[Bug c++/78489] [6/7/8 Regression] Substitution failure does not happen in lexical order for NTTP declarations.

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78489

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This started to fail with r223301

        DR 1391
        * pt.c (type_unification_real): Check convertibility here.
        (unify_one_argument): Not here.