ClassCastException when summoning Mirror for hierarchical sum compiled by 3.0.x

Compiler version

3.1.0

Minimized code

compile with Scala 3.0.2 to out dir

sealed trait Top
object Top // companion is necessary

case class Middle() extends Top with Bottom
sealed trait Bottom extends Top

compile with Scala 3.1.0 and with out on classpath

val m = summon[deriving.Mirror.SumOf[Top]]

Output

runtime error:

java.lang.ClassCastException: Top$ cannot be cast to scala.deriving.Mirror$Sum
  ... 28 elided

This happens because Top is an eligible sum, and it has a companion object (which if compiled by Scala 3.1.0 would extend deriving.Mirror.Sum), so the companion object is used as the mirror and is cast to Sum.

Expectation

prevent summoning mirrors for hierarchical sums with companion objects compiled before 3.1.0

[smarter]

Currently, the community build rebuilds all transitive dependencies from sources, this is a great showcase of why we may want to also have a version of the community build which keeps the binary dependencies.

[bishabosha]

I’d say don’t use the companion object as the mirror if it doesn’t extend Mirror.Sum, but rather summon a local mirror. Any reason that can’t be made to work?

This sounds very reasonable to me.

Summarising a Slack discussion: @dwijnand and @julienrf also asked if we can check if the companion is a subtype of Mirror.Sum, I said this is not possible with code compiled from source, as mirror synthesis happens too early in the compiler currently, so @dwijnand suggested that if we can check if a type is a compiled dependency, then check that its companion extends Mirror.Sum, else keep the current behavior.

this change can be implemented in SymUtils.useCompanionAsMirror