Rust error E0222: ambiguous associated type `Color` in bounds of `BoxCar`

The compiler team's explanation

An attempt was made to constrain an associated type.

Erroneous code example:

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:

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>
{}

From rustc --explain E0222 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.