A macro annotation on a class should also allow modifying the companion object

[smarter]

This is especially useful when the companion object is synthetically generated by the compiler, in Scala 3 this is true for all classes since I can write:

class Foo(x: Int)
Foo.apply(1) // Synthetic object Foo with synthetic apply

Therefore I'd like to be able to put an annotation on class Foo to modify the behavior of the companion Foo.apply, e.g.:

@memo class Foo(x: Int)
// generates:
object Foo:
  private val cache$macro$1: HashMap[Int, Foo] = HashMap.empty
  def apply(x: Int): Foo = cache$macro$1.getOrElseUpdate(x, new Foo(x))

@nicolasstucki I noticed that the description of #16454 mentions "Can annotate object to transform the companion class" which seems related, but I didn't find any test matching this behavior?