https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71965
--- Comment #5 from Andrew Sutton <andrew.n.sutton at gmail dot com> --- Hmm... I haven't looked at this in a while. It looks like the expansion of ConstructibleObject<int[2][2], int[2][2]> is triggering the diagnostic. I'm a little surprised that this gets diagnosed -- especially with a "sorry". But it still shouldn't happen. The implementation (I suspect this comes out of satisfy_check_constraint) appears to be substituting through too aggressively. It expands the concept using the instantiated arguments, the pair of int[2][2]s, around constraint.cc:2056. I'm guessing that that ultimately triggers a substitution of those arguments into the requirements of the requires-expression. Bang! error. One solution would be to make expand_concept() simply return the definition and then decompose that expression, substituting when needed.
