failed Send inference resulting in "one type is more general than the other" E0308

this might be related to #90656 but since that is open since 2021 I document my case.
I'm running into E0308's when trying to compile an async fn that uses

• rust 1.98.1 (48a229cea 2026-09-01)
• uom v0.38
• ndarray v0.17.2 (NOTE - v0.16 seems to work)

The problem can be isolated into the following minimal code, which I would expect to compile. Please note it works with ndarray v0.16 so I will also report it there but at least the error message is not helpful

use uom::si::f64::Length;
use uom::ConstZero;
use ndarray::Array2;

fn assert_send<T: Send>(_t: T) {}

fn main() {
    assert_send(check());
}

async fn check() {
    let lengths: Array2<Length> = Array2::from_elem((2, 3), Length::ZERO);
    std::future::pending::<()>().await;
    println!("{}", lengths[(0, 0)].get::<uom::si::length::meter>());
}

The compiler error message is:

error[E0308]: mismatched types
 --> src/main.rs:8:5
  |
8 |     assert_send(check());
  |     ^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
  |
  = note: expected struct `Quantity<..., ..., f64>`
             found struct `Quantity<..., ..., f64>`
note: the lifetime requirement is introduced here
 --> src/main.rs:5:19
  |
5 | fn assert_send<T: Send>(_t: T) {}
  |                   ^^^^

If you are interested I have a workaround wrapping the Array2 into a newtype with a at(&self,usize,usize)->Length fn and using that to obtain Length quantities inside the async fn. This comes from a bigger project though (https://github.com/ODIN-fire/odin-rs) where such workarounds are clearly suboptimal since it is highly async.

Start with the minimal reproducer in src/main.rs and compile it using Rust 1.98.1, comparing ndarray v0.17.2 with v0.16. Read the related issue #90656 and investigate the E0308 diagnostic around the async Send requirement. Done means the cause is identified and the compiler either accepts the valid case or reports a more useful, accurate error.