[Bug libstdc++/90745] [11/12/13/14 Regression] std::tuple::operator= parameter causes error outside immediate context

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90745

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Replacing delctype(auto) on the cell::operator=(X&&) function (or constraining
that function to not be instantiated for non-assignable cells) will fix the
code.