[Bug c++/89793] Implicit conversion to std::string is ambiguous on GCC 8.2 but not GCC 7.3

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89793

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I haven't analysed whether it's valid yet, but it started to fail with r258755

            PR c++/81311 - wrong C++17 overload resolution.

            * call.c (build_user_type_conversion_1): Remove C++17 code.
            (conv_binds_ref_to_prvalue): New.
            (build_over_call): Handle C++17 copy elision.
            (build_special_member_call): Only do C++17 copy elision here if the
            argument is already the right type.