#![feature(type_alias_impl_trait)] #![allow(dead_code)] use std::fmt::Debug; type FooX = impl Debug; trait Foo { } impl Foo<()> for () { } fn foo() -> impl Foo { //~^ ERROR: the trait bound `(): Foo` is not satisfied // FIXME(type-alias-impl-trait): We could probably make this work. () } fn main() { }