might print just the inner type there as well ("cast from const char* to
> > char*") - is that what GCC does? Or does it print the full outer types
> > always? Is your patch's behavior going to be better for users (probably,
> > but I'm not sure).
> > 
> > 
> > > For the const int **bahc = (const int **)bah; case gcc5 emits a warning
> > > with
> > > a different warning (to be safe all intermediate pointers in cast from
> > > 'int **'
> > > to 'volatile int **' must be 'const' qualified [-Wcast-qual]). Do you think
> > > it's worth adding similarly worded warning for that case or is the
> > > "dropping
> > > const" ok?
> > >
> > 
> > I think it's OK, but confusing - it'd be good to make it better, but
> > possibly as a follow-up patch. Could you include FIXMEs in the test case
> > for both the const and volatile cases for this showing what GCC does (or
> > otherwise suggesting better wording). I'm assuming in even the volatile
> > case, the correct (and GCC5) text is that you need to add const on the
> > intermediate pointers?
> > 
> > Actually, now that I look at the test case, it seems a bit worse - it's not
> > printing the top-level type in this case (int**->const int**) it's printing
> > the nested type (int*->const int*) and saying it drops const, which is even
> > more confusing... hrm. So maybe it makes sense to fix this in the current
> > patch rather than a follow-up, given how confusing it is.
> > 
> > One thing to do with this current patch, would be to use a %select rather
> > than %2 and %3 being directly substituted (having to add that 's' is a bit
> > weird). Something like this:
> > 
> > Warning<"cast from %0 to %1 drops %select{const and volatile
> > qualifiers|const qualifier|volatile qualifier}2">
> > 
> > int qualifiers = 0;
> > if (const && volatile)
> >   qualifiers = 0;
> > else if (const)
> >   qualifiers = 1;
> > else if (volatile)
> >   qualifiers = 2;
> > 
> > etc.
> > 
> > & doing the better "all intermediate pointers in cast" thing shouldn't be
> > too hard, right? Just another warning in the .td file, plus a condition
> > before all that to say "if !const && !volatile <do the other warning>" but
> > you'll need to get those types right (providing the top level types, not
> > the nested types, especially in that instance).
> > 
> > 
> > 
> > 
> > >
> > > On Tue, Nov 18, 2014 at 09:27:17AM -0800, David Blaikie wrote:
> > > > The last line should have a warning on it (and assuming the
> > > > expected-warning comments in the test are correct, it doesn't):
> > > >
> > > > GCC 4.9:
> > > >
> > > > warning: cast from type 'int**' to type 'const int**' casts away
> > > qualifiers
> > > > [-Wcast-qual]
> > > >  const int **bahc = (const int **)bah;
> > > >
> > > > On Tue, Nov 18, 2014 at 6:11 AM, Roman Divacky <rdivacky at vlakno.cz>
> > > wrote:
> > > >
> > > > > Scrap the previous patch. It doesnt work when we add qualifiers. The
> > > new
> > > > > version is fixed.
> > > > >
> > > > > On Tue, Nov 18, 2014 at 12:49:59PM +0100, Roman Divacky wrote:
> > > > > > Perhaps like this?
> > > > > >
> > > > > > On Fri, Nov 14, 2014 at 01:17:38PM -0800, David Blaikie wrote:
> > > > > > > Yep. Though the diagnostic for dropping const and volatile is a
> > > bit off
> > > > > > > ("drops const volatile qualifier" should probably read "drops
> > > const and
> > > > > > > volatile qualifiers"? (maybe you'll need a %select for this - you
> > > could
> > > > > > > probably use a %select for the const and volatile separately too
> > > if you
> > > > > > > like)). What does GCC do here? Does it warn on dropping volatile at
> > > > > all?
> > > > > > >
> > > > > > > Chatting to Richard over lunch he mentioned an interesting case
> > > where
> > > > > we
> > > > > > > might want to warn:
> > > > > > >
> > > > > > >   int **x;
> > > > > > >   auto y = (const int **)x;
> > > > > > >
> > > > > > > which, if it were a static_cast, would warn:
> > > > > > >
> > > > > > >   cast.cpp:2:10: error: static_cast from 'int **' to 'const int
> > > **' is
> > > > > not
> > > > > > > allowed
> > > > > > >   auto y = static_cast<const int **>(x);
> > > > > > >            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > > > > >
> > > > > > > I'm just not sure we'll be able to get a good diagnostic in both
> > > these
> > > > > > > cases. But as I think/type this out I think:
> > > > > > >
> > > > > > > We should just use the same machinery that powers static_cast here,
> > > > > > > (Richard mentioned there should be a way to test whether a
> > > conversion
> > > > > is a
> > > > > > > "qualification conversion" which is the description of the valid
> > > > > implicit
> > > > > > > qualification changes, and not the above const-changing cases))
> > > and we
> > > > > > > should teach the machinery to give us enough information to create
> > > good
> > > > > > > diagnostics - telling the user where the missing const, volatile,
> > > etc,
> > > > > is.
