Rust error E0221: ambiguous associated type `A` in bounds of `Self`

What it meansAn attempt was made to retrieve an associated type, but the type was ambiguous.

What rustc prints

error[E0221]: ambiguous associated type `A` in bounds of `Self`
  --> main.rs:11:16
   |
 5 |     type A: T1;
   |     ---------- ambiguous `A` from `Foo`
...
 9 |     type A: T2;
   |     ---------- ambiguous `A` from `Bar`
10 |     fn do_something() {
11 |         let _: Self::A;
   |                ^^^^^^^ ambiguous associated type `A`
   |
help: use fully-qualified syntax to disambiguate
   |
11 -         let _: Self::A;
11 +         let _: <Self as Foo>::A;
   |
help: use fully-qualified syntax to disambiguate
   |
11 -         let _: Self::A;
11 +         let _: <Self as Bar>::A;
   |

Real output of rustc 1.99.0 compiling the official example below (2021 edition). Your names and line numbers will differ; the first line is the part to match.

E0221Types and traits

Activein rustc 1.99.0

8related errors linked below

The compiler team's explanation

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

Erroneous code example:

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;
    }
}

From rustc --explain E0221 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.

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Often hit nextE0308: mismatched typesA related error from the same family.Read it next →

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Rust error E0221: ambiguous associated type `A` in bounds of `Self`

What rustc prints

The compiler team's explanation

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