[Bug c++/88815] [9 Regression] is_constexpr (based on narrowing conversion and expression SFINAE) broken

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88815

--- Comment #9 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
(In reply to Marek Polacek from comment #4)
> So one idea would to be to walk_tree on the expression at the end of
> finish_decltype_type, looking for compound literals and call check_narrowing.

Scratch this -- I don't know what I did last night but this is not the
solution.  Perhaps we should create DECLTYPE_TYPE in finish_decltype_type when
there's narrowing.