An attempt was made to constrain an associated type. For 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 super-traits: Vehicle and Box. Both of these traits define an associated type Color. BoxCar inherits two types with that name from both super-traits. 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>
{}