[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 #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Eric Fiselier from comment #2)
> Created attachment 43737 [details]
> reproducer2.cpp
> 
> Another reproducer: This one is a regression from 7.3
> https://godbolt.org/g/UEti9f
> 
> Could somebody please take a look at this?

This one started to fail on trunk with r258039

        PR c++/84489 - dependent default template argument

        * pt.c (type_unification_real): Handle early substitution failure.

That fix has just been backported to the 6 and 7 branches, and so also fails
with the tip of those branches.