[Bug libstdc++/87982] No error for std::generate_n(ptr, ptr, f)

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87982

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
std::fill_n has the same problem (in both overloads of __fill_n_a).