can't summon mirror for MirroredElemTypes as it is generic tuple

[anatoliykmetyuk]

import scala.deriving._

case class Foo(x: Int, y: String)

def toTuple[T <: Product](x: T)(using m: Mirror.ProductOf[T], mt: Mirror.ProductOf[m.MirroredElemTypes]) =
  mt.fromProduct(x)

def weird = {
  val m = summon[Mirror.ProductOf[Foo]]
  val mt1 = summon[Mirror.ProductOf[(Int, String)]]
  type R = (Int, String)
  val mt2 = summon[Mirror.ProductOf[R]]
  val mt3 = summon[Mirror.ProductOf[m.MirroredElemTypes]]

  val f = Foo(1, "foo")
  val g: (Int, String) = toTuple(f)// (using m, mt1)
}

Says:

-- Error: ../pg/Main.scala:20:54 -----------------------------------------------
20 |  val mt3 = the[Mirror.ProductOf[m.MirroredElemTypes]]
   |                                                      ^
   |no implicit argument of type deriving.Mirror.ProductOf[m.MirroredElemTypes] was found for parameter x of method the in object DottyPredef
-- Error: ../pg/Main.scala:23:35 -----------------------------------------------
23 |  val g: (Int, String) = toTuple(f)// given (m, mt1)
   |                                   ^
   |no implicit argument of type deriving.Mirror.ProductOf[m.MirroredElemTypes] was found for parameter mt of method toTuple
two errors found

That is, we can resolve the mirror of the tuple, but not if it is obtained from another mirror. To make it work:

def weird = {
  val m = the[Mirror.ProductOf[Foo]]
  val mt1 = the[Mirror.ProductOf[(Int, String)]]
  type R = (Int, String)
  val mt2 = the[Mirror.ProductOf[R]]
  // val mt3 = the[Mirror.ProductOf[m.MirroredElemTypes]]

  val f = Foo(1, "foo")
  val g: (Int, String) = toTuple(f) given (m, mt1)
}

[milessabin]

I think you're experiencing some strange implicit/given scope related issues rather than anything specific to Mirror ... see my comments on #7050.

[bishabosha]

I think this is caused by #14127, since we improved error messages, they now show class *: is not a generic product because it is not a case class, full output here:

-- Error: ----------------------------------------------------------------------
13 |  val mt3 = summon[Mirror.ProductOf[m.MirroredElemTypes]]
   |                                                         ^
   |No given instance of type deriving.Mirror.ProductOf[m.MirroredElemTypes] was found for parameter x of method summon in object Predef. Failed to synthesize an instance of type deriving.Mirror.ProductOf[m.MirroredElemTypes]: class *: is not a generic product because it is not a case class
-- Error: ----------------------------------------------------------------------
16 |  val g: (Int, String) = toTuple(f)// (using m, mt1)
   |                                   ^
   |No given instance of type deriving.Mirror.ProductOf[(Int, String)] was found for parameter mt of method toTuple. Failed to synthesize an instance of type deriving.Mirror.ProductOf[(Int, String)]: class *: is not a generic product because it is not a case class
2 errors found