trait Id { type This<'a>; } #[derive(Copy, Clone)] struct W<T>(T); impl Id for W<u32> { type This<'a> = W<u32>; } impl Id for W<i32> { type This<'a> = W<i32>; } trait Trait<T> {} // when using this impl we only normalize after instantiating // with infer vars, at this point `<?t as Id>::This<'a>` is still // ambig, so we keep the alias around and require it to structurally // relate. impl<T: Id> Trait<for<'a> fn(T::This<'a>)> for T {} fn is_trait<T: Trait<U>, U>(_x: T) {} fn main() { let x = W(1); // We prove `W<_>: Trait<for<'a> fn(<W<_>>::This<'a>)>` during // typeck, and `W<i32>: Trait<for<'a> fn(<W<i32>)>` during borrowck. // This then fails. is_trait(x); let _: W<i32> = x; }