Avoid orphan param from default arg

[dwijnand]

Fixes #21558

[dwijnand]

@EugeneFlesselle ping, any chance you can review this please?

[SethTisue]

I don't know these APIs well, but I can't think of any objection to raise, either.

[WojciechMazur]

@dwijnand
Bisect points to this PR as cause of OpnCB failures in geirolz/toolkit - build logs
I was not able to correctly minimize the issue - the copyWith method seems to have some weird inference with the rest of the repository - copy of this method under a different name leads to compilation failure (missing implicit of AppContext.NoDeps)
My guess is that project previously compiled only because it was taking advantage of hidden unsoundness bug.

What's also interesting when using = this.depsLoader.asInstanceOf leads to a crash of the compiler in pickler due to orphan paramterer refrence. Here's reproducer

trait AppContext[CONFIG, DEPENDENCIES]
object AppContext:
  sealed trait NoDependencies
  type NoDeps[CONFIG] = AppContext[CONFIG, NoDependencies]

trait Resource[F[_], +T]
type \/[+A, +B] = Either[A, B]
given [A, B]: AppContext[A, B] = ???

class App[F[+_], FAILURE, CONFIG, DEPENDENCIES](
    val depsLoader: AppContext.NoDeps[CONFIG] ?=> Resource[F, FAILURE \/ DEPENDENCIES]
) {
  def copyWith[G[+_], FAILURE2, CONFIG2, DEPS2](
      dependenciesLoader: AppContext.NoDeps[CONFIG2] ?=> Resource[G, FAILURE2 \/ DEPS2] = this.depsLoader.asInstanceOf
  ): App[G, FAILURE2, CONFIG2, DEPS2] = ???
}

FYI @geirolz