Typeclass variance cannot be expressed with language.experimental.modularity Self types

[propensive]

Compiler version

Latest main, 3393f7ea7704eb1dd553c96a805906821c61db5f.

Minimized example

I'm not sure if I've missed a critical detail somewhere, but it's not clear if there's an intended way to support definition-site variance on a typeclass's Self type. (The word "variance" doesn't appear in the documentation, and I don't recall it being discussed during the long discussion about the feature.)

What this seems to mean is that the old-style typeclass and given instance,

trait Foo[-T]:
  def apply(): String

given Foo[Char] = () => "char"

would need its given instance rewritten as,

given [C <: Char] => C is Foo = () => "char"

rather than,

given Char is Foo = () => "char"

in order for summon[Foo['x']] to resolve.

Expectation

I would expect there to be a way to specify the Self type's variance at the typeclass definition site, or for the documentation to offer a solution.