Scala 3: Define a typeclass instance for an inner class

Question

I am trying to define typeclass instances for an inner class.

As a minimal example, I have code that looks like this:

trait Typeclass[T]

class Outer:

  class Inner

I would like to define an instance of Typeclass for Inner, so that this compiles:

object Main:

  import Instances.given

  val outer = new Outer
  val x = summon[Typeclass[outer.Inner]]

It should be possible for the instance to depend on the value of Outer.

I would prefer the instance to reside in a different file (possibly a different package) from both Typeclass and Outer. In this case, defining a given instance inside Outer is not an option.

Is it possible to achieve this in Scala 3?

Below I will outline what I have tried:

Attempt 1: Parameterisation on Outer

object Instances:

  given instance(using outer: Outer): Typeclass[outer.Inner] =
    new Typeclass { }

This kind of works, but unfortunately the using clause requires that the value of Outer be marked as given.

Attempt 2: Type projection

object Instances:

  given instance: Typeclass[Outer#Inner] =
    new Typeclass { }

This doesn't work at all. The compiler refuses to use instance. Also, even if it did work, it wouldn't allow instance to depend on the value of Outer.

Attempt 3: Extension method

object Instances:

  extension (outer: Outer)
    given instance: Typeclass[outer.Inner] =
      new Typeclass { }

It seems extension givens aren't even syntactically valid in Scala 3. (I suspect for good reason!)