Corner cases of "modint" when mod = 1

[rsk0315]

ac-library-rs/src/modint.rs

Line 676 in d2b35ac

let mut r = Self::raw(1);

ac-library-rs/src/modint.rs

Line 1046 in d2b35ac

impl<M: Modulus> Product<_> for StaticModInt<M> { fn product(_) -> _ { _(Self::raw(1), Mul::mul) } }

ac-library-rs/src/modint.rs

Line 1048 in d2b35ac

impl<I: Id > Product<_> for DynamicModInt<I> { fn product(_) -> _ { _(Self::raw(1), Mul::mul) } }

The identity of multiplication should be Self::new(1)(not raw but new), because of following (corner) cases:

use ac_library_rs::ModInt;

ModInt::set_modulus(1); // !!

let x: ModInt = std::iter::empty::<ModInt>().product();
assert_eq!(x.val(), 0);

let y = ModInt::new(123).pow(0);
assert_eq!(y.val(), 0);

Note that the original ACL allows to use modint::set_mod(1).