trait Id { type This<'a>; } impl<T> Id for T { type This<'a> = T; } trait Trait<T> {} impl<T: Id> Trait<for<'a> fn(T::This<'a>)> for T {} trait Method<T: Id> { fn call_me(&self) where T: Trait<for<'a> fn(T::This<'a>)>; } impl<T, U> Method<U> for T { fn call_me(&self) {} } fn generic<T: Id>(x: &dyn Method<T>) { // Proving `T: Trait<for<'a> fn(T::This<'a>)>` holds. x.call_me(); } fn main() { // Proving `u32: Trait<fn(u32)>` fails due to incompleteness. // We don't add the method to the vtable of `dyn Method`, so // calling it causes UB. generic::<u32>(&()); }