`#[const_trait]` and super trait error

[fee1-dead]

I tried this code:

#![feature(const_trait_impl)]

#[const_trait]
pub trait Index {
    type Output;
}

#[const_trait]
pub trait IndexMut: Index {
    fn foo(&mut self) -> <Self as Index>::Output;
}

I expected to see this happen: compiles
Instead, this happened: errors

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