[Bug c++/120555] [15/16 Regression] Use of auto func before deduction of auto

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120555

--- Comment #2 from Patrick Palka <ppalka at gcc dot gnu.org> ---
The comment #1 testcase might be over-reduced, it's more interesting if the two
branches of the constexpr have different return types:

// PR c++/120555
// { dg-do compile { target c++17 } }

struct A { int m; };

template<class T>
constexpr auto f() {
  if constexpr (__is_same(T, int))
    return 1;
  else
    return A{f<int>()};
}

static_assert(f<bool>().m == 1);
static_assert(f<int>() == 1);