[PATCH] libstdc++: Make syncbuf _S_get_mutex instantiation extern.

[Jonathan Wakely]

On Thu, 28 Aug 2025 at 18:35, Nathan Myers wrote:
>
> [Note: do we need entries in config/abi/pre/gnu.ver for these?]

Aren't there test failures without them?

> This declares and defines explicit instantiations of std::syncbuf<T>::
> __mutex::_S_get_mutex for T=char and T=wchar_t, where the function
> is defined at all, the wchar_t case gated on _GLIBCXX_USE_WCHAR_T.

I was thinking we'd just have one set of mutexes for all basic_syncbuf
specializations, e.g. in a non-dependent base class.