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 constraintIn 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>
{}