#![feature(const_generics)]
#![feature(const_evaluatable_checked)]
struct A<const N: usize>;
impl<const N: usize> A<N> {
    const fn f() -> usize {
        if N == 0 {
            0
        } else {
            N + A::<{ N - 1 }>::f()
        }
    }
}
fn main() {
    const a: usize = A::<{ 5 }>::f();
}

报错unconstrained generic constant