https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86138
--- Comment #15 from Jonathan Wakely <redi at gcc dot gnu.org> --- (In reply to Christian Franke from comment #13) > This patch prevents duplicate _S_empty_rep_storage[] even on Cygwin (char > only, wchar_t missing). Testcase works as expected then: > > --- basic_string.tcc.orig 2018-05-03 06:22:46.000000000 +0200 > +++ basic_string.tcc 2018-06-21 13:00:03.370070700 +0200 > @@ -1597,7 +1597,8 @@ > > // Inhibit implicit instantiations for required instantiations, > // which are defined via explicit instantiations elsewhere. > -#if _GLIBCXX_EXTERN_TEMPLATE > 0 && __cplusplus <= 201402L > +#if _GLIBCXX_EXTERN_TEMPLATE > 0 > +#if __cplusplus <= 201402L > extern template class basic_string<char>; > extern template > basic_istream<char>& > @@ -1627,6 +1628,12 @@ > basic_istream<wchar_t>& > getline(basic_istream<wchar_t>&, wstring&); > #endif > + > +#else > + extern template > + basic_string<char>::size_type > + basic_string<char>::_Rep::_S_empty_rep_storage[]; > +#endif > #endif This will only work for the old ABI, if you compile with _GLIBCXX_USE_CXX11_ABI=1 those declarations are invalid. But I like the basic idea, that will ensure that the part that must be unique is unique, but the member functions that are C++17-only will still be implicitly instantiated as needed. I'll prepare a complete and correct version of this patch.
