Summoning a mirror for a Tuple produces a class-cast-exception during runtime

[deusaquilus]

Compiler version

Latest nightly (i.e. dottyLatestNightlyBuild.get)

Minimized code

Create a simple mirror type object which summons a product mirror, add an extension method for it to be able to be applied to any type:

object MirrorType {
  class Container[T]

  inline def decode[T]: String =
    summonFrom {
      case ev: Mirror.ProductOf[T] =>
        s"Product-${new Container[ev.MirroredElemLabels]}" // This is the part that splices in the cast
      case m: Mirror.SumOf[T] =>
        "Sum"
    }

  inline def generic[T]: MirrorType[T] = 
    new MirrorType[T] {
      def mirrorType: String = decode[T]
    }

  extension[T](inline value: T)
    inline def mirrorType = summonFrom {
      case mt: MirrorType[T] => mt.mirrorType
      case _ => "mirror not found"
    }
}

Then import the context and run it:

object ContextUse {
  def main(args: Array[String]): Unit = {
    val ctx = new MyContext();
    import ctx._
    val tup = ("foo", 1)
    println( tup.mirrorType )
  }
}

Output

A class cast exception will occur

[error] (run-main-2) java.lang.ClassCastException: io.getquill.mytest.ContextUse$$anon$2 cannot be cast to scala.deriving.Mirror$Product
[error] java.lang.ClassCastException: io.getquill.mytest.ContextUse$$anon$2 cannot be cast to scala.deriving.Mirror$Product
[error]         at io.getquill.mytest.ContextUse$$anon$1.mirrorType(Use.scala:10)
[error]         at io.getquill.mytest.ContextUse$.main(Use.scala:10)
[error]         at io.getquill.mytest.ContextUse.main(Use.scala)
[error]         at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[error]         at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
[error]         at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
[error]         at java.lang.reflect.Method.invoke(Method.java:498)

This is probably because I can see that the following code is being spliced:

{
  val ev: Product {
    type MirroredType >: Tuple2[String, Int] <: Tuple2[String, Int]
    type MirroredMonoType >: Tuple2[String, Int] <: Tuple2[String, Int]
    type MirroredElemTypes >: Nothing <: Tuple
  } & Product {
    type MirroredMonoType >: Tuple2[String, Int] <: Tuple2[String, Int]
    type MirroredType >: Tuple2[String, Int] <: Tuple2[String, Int]
    type MirroredLabel >: "Tuple2" <: "Tuple2"
  } {
    type MirroredElemTypes >: *:[String, *:[Int, EmptyTuple]] <: *:[String, *:[Int, EmptyTuple]]
    type MirroredElemLabels >: *:["_1", *:["_2", EmptyTuple]] <: *:["_1", *:["_2", EmptyTuple]]
  } = {
    final class $anon() {
      type MirroredMonoType = Tuple2[String, Int]
    }

    (new $anon(): Object)
  }.$asInstanceOf$[Product {      // WHAT??? You can't cast this to a Product
    type MirroredType >: Tuple2[String, Int] <: Tuple2[String, Int]
    type MirroredMonoType >: Tuple2[String, Int] <: Tuple2[String, Int]
    type MirroredElemTypes >: Nothing <: Tuple
  } & Product {
    type MirroredMonoType >: Tuple2[String, Int] <: Tuple2[String, Int]
    type MirroredType >: Tuple2[String, Int] <: Tuple2[String, Int]
    type MirroredLabel >: "Tuple2" <: "Tuple2"
  } {
    type MirroredElemTypes >: *:[String, *:[Int, EmptyTuple]] <: *:[String, *:[Int, EmptyTuple]]
    type MirroredElemLabels >: *:["_1", *:["_2", EmptyTuple]] <: *:["_1", *:["_2", EmptyTuple]]
  }]
  _root_.scala.StringContext.apply("Product-", "").s(new Container[MirroredElemLabels]())
}

Expectation

Code should compile and run correctly.

Repository

Code is available here:
https://github.com/deusaquilus/class_cast_issue

[odersky]

Looks like a duplicate of #11961 and #11542.

[deusaquilus]

Makes sense. Whatever the case, I checked this against the latest Dotty build and it is happening.

[deusaquilus]

Also, making my methods transparent doesn't fix the problem.

[deusaquilus]

@odersky @bishabosha I see that the original issue #11542 was created Feb 26th. What are the chances that this will be fixed by the 3.0.0 release? This issue is making it impossible for me to return Tuples from any Quill queries. (Sorry if I sound demanding, I'm just really worried)

E.g. the following is impossible due to this issue:

inline def q =
  quote {
    for {
      p <- query[Person]
      a <- query[Address] if (p.id == a.fk)
    } yield {
      (p.name, a.street) // Summons a mirror for this tuple which blows up with ClassCast Exception
    }
  }

[deusaquilus]

This is great. Thank you!

[odersky]

You are welcome! Glad that we could fix it for 3.0.