[Bug c++/92771] [9/10 Regression] Concept won't use default template argument

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92771

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #1)
> trunk accepts with -fconcepts (?!?)

At r276764 it started to be accepted with -std=gnu++14 -fconcepts but is still
rejected with -std=gnu++17 -fconcepts or -std=gnu++2a -fconcepts. Odd.