[Bug libstdc++/85965] [8 Regression] G++ gives cryptic error instead of incomplete type

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85965

--- Comment #14 from Jonathan Wakely <redi at gcc dot gnu.org> ---
A new patch to fix this was posted to:
https://gcc.gnu.org/ml/gcc-patches/2019-05/msg00863.html