`js.await(promise)` does not compile in Scala 3.8.x when the type of `promise` is `js.Promise[Unit]`

[DanielMoss]

Compiler version

3.8.1, 3.8.2

Minimized code

Scastie

import scala.scalajs.js

// Does not compile in Scala 3.8.x, but does compile in Scala 2.13.8
js.async(
  js.await(js.Promise.resolve[Unit](()))
)

// Compiles in both Scala 3.8.x and 2.13.8
js.async(
  js.await(js.Promise.resolve[Int](5))
)

Output

        js.await(js.Promise.resolve[Unit](()))
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Illegal use of js.await().
It can only be used inside a js.async {...} block, without any lambda,
by-name argument or nested method in-between.
If you compile for WebAssembly, you can allow arbitrary js.await()
calls by adding the following import:
import scala.scalajs.js.wasm.JSPI.allowOrphanJSAwait

Expectation

Consistent behaviour between Scala 2 and Scala 3 when using js.async/js.await.

Potentially related PRs:

• Scala.js: Support js.async and js.await, including JSPI on Wasm. #23846
• Fix #5064: Add js.async { ... js.await(p) ... }, and support JSPI on WebAssembly. scala-js/scala-js#5130

[sjrd]

FTR, the problem is not linked to the type of the js.await. It happens when js.async takes a Unit block. Workaround: explicitly write js.async[Any] { ... }.