pub struct Bar<T: Foo> {}

impl<T> Bar<T>
where
    T: Foo,
{
    pub fn dumb(&self) {}
}

pub trait Foo {
    fn qux(bar: &Bar<impl Foo>) -> impl Send + Sync;
}

pub struct Baz;

impl Foo for Baz {
    fn qux(bar: &Bar<impl Foo>) -> impl Send {
        bar.dumb();
    }
}

fn main() {}

error[E0392]: type parameter `T` is never used
 --> src/main.rs:1:16
  |
1 | pub struct Bar<T: Foo> {}
  |                ^ unused type parameter
  |
  = help: consider removing `T`, referring to it in a field, or using a marker such as `PhantomData`

error[E0283]: type annotations needed
  --> src/main.rs:18:13
   |
18 |         bar.dumb();
   |             ^^^^ cannot infer type for type parameter `T`
   |
   = note: cannot satisfy `_: Foo`
   = help: the trait `Foo` is implemented for `Baz`
note: required by a bound in `Bar::<T>::dumb`
  --> src/main.rs:5:8
   |
5  |     T: Foo,
   |        ^^^ required by this bound in `Bar::<T>::dumb`
6  | {
7  |     pub fn dumb(&self) {}
   |            ---- required by a bound in this associated function