[Bug c++/85569] [8/9 Regression] is_invocable(F, decltype(objs)...) fails with "not supported by dump_expr#" unless via indirection

[aoliva at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85569

--- Comment #7 from Alexandre Oliva <aoliva at gcc dot gnu.org> ---
I realized the preprocessed testcase was the work-around rather than the base
failure mode.  What confused me was that even the work-around was failing, and
that's what my patch is for.

https://gcc.gnu.org/ml/gcc-patches/2018-11/msg01969.html

I spent some time looking into the initial testcase, but to no avail so far.