[cmake] Make sure that -Wcast-qual is not a error in preparation for clang's -Wcast-qual for C++

[lebedev.ri]

Summary

Diagnostic -Wcast-qual is not enabled by default in gcc/clang, It was enabled explicitly some time ago.
Later, clang has learned to do -Wcast-qual, like gcc does.
While that clang diagnostic is compatible with gcc's, it additionally catches one more case, see D34153.
And last but not least, clang's diagnostic -Wcast-qual only works for C code (sic!)
In D33102, i have extended it to work for C++ code too.
Once that landed, -Werror buildbots broke. That is when i was made aware that this diagnostic is enabled in llvm builds.
I have fixed that particular buildbot-discovered failure in D34153.
However, when i tried building clang stage2 with self-built clang stage1 (with D33102), i have quickly discovered that some sub-projects also exhibit this warning.
Specifically, openmp and libcxx subprojects produce 401 -Wcast-qual warnings, and i believe not all of these warnings are clang-specific.
(i don't think i have all subprojects checked-out, so there may be even more)
This tells two things:

• these subprojects are not being built by -Werror buildbots. (at least as part of LLVM, not standalone)
• no one builds LLVM by GCC with these subprojects with -Werror enabled.

So, if D33102 is to land, this may not break -Werror buildbots. This is bad and good.
But. If someone wants to build LLVM with these subprojects and with -Werror, he/she/it will face a lot of fatal warnings.
It is unfeasible for me to fix all these warnings before re-landing D33102, thus i propose this diff.

After D33102 has landed, the buildbots should probably be adjusted to catch these cases, and i will see if i can help fix these issues and revert this patch.

Thoughts?

[mclow.lists]

As of r305410, libc++ passes all the tests w/ -Wcast-qual enabled.

[lebedev.ri]

Ok, so i suppose libc++ is fine now, thanks to @mclow.lists
I can not build lldb, so i can not check whether it has similar issues.
This leaves only the OpenMP?

To reiterate, there are several solutions:

1. Do nothing, just land D33102, i'm under impression no current buildbot will break. That, however, will break anyone who builds with openmp subproject and with -DLLVM_ENABLE_WERROR=ON
2. Disable -Wcast-qual in openmp
3. This approach, which is wrong - disable it globally
4. Wait until openmp is fixed. I suppose i could help, though it is really better if someone with the actual knowledge about the code fixes these warnings...

I'd really like to hear any thoughts from the openmp people :)

[AndreyChurbanov]

I will try to make cleanup of the OpenMP runtime sources, hopefully this week.

• Andrey

[lebedev.ri]

In D34176#786840, @AndreyChurbanov wrote:

I will try to make cleanup of the OpenMP runtime sources,

That would be lovely :)
I sure can try to help, if needed.

hopefully this week.

• Andrey

[lebedev.ri]

Will be superseded by D34759

[lebedev.ri]

Abandoning.
All preparatory fixes has landed.

cmake/modules/HandleLLVMOptions.cmake

if (MSVC AND NOT CLANG_CL)
   foreach(flag ${msvc_warning_flags})
     append("${flag}" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
   endforeach(flag)
 endif (MSVC AND NOT CLANG_CL)
 
 if (LLVM_ENABLE_WARNINGS AND (LLVM_COMPILER_IS_GCC_COMPATIBLE OR CLANG_CL))
   append("-Wall -W -Wno-unused-parameter -Wwrite-strings" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
   append("-Wcast-qual" CMAKE_CXX_FLAGS)
+  append("-Wno-error=cast-qual" CMAKE_CXX_FLAGS)
 
   # Turn off missing field initializer warnings for gcc to avoid noise from
   # false positives with empty {}. Turn them on otherwise (they're off by
   # default for clang).
   check_cxx_compiler_flag("-Wmissing-field-initializers" CXX_SUPPORTS_MISSING_FIELD_INITIALIZERS_FLAG)
   if (CXX_SUPPORTS_MISSING_FIELD_INITIALIZERS_FLAG)
     if (CMAKE_COMPILER_IS_GNUCXX)
       append("-Wno-missing-field-initializers" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)