StackOverflowError when summoning a Mirror.Of within Companion object nested in method

[zarthross]

Compiler version

3.x.x, Tested specifically 3.0.0 and 3.2.2 (via Scastie) and saw the same behavior.

Minimized code

Scastie extended reproduction

import scala.deriving.Mirror

object Main extends App {
  def badWrapper() = {
    case class Baz(i: Int)
    object Baz { // It seems to have to do with the companion object
      val forceInit = 3
      summon[Mirror.Of[Baz]] // This causes a stack overflow of the init...
    }
    Baz.forceInit
  }

  badWrapper()
}

Output

➜ scala bug2.scala
Exception in thread "main" java.lang.StackOverflowError
        at Main$.Main$$$_$Baz$2(bug2.scala:6)
        at Main$Baz$3$.<init>(bug2.scala:8)
        at Main$.Baz$lzyINIT1$1(bug2.scala:6)
        at Main$.Main$$$_$Baz$2(bug2.scala:6)
        at Main$Baz$3$.<init>(bug2.scala:8)
        at Main$.Baz$lzyINIT1$1(bug2.scala:6)
        at Main$.Main$$$_$Baz$2(bug2.scala:6)

Expectation

I would not expect a stack overflow at runtime in this situation. I would expect the summon to occur, and the number 3 to be returned.

The extended example I provided in the scastie link shows that this only seems to occur if the mirror summon happens in a companion object within a method. If you do it in a singleton its fine, or some other case classes' companion object.

[armanbilge]

Also happens in 3.3.1-RC1-bin-20230208-1fce02e-NIGHTLY.

//> using scala "3.3.1-RC1-bin-20230208-1fce02e-NIGHTLY"

import scala.deriving.Mirror

@main def badWrapper() = {
  case class Baz(i: Int)
  object Baz { // It seems to have to do with the companion object
    val forceInit = 3
    summon[Mirror.Of[Baz]] // This causes a stack overflow of the init...
  }
  Baz.forceInit
}

[bishabosha]

this is not to do with mirrors, rather that you reference the companion inside itself:

object Main extends App {
  def badWrapper() = {
    object Baz {
      val forceInit = 3
      Baz // This causes a stack overflow of the init...
    }
    Baz.forceInit
  }

  badWrapper()
}

[zarthross]

In order to reference the companion inside itself, would you not have to say summon[Mirror.Of[Baz.type]] ?

[bishabosha]

the mirror of Baz case class is its companion object, to stop the stack overflow we would have to special case the summon to perhaps use this instead of Baz, when inside of Baz

[zarthross]

I think the current behavior is a bit surprising... we basically noticed it when using TypeClass.derived[Baz] from the companion.