[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 #9 from Jonathan Wakely <redi at gcc dot gnu.org> ---
There's no assignment operator from std::allocator<char>, so it's not
equivalent to construction.