I don't think pattern matching and union-types makes narrow typing useless. Rust has both pattern matching and union types but still implements some specific forms of control-flow based typing.
For example, consider this Rust snippet :
pub fn positive1(x: isize) -> Option<usize> {
if x > 0 { Some(x as usize) } else { None }
}
Unless I'm mistaken, without narrow typing, this cast would be impossible, and there would be no way to avoid the redundant runtime check caused by try_into().unwrap(), that is not even optimized unless you trick the compiler.
> Unless I'm mistaken, without narrow typing, this cast would be impossible
same sized integer casts in Rusts are no-ops [0]; the conditional isn't type narrowing, it just avoids the cases where the cast would not preserve the same semantic value.
For example, consider this Rust snippet :
Unless I'm mistaken, without narrow typing, this cast would be impossible, and there would be no way to avoid the redundant runtime check caused by try_into().unwrap(), that is not even optimized unless you trick the compiler.