Cannot translate usage of wildcard type from Scala 2

[oyvindberg]

Minimized code

type WatchHandler[T] = Function2[/* val */ T, /* oldVal */ T, scala.Unit]
val handlers: Seq[WatchHandler[_]] = Nil

Output

unreducible application of higher-kinded type [T] =>> (T, T) => Unit to wildcard arguments

Expectation

In Scala 2 this compiles. I tried replacing with ? instead, but got the same error.

[smarter]

This is intentionally not supported: we only allow wildcards on class types (because anything else would be equivalent to existential types which we don't support), some alternatives you could use:

• wrap the type in a class: class WatchHandler[T](f: Function2[T, T, scala.Unit]
• extend Function2: trait WatchHandler[T] extends Function2[T, T, scala.Unit]
• not sure if it would make sense here, but one could use the new-in-dotty polymorphic function types: val handlers: Seq[[T] => () => WatchHandler[T]] = Nil, cf [Proof of concept] Polymorphic function types #4672