[Bug c++/91678] [9 Regression] decltype returns wrong type under certain conditions

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91678

--- Comment #9 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
I'm not sure yet -- I remember issues I'd seen with the patch, but those issues
were somehow resolved, so the maybe_lvalue_p bit was all that was needed on
trunk.  I need to do a regtest/bootstrap on 9 to see if there's anything broken
by this change.  If not, then this should be fine to backport.