Implied bounds don't work for traits on &Self #47670
Description
If a trait has a bound on &Self, it doesn't seem to work as an implied bound.
For example (also https://gist.github.com/671c7746b96c86f9eb0e39176325157a):
use std::ops::Add;
pub trait Numoid where
Self: Sized,
for<'a> &'a Self: Add<Self, Output = Self>
{}
impl<N> Numoid for N where
for<'a> &'a N: Add<N, Output = N>
{}
pub fn compute<N: Numoid>(a: N, b: N) -> N
// where for<'a> &'a N: Add<N, Output = N> // redundant bound is required
{ &a + b }This produces:
error[E0277]: the trait bound `for<'a> &'a N: std::ops::Add<N>` is not satisfied
--> src/main.rs:20:1
|
20 | / pub fn compute<N>(a: N, b: N) -> N
21 | | where
22 | | N: Numoid,
23 | | // for<'a> &'a N: Add<N, Output = N>, // redundant bound is required
... |
27 | | b + d
28 | | }
| |_^ no implementation for `&'a N + N`
|
= help: the trait `for<'a> std::ops::Add<N>` is not implemented for `&'a N`
= help: consider adding a `where for<'a> &'a N: std::ops::Add<N>` bound
= note: required by `Numoid`
It works if the 'where' on fn compute is uncommented. Other implicit bounds in the same trait also work (e.g. for<'a> Self: Add<&'a Self> is fine).
This comes up when trying to write generic numeric code. I want to be explicit about using moves/refs to avoid unnecessary clones of things like BigInt, which requires binary operators to have the full set of overloads (i.e. a+b, &a+b, a+&b, &a+&b). BigInt and other numeric types support all these, and they work as explicit bounds, but repeating them on every function gets unwieldy quickly.