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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;
}
}
```
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