> > > > > > >
> > > > > > > Sorry to go through so many iterations - it didn't occur to me
> > > until
> > > > > > > Richard mentioned it that there was this more general approach.
> > > > > > >
> > > > > > > (wonder what GCC does here? - hmm, looks like it gets the "int** ->
> > > > > const
> > > > > > > int**" right: cast from type ???int**??? to type ???const int**???
> > > > > casts away
> > > > > > > qualifiers)
> > > > > > >
> > > > > > > On Fri, Nov 14, 2014 at 12:02 PM, Roman Divacky <
> > > rdivacky at vlakno.cz>
> > > > > wrote:
> > > > > > >
> > > > > > > > Like this?
> > > > > > > >
> > > > > > > > On Fri, Nov 14, 2014 at 11:20:49AM -0800, David Blaikie wrote:
> > > > > > > > > I take it this is consistent with the GCC warning - in terms of
> > > > > warning
> > > > > > > > on
> > > > > > > > > the innermost issue, reporting const or volatile - what about
> > > > > dropping
> > > > > > > > > const and volatile at the same time?
> > > > > > > > >
> > > > > > > > > Issues with the current code:
> > > > > > > > >
> > > > > > > > > * DestPtr and SrcPtr don't need to be initialized to null,
> > > they'll
> > > > > be
> > > > > > > > > written to on the first loop iteration as needed - avoiding
> > > excess
> > > > > > > > > initialization helps tools like MSan find more bugs rather
> > > than the
> > > > > > > > program
> > > > > > > > > silently using unintended default values
> > > > > > > > >
> > > > > > > > > * InnerMostDestType and InnerMostSrcType will be dangling
> > > pointers
> > > > > after
> > > > > > > > > the while loop (so accessing them in the proceeding 'if' is UB)
> > > > > > > > >
> > > > > > > > > * you don't need to check both InnerMostDestType and
> > > > > InnerMostSrcType in
> > > > > > > > > the following if - it's invariant that if one is non-null (you
> > > can
> > > > > use
> > > > > > > > > QualType values rather than QualType* to address the previous
> > > bug,
> > > > > and
> > > > > > > > use
> > > > > > > > > QualTypes "isNull()" member function here) so is the other
> > > > > > > > >
> > > > > > > > > On Fri, Nov 14, 2014 at 11:07 AM, Roman Divacky <
> > > > > rdivacky at vlakno.cz>
> > > > > > > > wrote:
> > > > > > > > >
> > > > > > > > > > Actually, try this patch. It includes check for volatile as
> > > well.
> > > > > > > > > >
> > > > > > > > > > On Wed, Nov 12, 2014 at 12:39:20PM -0800, David Blaikie
> > > wrote:
> > > > > > > > > > > [+Richard for oversight]
> > > > > > > > > > >
> > > > > > > > > > > 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}}
> > > > > > > > > > >
> > > > > > > > > > > I think if we're going to warn on multiple layers (I'm not
> > > sure
> > > > > > > > that's
> > > > > > > > > > > ideal - is that consistent with GCC's warning? Does GCC
> > > warn on
> > > > > > > > > > mismatched
> > > > > > > > > > > types too - "const T*" -> "U*"? - do we warn there too, or
> > > > > only when
> > > > > > > > > > > there's a valid implicit conversion like the void*
> > > example?)
> > > > > then we
> > > > > > > > > > should
> > > > > > > > > > > probably drop the top level const, "const char *const" ->
> > > > > "char*" -
> > > > > > > > the
> > > > > > > > > > top
> > > > > > > > > > > level const on the first type is confusing/misleading, it's
> > > > > only
> > > > > > > > relevant
> > > > > > > > > > > to show "const char*" and "char*".
