Const parameter defaults are not used as a fallback during type inference

[SUPERCILEX]

I tried this code:

struct A<const N: usize = 64> {}

// impl A { // Works because it's specialized to 64
impl<const N: usize> A<N> { // Fails
    fn new() -> Self {
        todo!();
    }
}

pub fn blah() {
    A::new();
}

I expected to see this happen: the const generic would be inferred to 64.

Instead, this happened: fails to compile. An attempt to fix it by doing impl<const N: usize = 64> doesn't work because it's disallowed.

[fmease]

This has nothing to do with “specialization” (#![feature(specialization)]) but with the known issue of type / const parameter defaults not being used as a fallback for failed inference. Duplicate of the closed issue #83687.

[SUPERCILEX]

Whoops, is there more accurate terminology?

About that issue, why was it closed? This seems like something worth fixing.

[fmease]

Closing in favor of #98931 which contains a bit more discussion.