#![feature(coerce_unsized)] use std::ops::CoerceUnsized; pub struct Foo<T: ?Sized> { field_with_unsized_type: T, } pub struct Bar<T: ?Sized> { field_with_unsized_type: T, } impl<T, U> CoerceUnsized<Bar<U>> for Foo<T> where T: CoerceUnsized<U> {} //~ ERROR E0377 fn main() {}