Assigning js.undefined to a type alias to js.UndefOr does not compile

import scala.scalajs.js.|
import scala.scalajs.js
type Foo = Int | String
type FooOpt = js.UndefOr[Foo]
val foo: js.UndefOr[Foo] = js.undefined // Works
val foo2: FooOpt = js.undefined // Doesn't work: "Expected `Nothing | Unit`,  got `Int | String | Unit`"

Unclear if we can do anything about this, but opening this so we have a centralized discussion point.

[sjrd]

I believe it would be worth creating a reproduction independent of Scala.js, and report it to Scala/JVM instead. AFAICT there is no reason for the implicit resolution to fail in the original scenario. Fixing it there upstream has the highest chance not to break zillions of other things.

[frobinet]

Sure. I haven’t had the bandwidth to reproduce and submit this issue to Scala/JVM, but closing it here is legitimate. I’ll reference this issue if I ever get to reporting to Scala/JVM.