Lambdas have unique types, and you can't use a generic parameter in a return type (which is impl Trait's raison d'etre), so I think it would have to be this:

    fn compose<T, F: Fn(T) -> T, G: Fn(T) -> T>(f: F, g: G) -> impl Fn(T) - > T

or:

    fn compose<T, F, G>(f: F, g: G) -> impl Fn(T) -> T
      where F: Fn(T) -> T, G: Fn(T) -> T