Cannot Pass Mirror to Helper Object

[deusaquilus]

minimized code

Say that I create a simple CSV encoder like this:

trait CsvEncoder[T] {
  def encode(elem: T): String
}

object CsvEncoder {
  import scala.compiletime.{erasedValue, summonFrom}
  import compiletime._
  import scala.deriving._

  inline def encodeElem[T](elem: T): String = summonFrom {
    case encoder: CsvEncoder[T] => encoder.encode(elem)
  }

  inline def encodeElems[Elems <: Tuple](idx: Int)(value: Any): List[String] =
    inline erasedValue[Elems] match {
      case _: (elem *: elems1) => 
        encodeElem[elem](productElement[elem](value, idx)) :: encodeElems[elems1](idx + 1)(value)
      case _ => Nil
    }

  inline def derived[T](implicit ev: Mirror.Of[T]): CsvEncoder[T] = new CsvEncoder[T] {
    def encode(value: T): String = 
      inline ev match {
        case m: Mirror.ProductOf[T] =>
          encodeElems[m.MirroredElemTypes](0)(value).mkString(", ")
      }
  }

  given intEncoder: CsvEncoder[Int] { def encode(value: Int) = value + "" }
  given stringEncoder: CsvEncoder[String] { def encode(value: String) = value }
}

I want to be able to use this encoder in a simple macro that will summon whatever mirror is needed from the environment:

object SummonCsvEncoderTest {
  inline def summonMirrorAndUseEncoder[T](value: =>T): String = {
    val mirror = summonFrom {
      case m :Mirror.ProductOf[T] => m
    }
    CsvEncoder.derived(mirror).encode(value)
  }
}

... but then when I use it, it just returns an empty string:

case class ThePerson(name:String, age:Int)
println( SummonCsvEncoderTest.summonMirrorAndUseEncoder[ThePerson](ThePerson("Joe", 123)) )
// Returns ""

expectation

Now, let's say I'll just write the derive/encodeElems/encodeElem method directly in my summoning macro (I'm just copy-pasting those methods from above):

object SummonTest {
  inline def encodeElem[T](elem: T): String = summonFrom {
    case encoder: JsonEncoder[T] => encoder.encode(elem)
  }

  inline def encodeElems[Elems <: Tuple](idx: Int)(value: Any): List[String] =
    inline erasedValue[Elems] match {
      case _: (elem *: elems1) => 
        encodeElem[elem](productElement[elem](value, idx)) :: encodeElems[elems1](idx + 1)(value)
      case _ => Nil
    }

  inline def derived[T](implicit ev: Mirror.Of[T]): CsvEncoder[T] = new CsvEncoder[T] {
      def encode(value: T): String = 
        inline ev match {
          case m: Mirror.ProductOf[T] =>
            encodeElems[m.MirroredElemTypes](0)(value).mkString(", ")
        }
    }

  inline def summonMirrorAndManuallyEncode[T](value: =>T): String = {
    summonFrom {
      case m: Mirror.ProductOf[T] => derived(m).encode(value)
      case _ => "cannot get mirror"
    }
  }
}

Then when I call it, it actually returns the right thing:

println( SummonTest.summonMirrorAndManuallyEncode[ThePerson](ThePerson("Joe", 123)) )
// Returns: "Joe, 123"

How come it works normally here but when I try to pass it into the EncodeCsv helper it returns an empty record???

[milessabin]

Could you try and minimize this for me please?

[deusaquilus]

Please give me a few days and I will reproduce in Scastie

[deusaquilus]

Closing in favor of #7974