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

[jason at gcc dot gnu.org]

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

--- Comment #11 from Jason Merrill <jason at gcc dot gnu.org> ---
(In reply to Marek Polacek from comment #10)
> Don't know if I should pursue this backport.

Seems like it depends on other fixes, not sure how hard they will be to find. 
Your call.