> > > > > > > > > > >
> > > > > > > > > > >
> > > > > > > > > > > On Wed, Nov 12, 2014 at 10:41 AM, Roman Divacky <
> > > > > rdivacky at vlakno.cz>
> > > > > > > > > > wrote:
> > > > > > > > > > >
> > > > > > > > > > > > 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 <
> > > > > > > > rdivacky at vlakno.cz>
> > > > > > > > > > > > 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
> > > > > > > > > > > > > > cfe-commits at cs.uiuc.edu
> > > > > > > > > > > > > >
> > > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
> > > > > > > > > > > > > >
> > > > > > > > > > > > > >
> > > > > > > > > > > >
> > > > > > > > > >
> > > > > > > >
> > > > >
> > > > > > Index: include/clang/Basic/DiagnosticGroups.td
> > > > > > ===================================================================
> > > > > > --- include/clang/Basic/DiagnosticGroups.td   (revision 222228)
> > > > > > +++ 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
> > > 222228)
> > > > > > +++ include/clang/Basic/DiagnosticSemaKinds.td        (working copy)
> > > > > > @@ -6104,6 +6104,8 @@
> > > > > >  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 %2
> > > qualifier%3">,
> > > > > > +  InGroup<CastQual>, DefaultIgnore;
> > > > > >  } // End of general sema category.
> > > > > >
> > > > > >  // inline asm.
> > > > > > Index: lib/Sema/SemaCast.cpp
> > > > > > ===================================================================
> > > > > > --- lib/Sema/SemaCast.cpp     (revision 222228)
> > > > > > +++ lib/Sema/SemaCast.cpp     (working copy)
> > > > > > @@ -143,7 +143,10 @@
> > > > > >  }
> > > > > >
> > > > > >  static bool CastsAwayConstness(Sema &Self, QualType SrcType,
> > > QualType
> > > > > DestType,
> > > > > > -                               bool CheckCVR, bool
> > > CheckObjCLifetime);
> > > > > > +                               bool CheckCVR, bool
> > > CheckObjCLifetime,
> > > > > > +                               QualType *TheOffendingSrcType =
> > > nullptr,
> > > > > > +                               QualType *TheOffendingDestType =
> > > nullptr,
> > > > > > +                               Qualifiers *CastAwayQualifiers =
> > > > > nullptr);
> > > > > >
> > > > > >  // The Try functions attempt a specific way of casting. If they
> > > > > succeed, they
> > > > > >  // return TC_Success. If their way of casting is not appropriate for
> > > > > the given
> > > > > > @@ -462,7 +465,10 @@
> > > > > >  /// \param CheckObjCLifetime Whether to check Objective-C lifetime
> > > > > qualifiers.
> > > > > >  static bool
> > > > > >  CastsAwayConstness(Sema &Self, QualType SrcType, QualType DestType,
> > > > > > -                   bool CheckCVR, bool CheckObjCLifetime) {
> > > > > > +                   bool CheckCVR, bool CheckObjCLifetime,
> > > > > > +                   QualType *TheOffendingSrcType,
> > > > > > +                   QualType *TheOffendingDestType,
> > > > > > +                   Qualifiers *CastAwayQualifiers) {
> > > > > >    // If the only checking we care about is for Objective-C lifetime
> > > > > qualifiers,
> > > > > >    // and we're not in ARC mode, there's nothing to check.
> > > > > >    if (!CheckCVR && CheckObjCLifetime &&
> > > > > > @@ -487,6 +493,8 @@
> > > > > >    // Find the qualifiers. We only care about cvr-qualifiers for the
> > > > > >    // purpose of this check, because other qualifiers (address
> > > spaces,
> > > > > >    // Objective-C GC, etc.) are part of the type's identity.
> > > > > > +  QualType PrevUnwrappedSrcType = UnwrappedSrcType;
> > > > > > +  QualType PrevUnwrappedDestType = UnwrappedDestType;
> > > > > >    while (UnwrapDissimilarPointerTypes(UnwrappedSrcType,
> > > > > UnwrappedDestType)) {
> > > > > >      // Determine the relevant qualifiers at this level.
