https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90397
--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
G++ allowed it because the mismatch was in a system header. If you use
-Wsystem-headers then GCC 9.1 gives a warning about it:
/xhome/jwakely/gcc/9.1.0/include/c++/9.1.0/variant: In instantiation of 'class
std::variant<A, B>':
var.cc:14:44: required from here
/xhome/jwakely/gcc/9.1.0/include/c++/9.1.0/variant:1559:34: warning:
declaration of 'template<long unsigned int _Np, class _Vp> constexpr
decltype(auto) std::__detail::__variant::__get(_Vp&&)' has a different
exception specifier [-Wsystem-headers]
1559 | friend constexpr decltype(auto) __detail::__variant::__get(_Vp&& __v);
| ^~~~~~~~
/xhome/jwakely/gcc/9.1.0/include/c++/9.1.0/variant:263:5: note: from previous
declaration 'template<long unsigned int _Np, class _Variant> constexpr
decltype(auto) std::__detail::__variant::__get(_Variant&&) noexcept'
263 | __get(_Variant&& __v) noexcept
| ^~~~~
If you use -pedantic-errors then you get an error.
