E0222 - Error codes index
Description: Book listing all Rust error codes

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# Error code E0222

An attempt was made to constrain an associated type.

Erroneous code example:

    #![allow(unused)]
    fn main() {
    pub trait Vehicle {
        type Color;
    }

    pub trait Box {
        type Color;
    }

    pub trait BoxCar : Box + Vehicle {}

    fn dent_object<COLOR>(c: dyn BoxCar<Color=COLOR>) {} // Invalid constraint
    }

In this example, `BoxCar` has two supertraits: `Vehicle` and `Box`. Both of these traits define an associated type `Color`. `BoxCar` inherits two types with that name from both supertraits. Because of this, we need to use the fully qualified path syntax to refer to the appropriate `Color` associated type, either `<BoxCar as Vehicle>::Color` or `<BoxCar as Box>::Color`, but this syntax is not allowed to be used in a function signature.

In order to encode this kind of constraint, a `where` clause and a new type parameter are needed:

    #![allow(unused)]
    fn main() {
    pub trait Vehicle {
        type Color;
    }

    pub trait Box {
        type Color;
    }

    pub trait BoxCar : Box + Vehicle {}

    // Introduce a new `CAR` type parameter
    fn foo<CAR, COLOR>(
        c: CAR,
    ) where
        // Bind the type parameter `CAR` to the trait `BoxCar`
        CAR: BoxCar,
        // Further restrict `<BoxCar as Vehicle>::Color` to be the same as the
        // type parameter `COLOR`
        CAR: Vehicle<Color = COLOR>,
        // We can also simultaneously restrict the other trait's associated type
        CAR: Box<Color = COLOR>
    {}
    }