[Bug c++/120917] warning: use of 'auto' in template argument only available with '-fconcepts-ts'

[jason at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120917

--- Comment #23 from Jason Merrill <jason at gcc dot gnu.org> ---
(In reply to Frank Heckenbach from comment #21)
> So my question stands.

Then yes, it does look like it should work; libstdc++ uses that 'requires
function call' pattern in

  template<typename _Tp>
    concept __is_derived_from_optional = requires (const _Tp& __t) {
      []<typename _Up>(const optional<_Up>&){ }(__t);
    };