fn foo<'a>() -> impl Send {
    if false {
        foo();
    }
    async {}
}

results in

error[E0283]: type annotations needed: cannot satisfy `{async block@src/main.rs:5:5: 5:10}: Send`
 --> src/main.rs:3:9
  |
3 |         foo();
  |         ^^^^^
  |
  = note: cannot satisfy `{async block@src/main.rs:5:5: 5:10}: Send`

we first take stalled coroutine obligations, this encounters foo<'unconstrained> normalizes-to ?infer and foo<'a> normalizes-to {coroutine}. It only takes the second one.

We then apply the defining use to foo<'unconstrained> normalizes-to ?infer which constrains ?infer to {coroutine}. We do not take stalled obligations again, meaning that this normalizes-to goal stays ambiguous :<