No warn for evidence params of marker traits such as NotGiven

[som-snytt]

Fixes #22969

[som-snytt]

The test shows I'd made the opposite choice previously:

trait Context[A]
trait Implicits {
  def f[A](implicit ctx: Context[A]) = answer // warn implicit param even though only marker
  def g[A: Context] = answer // no warn bound that is marker only
}
class Bound[A: Context] // no warn bound that is marker only

Possibly that was intended for implicit but not using parameters. It's better not to special-case "old-style" implicit, especially since the consequence would be to "underwarn".

[odersky]

I am not sure how much it's worth to run down the rabbit hole with this. The proper way to handle this is to use erased. erased implicit parameters should not need to be used. In fact, they can't be used except as arguments to other erased parameters.

erased is still experimental. This goes to show that the language proper moves a lot slower than annotations since annnotations currently don't require SIPs. But that should not be a reason to push everything into annotations now.

[som-snytt]

I forgot about erased. These "mitigations" try to "guess user intentions" that aren't expressed directly in the language.