Trait information is discarded

[yongqli]

I would expect this to compile, but it doesn't. Is this intentional? It seems that M2 discards the f64: Mul<Self, Output=Self> constraint.

use std::ops::{Mul};

pub trait M: Sized where f64: Mul<Self, Output=Self> {
}

pub trait M2: M {
    fn f(self, a: f64) -> Self {
        a * self
    }
}

Error is:

error: the trait `core::ops::Mul<Self>` is not implemented for the type `f64` [E0277]
<anon>:11         a * self

[arielb1]

Non-supertrait predicates (i.e. predicates not of the form Self: Trait<..>) are not automatically elaborated. You have to add them manually:

use std::ops::{Mul};

pub trait M: Sized where f64: Mul<Self, Output=Self> {
}

pub trait M2: M
        where f64 : Mul<Self, Output=Self> // <- ADDED
{
    fn f(self, a: f64) -> Self {
        a * self
    }
}

fn main() {}

We should document this better, through.

[yongqli]

Should this be an RFC? Because it's quite tedious to repeat things the compiler already knows about.

[steveklabnik]

Yes, this is currently expected behavior (at least, as I understand it) and would require an RFC to change.