Regression in typer when copying case class with generic parameters

[WojciechMazur]

Compiler version

Fails since 3.1.2

Minimized code

Minimized using scala-cli

//> using scala "3.2.0-RC1-bin-20220429-400427d-NIGHTLY"

// Last working version
////> using scala "3.1.1"

trait RType
trait TypeEntry{
  def typeAdapter: TypeAdapter[_] = ???
}

trait TypeAdapter[E]
case class OptionTypeAdapter[E](nullIsNone: Boolean, valueTypeAdapter: TypeAdapter[E]) extends TypeAdapter[E]
case class JavaOptionalTypeAdapter[E](nullIsNone: Boolean, valueTypeAdapter: TypeAdapter[E]) extends TypeAdapter[E]

def typeAdapterOf(concreteType: RType): TypeAdapter[_] = ???

@main def test() = {
  // https://github.com/gzoller/scalajack/blob/4a29366e28fbd594d7c21b4eb969ca14626ac0d1/core/src/main/scala/co.blocke.scalajack/typeadapter/TupleTypeAdapter.scala#L21-L30
  val seq = List.empty[RType]
  seq map { f =>
    typeAdapterOf(f) match {
      case ota: OptionTypeAdapter[_] => ota.copy(nullIsNone = true)
      case jota: JavaOptionalTypeAdapter[_] => jota.copy(nullIsNone = true)
      case other => other 
    }
  }
}
    

Output

[error] ./test.scala:21:41: Found:    TypeAdapter[_]
[error] Required: TypeAdapter[(avoid)E]
[error] 
[error] where:    (avoid)E is a type variable
[error]       case ota: OptionTypeAdapter[_] => ota.copy(nullIsNone = true)//.asInstanceOf[OptionTypeAdapter[_]]
[error]                                         ^^^^^^^^
[error] ./test.scala:22:48: Found:    TypeAdapter[_]
[error] Required: TypeAdapter[(avoid)E]
[error] 
[error] where:    (avoid)E is a type variable
[error]       case jota: JavaOptionalTypeAdapter[_] => jota.copy(nullIsNone = true)//.asInstanceOf[JavaOptionalTypeAdapter[_]]
[error]                                                ^^^^^^^^^```

Expectation

Should compile like in the Scala 3.1.1 or earlier
Should not need explicit casting of copy result to satisfy compiler

[nicolasstucki]

This issue started in 3ab18a9.

[smarter]

This looks like the same problematic code pattern as SnippetChecker.scala mentioned in the commit message of 3ab18a9, so the same workaround applies: pass an explicit type argument to map:

  seq.map[TypeAdapter[?]]{ f =>
    typeAdapterOf(f) match {
      case ota: OptionTypeAdapter[_] => ota.copy(nullIsNone = true)
      case jota: JavaOptionalTypeAdapter[_] => jota.copy(nullIsNone = true)
      case other => other 
    }
  }

Closing, since there is no plan to fix this so far.