absl::meta Alignment test, CMake-based build compilation issue with Visual Studio 2017 (>= v15.8)

[Mizux]

Description

With Visual Studio 2017 >= 15.8, there is a (breaking ABI) change on extended alignment.

Before VS 2017 15.8, the member type would non-conformingly have an alignment of only alignof(max_align_t). VS 2017 15.8 was fixed to handle this correctly, but the fix inherently changes layout and breaks binary compatibility (only for uses of aligned_storage with extended alignments). Please define either (1) _ENABLE_EXTENDED_ALIGNED_STORAGE to acknowledge that you understand this message and that you actually want a type with an extended alignment, or (2) _DISABLE_EXTENDED_ALIGNED_STORAGE to silence this message and get the old non-conformant behavior.

Protocol

This error C2338, can be easily reproduce with Appveyor CI jobs or as follow

call "C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Auxiliary\Build\vcvars64.bat"
cmake -H. -Bbuild -DABSL_RUN_TESTS=ON -DABSL_USE_GOOGLETEST_HEAD=ON -G "Visual Studio 15 2017 Win64"
cmake --build build --config Release --target ALL_BUILD
cmake --build build --config Release --target RUN_TESTS

Got error:

uilding Custom Rule C:/projects/abseil-cpp/absl/meta/CMakeLists.txt
  CMake does not need to re-run because C:/projects/abseil-cpp/build/absl/meta/CMakeFiles/generate.stamp is up-to-date.
  type_traits_test.cc
c:\projects\abseil-cpp\absl\base\throw_delegate_test.cc(64): warning C4101: 'e': unreferenced local variable [C:\projects\abseil-cpp\build\absl\base\throw_delegate_test_bin.vcxproj]
  c:\projects\abseil-cpp\absl\base\throw_delegate_test.cc(90): note: see reference to function template instantiation 'void `anonymous-namespace'::ExpectThrowNoWhat<std::bad_function_call>(void (__cdecl *)(void))' being compiled
  absl_symbolize.vcxproj -> C:\projects\abseil-cpp\build\absl\debugging\Release\absl_symbolize.lib
c:\projects\abseil-cpp\absl\meta\type_traits_test.cc(742): error C2338: You've instantiated std::aligned_storage<Len, Align> with an extended alignment (in other words, Align > alignof(max_align_t)). Before VS 2017 15.8, the member type would non-conformingly have an alignment of only alignof(max_align_t). VS 2017 15.8 was fixed to handle this correctly, but the fix inherently changes layout and breaks binary compatibility (*only* for uses of aligned_storage with extended alignments). Please define either (1) _ENABLE_EXTENDED_ALIGNED_STORAGE to acknowledge that you understand this message and that you actually want a type with an extended alignment, or (2) _DISABLE_EXTENDED_ALIGNED_STORAGE to silence this message and get the old non-conformant behavior.
...

src: https://ci.appveyor.com/project/Mizux/abseil-cpp/builds/19577145/job/xfqk597tlgih3uj1#L353