[Bug c++/96926] [9/10/11 Regression] Tuple element w/ member reference to incomplete template type rejected

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96926

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Hmm, this might be a bad reduction. __is_constructible_impl really *is*
incomplete here. In the real code it isn't.