emsdk wstring_convert deprecation probe

Bumping MeshLib/MeshInspector from emsdk 4.0.19 to 5.0.7 made this fail:

__locale_dir/wstring_convert.h:179:1: error: 'wstring_convert<std::codecvt_utf8<wchar_t>>'
    is deprecated [-Werror,-Wdeprecated-declarations]
  note: in instantiation of member function 'to_bytes' requested here
  MRStringConvert.cpp:28: note: in instantiation of member function 'to_bytes' requested here

even though the call sites are wrapped in #pragma clang diagnostic ignored "-Wdeprecated-declarations". The pragma cannot reach it: libc++ reports the diagnostic against its own header, at the declarator of the out-of-line to_bytes definition, not against our call.

We fixed it with a per-file -Wno-deprecated-declarations, then went looking for the upstream change that made it start happening. Source inspection did not find one:

• emscripten's vendored libc++ has the same construct in both versions - the out-of-line to_bytes, the same _LIBCPP_DEPRECATED_IN_CXX17 macro chain, the same #pragma GCC system_header. Only the file split (<locale> -> __locale_dir/wstring_convert.h) and a dropped _LIBCPP_TEMPLATE_VIS differ.
• upstream clang 21.1 and 22.1 both warn on the same code through libc++, and today's trunk does not. So the diagnostic is not new in the LLVM 23 that emsdk 5.0.7 ships; if anything it was removed afterwards.

So the premise needed testing rather than more reading. This repo compiles that one translation unit under every emsdk version across the range, with MeshLib's own warning flags, and reports whether the warning appears and whether -Werror rejects it.

Read the results by grepping the run log for RESULT and RESULT_WERROR.

Result

Not a toolchain change at all. Every emsdk from 4.0.19 to 6.0.9 behaves identically:

how the TU is compiled	4.0.19 ... 6.0.9
plain	warns, -Werror rejects
through a PCH	warns, -Werror rejects
through a PCH that included <locale> inside a -Wdeprecated-declarations-ignored region	clean

So whether the warning appears depends on where the header is first lexed, not on the compiler version: clang records diagnostic state per source location, and a header first pulled in inside an ignored region stays ignored for the rest of the build.

That is what MeshLib was relying on without knowing it. Its PCH reaches <locale> through MRHashMap.h / MRSpdlog.h, which ignore -Wdeprecated-declarations around their includes on emscripten. Two libc++ commits decide the outcome:

• llvm/llvm-project#146650 (ca697096a, 2025-07-05) granularized <locale>, moving wstring_convert into __locale_dir/wstring_convert.h. emsdk 4.0.19 vendors a libc++ from before that split, so the definition was still inside the file the PCH had stamped as ignored. From emsdk 5.0.0 the split is in, the header is first reached elsewhere (Eigen -> <complex> -> <sstream> -> <istream>), and the warning fires at the out-of-line to_bytes declarator - where a call-site pragma cannot reach it.
• llvm/llvm-project#201633 (merged 2026-06-04) fixes it properly, wrapping that definition in _LIBCPP_SUPPRESS_DEPRECATED_PUSH/POP. Its description is our exact complaint. emsdk 6.0.9 has the macro around the destructor and from_bytes but not around to_bytes, which is why 6.0.9 still warns here.

MeshLib therefore needs -Wno-deprecated-declarations on MRStringConvert.cpp for emsdk 5.0.0 through at least 6.0.9, and can drop it once a vendored libc++ carries #201633.