Consider the following code:
use std::ops::Add;
trait Trait
where Self::Assoc: Add<Self::Assoc, Output=Self::Assoc>
+ for <'a> Add<&'a Self::Assoc, Output=Self::Assoc>
{
type Assoc;
fn get(&self) -> Self::Assoc;
}
fn add1<T: Trait>(x: T, y: T) -> T::Assoc {
x.get() + y.get()
}
This fails to compile with:
error[E0308]: mismatched types
--> src/lib.rs:12:15
|
12 | x.get() + y.get()
| ^^^^^^^
| |
| expected reference, found associated type
| help: consider borrowing here: `&y.get()`
|
= note: expected reference `&<T as Trait>::Assoc`
found associated type `<T as Trait>::Assoc`
I'm able to work around the issue by explicitly specifying what trait I want to use:
fn add2<T: Trait>(x: T, y: T) -> T::Assoc {
<T::Assoc as Add<T::Assoc>>::add(x.get(), y.get())
}
But I'm wondering: why is this happening? Also, is there any more compact work around?
Note that the order of the trait bounds matters. If I change them to:
where Self::Assoc: for <'a> Add<&'a Self::Assoc, Output=Self::Assoc>
+ Add<Self::Assoc, Output=Self::Assoc>
... then add1 compiles fine, but this fails:
fn add3<T: Trait>(x: T, y: T) -> T::Assoc {
x.get() + &y.get()
}