> > > > > >      Qualifiers SrcQuals, DestQuals;
> > > > > > @@ -497,6 +505,13 @@
> > > > > >      if (CheckCVR) {
> > > > > >
> > > RetainedSrcQuals.setCVRQualifiers(SrcQuals.getCVRQualifiers());
> > > > > >
> > > RetainedDestQuals.setCVRQualifiers(DestQuals.getCVRQualifiers());
> > > > > > +
> > > > > > +      if (RetainedSrcQuals != RetainedDestQuals &&
> > > TheOffendingSrcType
> > > > > &&
> > > > > > +          TheOffendingDestType && CastAwayQualifiers) {
> > > > > > +        *TheOffendingSrcType = PrevUnwrappedSrcType;
> > > > > > +        *TheOffendingDestType = PrevUnwrappedDestType;
> > > > > > +     *CastAwayQualifiers = RetainedSrcQuals - RetainedDestQuals;
> > > > > > +      }
> > > > > >      }
> > > > > >
> > > > > >      if (CheckObjCLifetime &&
> > > > > > @@ -505,6 +520,9 @@
> > > > > >
> > > > > >      cv1.push_back(RetainedSrcQuals);
> > > > > >      cv2.push_back(RetainedDestQuals);
> > > > > > +
> > > > > > +    PrevUnwrappedSrcType = UnwrappedSrcType;
> > > > > > +    PrevUnwrappedDestType = UnwrappedDestType;
> > > > > >    }
> > > > > >    if (cv1.empty())
> > > > > >      return false;
> > > > > > @@ -2371,6 +2389,28 @@
> > > > > >
> > > > > >    if (Kind == CK_BitCast)
> > > > > >      checkCastAlign();
> > > > > > +
> > > > > > +  // -Wcast-qual
> > > > > > +  QualType TheOffendingSrcType, TheOffendingDestType;
> > > > > > +  Qualifiers CastAwayQualifiers;
> > > > > > +  if (CastsAwayConstness(Self, SrcType, DestType, true, false,
> > > > > > +                      &TheOffendingSrcType, &TheOffendingDestType,
> > > > > > +                      &CastAwayQualifiers)) {
> > > > > > +    const char *qualifiers;
> > > > > > +    const char *suffix = "";
> > > > > > +    if (CastAwayQualifiers.hasConst() &&
> > > > > CastAwayQualifiers.hasVolatile()) {
> > > > > > +      qualifiers = "const and volatile";
> > > > > > +      suffix = "s";
> > > > > > +    } else if (CastAwayQualifiers.hasConst())
> > > > > > +      qualifiers = "const";
> > > > > > +    else if (CastAwayQualifiers.hasVolatile())
> > > > > > +      qualifiers = "volatile";
> > > > > > +    else {
> > > > > > +       llvm_unreachable("Impossible qualifier");
> > > > > > +    }
> > > > > > +    Self.Diag(SrcExpr.get()->getLocStart(), diag::warn_cast_qual) <<
> > > > > > +      TheOffendingSrcType << TheOffendingDestType << qualifiers <<
> > > > > suffix;
> > > > > > +  }
> > > > > >  }
> > > > > >
> > > > > >  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 +1,19 @@
> > > > > > +// 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}}
> > > > > > +  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}}
> > > > > > +
> > > > > > +  volatile char *vol = 0;
> > > > > > +  char *vol2 = (char *)vol; // expected-warning {{cast from
> > > 'volatile
> > > > > char *' to 'char *' drops volatile qualifier}}
> > > > > > +  const volatile char *volc = 0;
> > > > > > +  char *volc2 = (char *)volc; // expected-warning {{cast from 'const
> > > > > volatile char *' to 'char *' drops const and volatile qualifiers}}
> > > > > > +}
> > > > >
> > > > >
> > >

> Index: include/clang/Basic/DiagnosticGroups.td
> ===================================================================
> --- include/clang/Basic/DiagnosticGroups.td	(revision 222421)
> +++ 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 222421)
> +++ include/clang/Basic/DiagnosticSemaKinds.td	(working copy)
> @@ -6112,6 +6112,11 @@
>  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 %select{const and"
> +  "volatile qualifiers|const qualifier|volatile qualifier}2">,
> +  InGroup<CastQual>, DefaultIgnore;
> +def warn_cast_qual2 : Warning<"cast from %0 to %1 must have all intermediate "
> +  "pointers const qualified to be safe">, InGroup<CastQual>, DefaultIgnore;
>  } // End of general sema category.
>  
>  // inline asm.
> Index: lib/Sema/SemaCast.cpp
> ===================================================================
> --- lib/Sema/SemaCast.cpp	(revision 222421)
> +++ lib/Sema/SemaCast.cpp	(working copy)
> @@ -142,9 +142,6 @@
>    };
>  }
>  
> -static bool CastsAwayConstness(Sema &Self, QualType SrcType, QualType DestType,
> -                               bool CheckCVR, bool CheckObjCLifetime);
> -
>  // The Try functions attempt a specific way of casting. If they succeed, they
>  // return TC_Success. If their way of casting is not appropriate for the given
>  // arguments, they return TC_NotApplicable and *may* set diag to a diagnostic
> @@ -462,7 +459,10 @@
>  /// \param CheckObjCLifetime Whether to check Objective-C lifetime qualifiers.
