What about the fromProduct method? That is not generated if there is no companion, right? How does that manifest itself?

We probably should have a test involving fromProduct

I have added a test using ordinal, and fromProduct but there should be no change in behaviour for fromProduct as it is only generated on Mirror.Product, which is not affected by this change.

Edit: I have added a test case to prevent a crash in the following:

class ClassWrapper {
  sealed trait Parent
  case class Foo(x: Int, y: Int, s: String) extends Parent
}

@main def Test =
  val cw = new ClassWrapper()
  val mirrorParent = summon[deriving.Mirror.Of[cw.Parent]] // error - dont synthesize mirror in this case

If we allow this case then the generated mirror will use paths such as ClassWrapper.this.Foo in def Test which is an illegal scope.

A temporary mitigation is included in the next commit to check accessibility from the scope of the generated mirror, a follow up PR could correct the paths generated when the mirror has a class instance in its path

I have added a test to check that the mirror can also access the children if it is an anonymous class

I think that's a serious degradation that is not at all obvious. So at least we should have a warning that this happens and an encouragement to create a companion object.

Should this warning be for all anonymous mirrors (i.e. including mirrors for Scala 2 types) or just the ones for types in Scala 3.

I would assume probably avoid noise for Scala 2 types

adding the warning is breaking shapeless though with an unrelated error

I think I misunderstood two cases. If a trait has a derived clause

sealed trait T derives C

it gets a companion automatically. That's the case I was after. I thought somehow that we would then fallback to anonymous classes, but that's obviously not the case. If we summon a mirror outside, we should be prepared that an anonymous class is created. So I think we can just revert the last commit and drop the warning. Sorry for the confusion!

I have reverted the changes to add the warning

the problem in shapeless was that it had fatal warnings turned on which causes an error in the macro to summon a shapeless.typeable.Typeable for a sealed trait with no companion, and so suppressed the error for summoning an anonymous Mirror in favour of displaying the Implicit not found error. If fatal warnings is turned off, you instead see the warning about an anonymous mirror but the implicit Typeable can still be found