https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99599
--- Comment #16 from danakj at orodu dot net --- Well for anyone who hits the same issue, it appears that GCC _does_ follow Clang and MSVC in not considering the overload and chasing through the concept resolution if the non-concept types are templates and do not match the caller's arguments. So you need to do: 1) For non-GCC just use: template<fooable T> auto invoke_tag(bar_tag, T it); 2) For GCC non-template type bar_tag use: template<std::same_as<bar_tag> T, fooable U> auto invoke_tag(T, U it); 3) For GCC template type bar_tag, back to 1) template<class P, fooable T> auto invoke_tag(bar_tag<P>, T it); Note also that 2) uses same_as, not convertible_to as in Comment #6, otherwise you can get ambiguous overload resolution if multiple types convert to one, which does not occur in Clang/MSVC with the regular type parameter. This _does_ again result in more code that will compile in Clang/MSVC than in GCC, as it prevents conversions from types that don't have an overload. The macros to do this get rather exciting, if that's of interest to someone in the future: https://github.com/chromium/subspace/pull/253/commits/719500c4d2cbfcfd238d7ee3c5b3d371f40e46c1
