Bad unconstrained type parameter error

[philberty]

I tried this code:

trait A<T> {                                                                                                         
    type Output;                                                                                                                         
                                                                        
    fn test(self, a: &T) -> &Self::Output;
}

struct Foo<T>{
    start: T,
    end: T,
}

impl<T> A<T> for Foo<usize> {
    type Output = T;

    fn test(self, a: &T) -> &Self::Output {
        a
    }
}

I expected to see this happen: compile without error

Instead, this happened:

<source>:12:6: error: unconstrained type parameter
   12 | impl<T> A<T> for Foo<usize> {
      |      ^

see: https://godbolt.org/z/aK4T4xEGq

Meta

• What version of Rust GCC were you using, git sha if possible. b82408f