[Bug c++/85765] [8/9 Regression] Missing SFINAE in default template argument

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85765

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Those volatile overloads in std::bind are a constant source of problems (most
recently PR 85118).

The regression happened with r258407:

            PR c++/84785 - ICE with alias template and default targs.

            * pt.c (type_unification_real): Set processing_template_decl if
            saw_undeduced == 1.