Silence new warning C4877 when overriding deprecated non-pure virtual functions

[StephanTLavavej]

• Pre-existing: ABI warning C5291 is off-by-default.
  • Followup to Silence ABI warning C5291 for the entire STL #5645 on 2025-07-15.
• Add new off-by-default warning C4877 to _STL_DISABLE_DEPRECATED_WARNING.
  • Implemented by MSVC-PR-734301 on 2026-05-01.
  • I should revert selfbuild fix MSVC-PR-734920 on 2026-05-04.

Example

C:\Temp>type meow.cpp

#include <__msvc_all_public_headers.hpp>

int main() {}

Before

C:\Temp>cl /EHsc /nologo /Wall /std:c++latest /MTd /Od /c meow.cpp
meow.cpp
D:\msvc\binaries\amd64chk\inc\xlocale(2321): warning C4877: overriding non-pure virtual function 'std::codecvt<char16_t,char,mbstate_t>::~codecvt': warning STL4020: std::codecvt<char16_t, char, mbstate_t>, std::codecvt<char32_t, char, mbstate_t>, std::codecvt_byname<char16_t, char, mbstate_t>, and std::codecvt_byname<char32_t, char, mbstate_t> are deprecated in C++20. You can define _SILENCE_CXX20_CODECVT_FACETS_DEPRECATION_WARNING or _SILENCE_ALL_CXX20_DEPRECATION_WARNINGS to suppress this warning.
D:\msvc\binaries\amd64chk\inc\xlocale(2335): warning C4877: overriding non-pure virtual function 'std::codecvt<char32_t,char,mbstate_t>::~codecvt': warning STL4020: std::codecvt<char16_t, char, mbstate_t>, std::codecvt<char32_t, char, mbstate_t>, std::codecvt_byname<char16_t, char, mbstate_t>, and std::codecvt_byname<char32_t, char, mbstate_t> are deprecated in C++20. You can define _SILENCE_CXX20_CODECVT_FACETS_DEPRECATION_WARNING or _SILENCE_ALL_CXX20_DEPRECATION_WARNINGS to suppress this warning.
D:\msvc\binaries\amd64chk\inc\xlocale(2350): warning C4877: overriding non-pure virtual function 'std::codecvt<char16_t,char8_t,mbstate_t>::~codecvt': warning STL4047: std::codecvt<char16_t, char8_t, mbstate_t>, std::codecvt<char32_t, char8_t, mbstate_t>, std::codecvt_byname<char16_t, char8_t, mbstate_t>, and std::codecvt_byname<char32_t, char8_t, mbstate_t> are deprecated by LWG-3767. You can define _SILENCE_CXX20_CODECVT_CHAR8_T_FACETS_DEPRECATION_WARNING or _SILENCE_ALL_CXX20_DEPRECATION_WARNINGS to suppress this warning.
D:\msvc\binaries\amd64chk\inc\xlocale(2364): warning C4877: overriding non-pure virtual function 'std::codecvt<char32_t,char8_t,mbstate_t>::~codecvt': warning STL4047: std::codecvt<char16_t, char8_t, mbstate_t>, std::codecvt<char32_t, char8_t, mbstate_t>, std::codecvt_byname<char16_t, char8_t, mbstate_t>, and std::codecvt_byname<char32_t, char8_t, mbstate_t> are deprecated by LWG-3767. You can define _SILENCE_CXX20_CODECVT_CHAR8_T_FACETS_DEPRECATION_WARNING or _SILENCE_ALL_CXX20_DEPRECATION_WARNINGS to suppress this warning.

For [[deprecated("reason")]], C4877 repeats the reason, otherwise it just says "was declared deprecated".

After

C:\Temp>cl /EHsc /nologo /Wall /std:c++latest /MTd /Od /c meow.cpp /I D:\GitHub\STL\stl\inc
meow.cpp

C:\Temp>

As we don't officially attempt to be /Wall-clean, we don't have /Wall test coverage, but the internal compiler selfbuild will be an effective test (after I remove the workaround in the compiler's sources).

[Copilot]

Pull request overview

Note

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Updates the STL’s deprecated-warning suppression macro to also silence a newly introduced MSVC /Wall warning (C4877) that appears when overriding deprecated non-pure virtual functions.

Changes:

• Clarify the classification comment for warning C5291 as /Wall-only.
• Extend _STL_DISABLE_DEPRECATED_WARNING to disable C4877 in addition to C4996.
• Add inline documentation describing C4877 and its relationship to C4996.

Show a summary per file

File	Description
stl/inc/yvals_core.h	Adds C4877 to the deprecated-warning suppression pragma and updates warning documentation comments.

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• Files reviewed: 1/1 changed files
• Comments generated: 1