[libcxx] Don't add -Wall when building in MSVC mode

[mstorsjo]

Summary

The MSVC -Wall (or /Wall) option maps (in clang-cl) to the GCC style
option -Weverything, which we don't really want.

This silences the build with clang-cl, which previously used to
output 100 warnings per translation unit.

[ldionne]

Does libcxxabi need a similar fix? Otherwise LGTM.

[• Quuxplusone]

libcxx/CMakeLists.txt

584

LGTM too. I'm in the habit of treating -W4 as the MSVC equivalent of -Wall; you might consider trying out -W4 here if you haven't already, just for curiosity's sake.

[curdeius]

libcxx/CMakeLists.txt

584

It would be nice indeed to use -W4 if possible, and otherwise -W3 (or lower level).
If -W4/-W3 is not ok, maybe you might add a FIXME comment and we will work on it later (when a Windows CI builder is available)?

[mstorsjo]

Added -W4 for MSVC

[mstorsjo]

In D98035#2606577, @ldionne wrote:

Does libcxxabi need a similar fix? Otherwise LGTM.

Not sure actually, one doesn't really use libcxxabi in MSVC mode, as one normally would run it on top of the MSVC ABI runtime.

libcxx/CMakeLists.txt

584

-W4 actually works fine, amended the commit to include that.

There's D96408 that tries to silence some warnings in -W4 mode, but when building with CMake, we try to add -Wno-unused-parameter anyway, so the warnings that are fixed there shouldn't show up as long as building with cmake instead of custom project files.

[curdeius]

libcxx/CMakeLists.txt

575–583

Nit: better to write if (MSVC) ... else ... (without negation+else).

[mstorsjo]

Avoided the negation+else, simplified the comment wording

libcxx/CMakeLists.txt

if (LIBCXX_HAS_RT_LIB)
       target_compile_definitions(${target} PRIVATE -D_LIBCPP_LINK_RT_LIB)
     endif()
   endif()
 endfunction()
 
 # Warning flags ===============================================================
 function(cxx_add_warning_flags target)
   target_compile_definitions(${target} PUBLIC -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-  target_add_compile_flags_if_supported(${target} PRIVATE -Wall -Wextra -W -Wwrite-strings
+  if (MSVC)
+    # -W4 is the cl.exe/clang-cl equivalent of -Wall. (In cl.exe and clang-cl,
+    # -Wall is equivalent to -Weverything in GCC style compiler drivers.)
+    target_add_compile_flags_if_supported(${target} PRIVATE -W4)
+  else()
+    target_add_compile_flags_if_supported(${target} PRIVATE -Wall)
+  endif()
+  target_add_compile_flags_if_supported(${target} PRIVATE -Wextra -W -Wwrite-strings
                                                           -Wno-unused-parameter -Wno-long-long
                                                           -Werror=return-type -Wextra-semi)
   if ("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
     target_add_compile_flags_if_supported(${target} PRIVATE
       -Wno-user-defined-literals
       -Wno-covered-switch-default
       -Wno-suggest-override
       -Wno-ignored-attributes # FIXME: Caused by _LIBCPP_NODEBUG_TYPE not being supported on older clangs
     )
     if (LIBCXX_TARGETING_CLANG_CL)