alias.rs

pub trait Service {
    type Request;

    fn call(&self, request: Self::Request);
}

pub trait OptionService: Service<Request = Option<<Self as OptionService>::Some>> {
    type Some;
}

impl<T, A> OptionService for T
where T: Service<Request = Option<A>>
{
    type Some = A;
}

pub struct SomeString;

impl Service for SomeString {
    type Request = Option<String>;

    fn call(&self, request: Self::Request) {
        println!("GOT: {:?}", request);
    }
}

fn call<T: OptionService<Some = String>>(s: &T) {
    s.call(Some("foo".to_string()));
}

pub fn main() {
    call(&SomeString);
}

gistfile1.txt

$ rustc blanket.rs
error[E0275]: overflow evaluating the requirement `<Self as OptionService>::Some`
 --> blanket.rs:7:1
  |
7 | / pub trait OptionService: Service<Request = Option<<Self as OptionService>::Some>> {
8 | |     type Some;
9 | | }
  | |_^
  |
  = note: required because of the requirements on the impl of `OptionService` for `Self`

error: aborting due to previous error