>  static bool
>  CastsAwayConstness(Sema &Self, QualType SrcType, QualType DestType,
> -                   bool CheckCVR, bool CheckObjCLifetime) {
> +                   bool CheckCVR, bool CheckObjCLifetime,
> +                   QualType *TheOffendingSrcType = nullptr,
> +                   QualType *TheOffendingDestType = nullptr,
> +                   Qualifiers *CastAwayQualifiers = nullptr) {
>    // If the only checking we care about is for Objective-C lifetime qualifiers,
>    // and we're not in ARC mode, there's nothing to check.
>    if (!CheckCVR && CheckObjCLifetime && 
> @@ -487,6 +487,8 @@
>    // Find the qualifiers. We only care about cvr-qualifiers for the 
>    // purpose of this check, because other qualifiers (address spaces, 
>    // Objective-C GC, etc.) are part of the type's identity.
> +  QualType PrevUnwrappedSrcType = UnwrappedSrcType;
> +  QualType PrevUnwrappedDestType = UnwrappedDestType;
>    while (UnwrapDissimilarPointerTypes(UnwrappedSrcType, UnwrappedDestType)) {
>      // Determine the relevant qualifiers at this level.
>      Qualifiers SrcQuals, DestQuals;
> @@ -497,6 +499,13 @@
>      if (CheckCVR) {
>        RetainedSrcQuals.setCVRQualifiers(SrcQuals.getCVRQualifiers());
>        RetainedDestQuals.setCVRQualifiers(DestQuals.getCVRQualifiers());
> +
> +      if (RetainedSrcQuals != RetainedDestQuals && TheOffendingSrcType &&
> +          TheOffendingDestType && CastAwayQualifiers) {
> +        *TheOffendingSrcType = PrevUnwrappedSrcType;
> +        *TheOffendingDestType = PrevUnwrappedDestType;
> +        *CastAwayQualifiers = RetainedSrcQuals - RetainedDestQuals;
> +      }
>      }
>      
>      if (CheckObjCLifetime &&
> @@ -505,6 +514,9 @@
>      
>      cv1.push_back(RetainedSrcQuals);
>      cv2.push_back(RetainedDestQuals);
> +
> +    PrevUnwrappedSrcType = UnwrappedSrcType;
> +    PrevUnwrappedDestType = UnwrappedDestType;
>    }
>    if (cv1.empty())
>      return false;
> @@ -2371,6 +2383,29 @@
>  
>    if (Kind == CK_BitCast)
>      checkCastAlign();
> +
> +  // -Wcast-qual
> +  QualType TheOffendingSrcType, TheOffendingDestType;
> +  Qualifiers CastAwayQualifiers;
> +  if (CastsAwayConstness(Self, SrcType, DestType, true, false,
> +                         &TheOffendingSrcType, &TheOffendingDestType,
> +                         &CastAwayQualifiers)) {
> +    int qualifiers = 0;
> +    if (CastAwayQualifiers.hasConst() && CastAwayQualifiers.hasVolatile()) {
> +      qualifiers = 0;
> +    } else if (CastAwayQualifiers.hasConst()) {
> +      qualifiers = 1;
> +    } else if (CastAwayQualifiers.hasVolatile()) {
> +      qualifiers = 2;
> +    } else {
> +      // This is a variant of int **x; const int **y = (const int **)x;
> +      Self.Diag(SrcExpr.get()->getLocStart(), diag::warn_cast_qual2) <<
> +        SrcType << DestType;
> +      return;
> +    }
> +    Self.Diag(SrcExpr.get()->getLocStart(), diag::warn_cast_qual) <<
> +      TheOffendingSrcType << TheOffendingDestType << qualifiers;
> +  }
>  }
>  
>  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 +1,29 @@
> +// 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}}
> +  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}}
> +
> +  volatile char *vol = 0;
> +  char *vol2 = (char *)vol; // expected-warning {{cast from 'volatile char *' to 'char *' drops volatile qualifier}}
> +  const volatile char *volc = 0;
> +  char *volc2 = (char *)volc; // expected-warning {{cast from 'const volatile char *' to 'char *' drops const and volatile qualifiers}}
> +
> +  int **intptrptr;
> +  const int **intptrptrc = (const int **)intptrptr; // expected-warning {{cast from 'int **' to 'const int **' must have all intermediate pointers const qualified}}
> +  volatile int **intptrptrv = (volatile int **)intptrptr; // expected-warning {{cast from 'int **' to 'volatile int **' must have all intermediate pointers const qualified}}
> +
> +  int *intptr;
> +  const int *intptrc = (const int *)intptr;    // no warning
> +
> +  const char **charptrptrc;
> +  char **charptrptr = (char **)charptrptrc; // expected-warning {{cast from 'const char *' to 'char *' drops const qualifier}}
> +}

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