https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102975
--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
(In reply to Patrick Palka from comment #2)
> (In reply to Andrew Pinski from comment #1)
> > Hmm, this is interesting:
> > template<class> concept Never = false;
> > template<class T> concept C = Never<typename T::type>;
> > void f() {
> > struct X {
> > // using type = int;
> > };
> > static_assert(not C<X>);
> > }
> >
> > is able to compile. I don't know enough about C++ concepts to say if this is
> > valid or not but it looks like the type is really unused in the above case
> > ...
> > This is different from "constexpr bool" which requires the type to be
> > defined ...
>
> Yeah, when evaluating a concept-id such as C<X> (which is done by
> substituting {X} into the normal form of C), the normalization process
> (https://eel.is/c++draft/temp.constr.normal) ignores arguments to unused
> template parameters.
>
> So since 'Never' doesn't use its template parameter, during normalization of
> C we discard the template argument 'typename T::type' passed to Never. The
> normal form of C ends up being 'false (with empty parameter mapping)' which
> is trivially satisfied for all T.
... trivially _unsatisfied_ rather
