[Bug c++/64969] generic functions do not work with placeholder return types

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64969

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Or even simpler:

auto f1(auto x) { return *x; }
decltype(auto) f2(auto x) { return *x; }
auto f3(auto x) -> int { return *x; }

int i;
auto r1 = f1(&i);
auto r2 = f2(&i);
auto r3 = f3(&i);