An attempt was made to retrieve an associated type, but the type was ambiguous.

Erroneous code example:

```compile_fail,E0221
trait T1 {}
trait T2 {}

trait Foo {
    type A: T1;
}

trait Bar : Foo {
    type A: T2;
    fn do_something() {
        let _: Self::A;
    }
}
```

In this example, `Foo` defines an associated type `A`. `Bar` inherits that type
from `Foo`, and defines another associated type of the same name. As a result,
when we attempt to use `Self::A`, it's ambiguous whether we mean the `A` defined
by `Foo` or the one defined by `Bar`.

There are two options to work around this issue. The first is simply to rename
one of the types. Alternatively, one can specify the intended type using the
following syntax:

```
trait T1 {}
trait T2 {}

trait Foo {
    type A: T1;
}

trait Bar : Foo {
    type A: T2;
    fn do_something() {
        let _: <Self as Bar>::A;
    }
}
```