Unsoundness with Mirrors: it is possible to summon a mirror with an incorrect MirrorElemTypes of your choice

[anatoliykmetyuk]

Example

import scala.deriving.Mirror

@main def Test =
  val mrr = summon[Mirror.Of[(Int, String)] { type MirroredElemTypes = (Int, Int, Int) }]
  val ev1 = summon[(Int, Int, Int) =:= mrr.MirroredElemTypes]
  val ev2 = summon[(Int, String) =:= mrr.MirroredElemTypes]
  // val ev3 = summon[(Int, String) =:= (Int, Int, Int)]  // Implicit not found (correct, but breaks transitivity)

  val tup: (Int, String) = (1, "foo")
  val res: mrr.MirroredElemTypes = tup

Run it:

Exception in thread "main" java.lang.ClassCastException: scala.Tuple2 cannot be cast to scala.Tuple3
	at Test$package$.Test(Test.scala:10)
	at Test.main(Test.scala:3)

Expectation

I would expect mrr to be impossible to summon because of the bad refinement type.

[milessabin]

Typically MirroredElemTypes would be unconstrained and computed by the summoning. Here it's been constrained with an incompatible type.

The fix for this is to verify that if MirroredElemTypes has been constrained then the computed type is compatible with the constraint.

[mpilquist]

BTW, I ran in to this issue in scodec when porting it to Dotty. I thought I had commented on this issue then but apparently not. I've been working around this limitation by not constraining MirroredElemTypes and taking a separate type equality, like here:

https://github.com/scodec/scodec/blob/main/shared/src/main/scala/scodec/Iso.scala#L61

I'd prefer to be able to write that as protected def toTuple[A, B <: Tuple](a: A)(using m: Mirror.ProductOf[A] { type MirroredElemTypes = B }): B