https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102012
--- Comment #4 from Patrick Palka <ppalka at gcc dot gnu.org> --- (In reply to 康桓瑋 from comment #3) > (In reply to Patrick Palka from comment #1) > > Hmm, I think this might actually be well-formed according to [temp.names]/9. > > I don't see why we'd need to check satisfaction of a concept-id inside > > decltype if its type is already prescribed to be bool. > > "The standard requires MSVC/Clang's behavior of erroring out." from > https://stackoverflow.com/a/68910627/11638718. Yes, evaluation of the concept-id should produce an error during satisfaction due to the non-bool constraint. But why should we evaluate a concept-id inside decltype in the first place, when the type of a concept-id is always prescribed to be bool?
