Tests for truly nested routing?

[jplatte]

After the discussion in #714, I'm wondering whether all those variants (the updated behavior as of #824) work the same way when the inner router is wrapped in another service, and we hit the Err branch of try_downcast::<Router<B>, _>(svc). Do the tests cover this scenario already?

[davidpdrsn]

In general it seems we need better test coverage for nested opaque routes. This one fails:

#[tokio::test]
async fn nest_at_capture() {
    let api_routes = Router::new().route("/:b", get(|| async {})).boxed_clone();

    let app = Router::new().nest("/:a", api_routes);

    let client = TestClient::new(app);

    let res = client.get("/foo/bar").send().await;
    assert_eq!(res.status(), StatusCode::OK);
}

It gets 404 when it should get 200.