E0221 - Error codes index
Description: Book listing all Rust error codes

# Error code E0221

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

Erroneous code example:

    #![allow(unused)]
    fn main() {
    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:

    #![allow(unused)]
    fn main() {
    trait T1 {}
    trait T2 {}

    trait Foo {
        type A: T1;
    }

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