src/test/ui/feature-gates/feature-gate-type_alias_impl_trait.rs

type Foo = impl std::fmt::Debug; //~ ERROR `impl Trait` in type aliases is unstable

trait Bar {
    type Baa: std::fmt::Debug;
    fn define() -> Self::Baa;
}

impl Bar for () {
    type Baa = impl std::fmt::Debug; //~ ERROR `impl Trait` in type aliases is unstable
    fn define() -> Self::Baa { 0 }
}

fn define() -> Foo { 0 }

fn main() {}