Mirror.SumOf not being provided when a direct subtype is also a sum

[japgolly]

Compiler version

3.0.0-RC1

Minimized code

import scala.deriving.*

sealed trait A
sealed trait B extends A
case class A1() extends A
case class B1() extends B

def test =
  summon[Mirror.Of[A]]
  summon[Mirror.Of[B]]
  summon[Mirror.Of[A1]]
  summon[Mirror.Of[B1]]

Output

[error] 9 |  summon[Mirror.Of[A]]
[error]   |                      ^
[error]   |no implicit argument of type deriving.Mirror.Of[A] was found for parameter x of method summon in object Predef

Expectation

It should compile.

Mirror.Of[A] should return a Mirror.SumOf[A] { type MirroredElemTypes = B *: A1 *: EmptyTuple }

[bishabosha]

this will be addressed by #11686, but currently it is explicitly part of the design to only support product cases as children

[dwijnand]

Duplicate of lampepfl/dotty-feature-requests#161