E0223 - Error codes index
Description: Book listing all Rust error codes

# Error code E0223

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

Erroneous code example:

    trait Trait { type X; }

    fn main() {
        let foo: Trait::X;
    }

The problem here is that we're attempting to take the associated type of `X` from `Trait`. Unfortunately, the type of `X` is not defined, because it's only made concrete in implementations of the trait. A working version of this code might look like:

    trait Trait { type X; }

    struct Struct;
    impl Trait for Struct {
        type X = u32;
    }

    fn main() {
        let foo: <Struct as Trait>::X;
    }

This syntax specifies that we want the associated type `X` from `Struct`'s implementation of `Trait`.

Due to internal limitations of the current compiler implementation we cannot simply use `Struct::X`.