[llvm-bugs] [Bug 44282] New: Diagnostic missing from -Wcast-qual in C++

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https://bugs.llvm.org/show_bug.cgi?id=44282

            Bug ID: 44282
           Summary: Diagnostic missing from -Wcast-qual in C++
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: warren_ristow at playstation.sony.com
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

The test-case below has two functions in it, both of which cast a const pointer
to a 'void *', dropping the const qualifier.  I believe they both should
produce a warning when compiled with '-Wcast-qual'.  The function 'foo()'
(which casts a 'const float *' to a 'void *') does not produce the warning,
whereas the function 'bar()' (which casts a 'const void *' to a 'void *') does.
Tested this with llvm 9.0.

$ cat test.cpp
////////////////////////////////////////////////////////////
extern void usePtr(const void *ptr);

void foo(const float *pf_const)
{
  // Casting away const-ness, but no warning appears here.
  usePtr((void *) pf_const);
}

void bar(const void *pv_const)
{
  // Casting away const-ness, and warning does appear here.
  usePtr((void *) pv_const);
}
////////////////////////////////////////////////////////////
$ clang++ -c -Wcast-qual test.cpp
test.cpp:13:19: warning: cast from 'const void *' to 'void *' drops const
        qualifier [-Wcast-qual]
  usePtr((void *) pv_const);
                  ^
1 warning generated.
$

I noticed that an appropriate diagnostic does appear on both of these cases if
the test is compiled as C, rather than C++. So maybe there is some C++ subtlety
that means the 'foo()' example above should not get a warning.  That said, GCC
produces the warning for both cases, in both C and C++ mode (tested GCC 7.4.0).

This looks to be similar to bug 23396, although not quite the same.

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