trait Sup<T> { type Assoc; } impl<T> Sup<T> for () { type Assoc = T; } impl<T, U> Dyn<T, U> for () {} trait Dyn<A, B>: Sup<A, Assoc = A> + Sup<B, Assoc = B> {} trait Trait { type Assoc; } impl Trait for dyn Dyn<(), ()> { type Assoc = &'static str; } impl<A, B> Trait for dyn Dyn<A, B> { type Assoc = usize; } fn call<A, B>(x: usize) -> <dyn Dyn<A, B> as Trait>::Assoc { x } fn main() { let x: &'static str = call::<(), ()>(0xDEADBEEF); println!("{x}"); }