[Bug c++/81239] std::__cxx11::string& visible in gcc warning output

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81239

--- Comment #9 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It's the ABI that meets the requirements of the C++11 standard, but that
doesn't mean it's enabled by the -std=c++11 flag. The -std flags never change
ABI, irrespective of the names used for attributes and namespaces.