https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67070
--- Comment #2 from Andrew Sutton <andrew.n.sutton at gmail dot com> --- > This is a very subtle point. It seems to me that it would be better if > creating the normal form of a constaint stops substituting into concept bodies > once it's clear that we're inside an atomic constraint. So f would have a > single atomic predicate constraint > > !A<T> > > and then when we consider this later we try to satisfy A<B>, which fails, so > f's constraint is satisfied and the testcase succeeds. Though this does > create > an inconsistency in argument substitution behavior across the ! operator, > which > might be confusing. > > Any thoughts, Andrew? I had a bug report filed against that behavior earlier this year (note Bugzilla, just an outside email). If you stop substituting through atomic constraints, then requirements in e..g, A<T> would not be substituted in the immediate context. Basically, it makes any concepts behind a ! not SFINAE friendly. If we had a rule that allowed the evaluation of a concept in any context, then we could avoid doing this. It would also guarantee the ability to write; static_assert(C<T>); This is probably a good issue to raise against the TS, and we should discuss it in Kona. Andrew
