https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98465
--- Comment #6 from Martin Sebor <msebor at gcc dot gnu.org> ---
Different -std= settings also affect the warning, again as a result of inlining
and the explicit instantiation declarations. The basic_string.tcc file has the
following block toward the end which explains when implicit instantiation is
suppressed:
// Inhibit implicit instantiations for required instantiations,
// which are defined via explicit instantiations elsewhere.
#if _GLIBCXX_EXTERN_TEMPLATE
// The explicit instantiation definitions in src/c++11/string-inst.cc and
// src/c++17/string-inst.cc only instantiate the members required for C++17
// and earlier standards (so not C++20's starts_with and ends_with).
// Suppress the explicit instantiation declarations for C++20, so C++20
// code will implicitly instantiate std::string and std::wstring as needed.
# if __cplusplus <= 201703L && _GLIBCXX_EXTERN_TEMPLATE > 0
extern template class basic_string<char>;
# elif ! _GLIBCXX_USE_CXX11_ABI
// Still need to prevent implicit instantiation of the COW empty rep,
// to ensure the definition in libstdc++.so is unique (PR 86138).
extern template basic_string<char>::size_type
basic_string<char>::_Rep::_S_empty_rep_storage[];
# endif
