Manual PartialEq impl not found for const generics.

[jafioti]

I tried this code:

#[derive(Eq)]
pub enum DimType {
    Static(usize),
    Dynamic
}

impl PartialEq for DimType {
    fn eq(&self, other: &Self) -> bool {
        match self {
            DimType::Static(s) => match other {
                DimType::Static(t) => s == t,
                DimType::Dynamic => true,
            },
            DimType::Dynamic => true,
        }
    }
}

I expected to see this happen: I expected to be able to use DimType as a const generic type. I am able to use it when I derive PartialEq and Eq, but when I manually derive PartialEq it doesn't work.

Instead, this happened: I get an error: tensor::DimType must be annotated with #[derive(PartialEq, Eq)] to be used as the type of a const parameter

Meta

rustc --version --verbose:

rustup 1.24.3 (ce5817a94 2021-05-31)
info: This is the version for the rustup toolchain manager, not the rustc compiler.
info: The currently active `rustc` version is `rustc 1.61.0-nightly (c274e4969 2022-03-05)`

[ChayimFriedman2]

This is not a bug, this is intended. PartialEq and Eq has to be derived so the compiler can ensure comparing types is sound.

See https://github.com/rust-lang/rfcs/blob/master/text/1445-restrict-constants-in-patterns.md.

[jafioti]

@ChayimFriedman2 Is there any way to manually implement PartialEq like I have it then? I am trying to get DimType::Dynamic to be equal to DimType::Static

[wwylele]

Your PartialEq implementation doesn't satisfy the transitivity requirement ( Static(1) == Dynamic == Static(2) => Static(1) == Static(2) ). Exactly because of the existence of this kind of ill-formed PartialEq implementation, the compiler cannot trust a custom PartialEq implementation for const generic dispatch.

[fmease]

Closing as works as intended.