`Send` bound not inferred on RPIT due to nonsensical unsatisfied higher ranked bound

Extracted from #159575

trait SomeTrait {} // doesnt matter which trait we use for this

trait Trait {
    type Assoc;
}

// error goes away when implementing for `dyn SomeTrait + '_`
impl Trait for dyn SomeTrait {
    type Assoc = ();
}

// The future returned here is obviously `Send`.
// Adding an explicit `+ Send` bound demonstrates this and makes the error go away.
fn future<T: Trait + ?Sized>() -> impl Future<Output = T::Assoc> {
    async { todo!() }
}

fn use_send(_: impl Send) {} // to assert the `Send` bound

fn main() {
    use_send(async { future::<dyn SomeTrait>().await }); // error
}

Compiler output

error: implementation of `Trait` is not general enough
  --> src/main.rs:21:5
   |
21 |     use_send(async { future::<dyn SomeTrait>().await }); // error
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `Trait` is not general enough
   |
   = note: `(dyn SomeTrait + '0)` must implement `Trait`, for any lifetime `'0`...
   = note: ...but `Trait` is actually implemented for the type `(dyn SomeTrait + 'static)`

With -Znext-solver:

error: higher-ranked subtype error
  --> src/main.rs:21:5
   |
21 |     use_send(async { future::<dyn SomeTrait>().await }); // error
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

There should not be any higher ranked logic involved here, since dyn SomeTrait in both cases is just the fixed type dyn SomeTrait + 'static (error still happens if written as such). Replacing both occurrences with a type like () makes the error go away. The error also happens if dyn SomeTrait is wrapped in Box or similar.

Adding an explicit + Send bound to the RPIT makes the error go away but can "cause" (unknown if this is the cause) an ICE on the old solver later on (see #159575).

Removing the seemingly redundant async { (...).await } also causes the error to go away.

Start by compiling the reduced example in src/main.rs and compare the default solver with -Znext-solver, focusing on the RPIT, dyn SomeTrait, and async-await interactions described here. Done means the example no longer reports an erroneous higher-ranked bound for dyn SomeTrait without requiring an explicit + Send; also check the related behavior noted in #159575.