Constraint information is not propagated

[yongqli]

I would expect this code to compile, but it does not:

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
    }
}

Having to repeat constraints leads to more verbose code and is only repeating information that the compiler already knows.

See previous discussion: rust-lang/rust#26564

[withoutboats]

I didn't even know constraining by something like f64: Mul<Self, Output=Self> was valid.

Binary operator overloads are somewhat unusual in that there is no inherent conceptual Self trait (lhs being chosen arbitrary), but for most traits there would be little reason to attempt to constrain it in this way. And really, there is not much reason I can see not to write:

pub trait M: Sized + Mul<f64, Output=Self>

[daxpedda]

I believe this is a duplicate of #2089.