[libcxx] Fix gcc 4.9 -Wcast-qual warning.

[halyavin]

Summary

C-style cast from const pointer to non-const pointer causes -Wcast-qual warning in gcc. Fix the problem by casting away const with const_cast. Additionally, replace C-style cast with appropriate C++-style cast.

[halyavin]

ping.

[halyavin]

Thank you for fixing most of warnings. Now it says "cc1plus: warning: unrecognized command line option "-Wno-c++14-compat"" though.

[EricWF]

In D23831#528637, @halyavin wrote:

Thank you for fixing most of warnings. Now it says "cc1plus: warning: unrecognized command line option "-Wno-c++14-compat"" though.

No worries, it was driving me nuts. The addition of -Wno-c++14-compat and -Wno-literal-suffix targets GCC 5.x+ and 7.0+ respectively. It's almost impossible to detect the validity of -Wno-<warning> flags, I fear these warnings are here to stay for older GCC versions. Thankfully GCC does not emit a "unrecognized command line option" diagnostic on otherwise warning-free code, so if we can fix the other diagnostics we can suppress this one.

[EricWF]

The reason I'm dragging my feet on this patch is I'm not sure the two formulations are *exactly* equivalent. I need to re-read the standard to be sure.

What's the motivation for fixing a warning which libc++ never tests against? Normally the headers are treated as system headers so all warnings are ignored.

[halyavin]

We have libc++ in our source tree and compile it as part of our build process. All code in the source tree uses -Werror by default, so I am having compile errors on gcc 4.9.

[EricWF]

In D23831#528666, @halyavin wrote:

We have libc++ in our source tree and compile it as part of our build process. All code in the source tree uses -Werror by default, so I am having compile errors on gcc 4.9.

Are you also compiling with -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER?

[halyavin]

! In D23831#528668, @EricWF wrote:

Are you also compiling with -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER?

No. Libc++ buildbots doesn't set this option either and we do see warnings there. So the pragma doesn't help with compiling libc++ itself.

[halyavin]

False alarm. I figured out that only -Wattributes warnings ignore gcc system_header pragma. So this -Wcast-qual and other warnings doesn't block us.

[EricWF]

Resigning as a reviewer since this revision is dead.

include/type_traits

 
 #else
 
 template <class _Tp>
 inline _LIBCPP_NO_CFI _LIBCPP_INLINE_VISIBILITY
 _Tp*
 addressof(_Tp& __x) _NOEXCEPT
 {
-    return (_Tp*)&reinterpret_cast<const volatile char&>(__x);
+    return reinterpret_cast<_Tp*>(const_cast<char*>(&reinterpret_cast<const volatile char&>(__x)));
 }
 
 #endif // __has_builtin(__builtin_addressof)
 
 #if defined(_LIBCPP_HAS_OBJC_ARC) && !defined(_LIBCPP_PREDEFINED_OBJC_ARC_ADDRESSOF)
 // Objective-C++ Automatic Reference Counting uses qualified pointers
 // that require special addressof() signatures. When
 // _LIBCPP_PREDEFINED_OBJC_ARC_ADDRESSOF is defined, the compiler