Regression (?) in softwaremill/tapir - refined type returned in a macro unable to be matched

[jchyb]

Compiler version

3.4.0-RC1-bin-20240115-31f837e-NIGHTLY, compiles successfully with 3.3.1

Minimized code

main.scala:

import scala.compiletime._
final class Greater[V]
final class Less[V]

// original codebase used non-transparent inlines in the defs below, but I think this would end up passing an abstract type parameter to the macro, so I don't think that should have worked (but transparent inlines should)
transparent inline given validatorForGreater[N: Numeric, NM <: N](using witness: ValueOf[NM]): ValidatorForPredicate[N, Greater[NM]] = ???
transparent inline given validatorForLess[N: Numeric, NM <: N](using witness: ValueOf[NM]): ValidatorForPredicate[N, Less[NM]] = ???
transparent inline def summonValidators[N, A <: Tuple]: List[ValidatorForPredicate[N, Any]] = {
  inline erasedValue[A] match
    case _: EmptyTuple => Nil
    case _: (head *: tail) =>
      summonInline[ValidatorForPredicate[N, head]]
        .asInstanceOf[ValidatorForPredicate[N, Any]] :: summonValidators[N, tail]
}
transparent inline given validatorForAnd[N, Predicates](using mirror: IntersectionTypeMirror[Predicates]): ValidatorForPredicate[N, Predicates] =
  new ValidatorForPredicate[N, Predicates] {
    summonValidators[N, mirror.ElementTypes]
  }

trait ValidatorForPredicate[Value, Predicate]

@main def main() = validatorForAnd[Int, Greater[1] & Less[3]]

macro.scala:

import scala.quoted.Quotes

import scala.annotation.implicitNotFound
import scala.quoted.*
import scala.collection.View.Empty

trait IntersectionTypeMirror[A] {
  type ElementTypes <: Tuple
}

class IntersectionTypeMirrorImpl[A, T <: Tuple] extends IntersectionTypeMirror[A] {
  override type ElementTypes = T
}

object IntersectionTypeMirror {

  transparent inline given derived[A]: IntersectionTypeMirror[A] = ${ derivedImpl[A] }

  private def derivedImpl[A](using Quotes, Type[A]): Expr[IntersectionTypeMirror[A]] = {
    import quotes.reflect.*

    val tplPrependType = TypeRepr.of[? *: ?]
    val tplConcatType = TypeRepr.of[Tuple.Concat]

    def prependTypes(head: TypeRepr, tail: TypeRepr): TypeRepr =
      AppliedType(tplPrependType, List(head, tail))

    def concatTypes(left: TypeRepr, right: TypeRepr): TypeRepr =
      AppliedType(tplConcatType, List(left, right))

    def rec(tpe: TypeRepr): TypeRepr = {
      tpe.dealias match
        case AndType(left, right) => concatTypes(rec(left), rec(right))
        case t                    => prependTypes(t, TypeRepr.of[EmptyTuple])
    }
    val tupled =
      TypeRepr.of[A].dealias match {
        case and: AndType => rec(and).asType.asInstanceOf[Type[Elems]]
        case tpe          => report.errorAndAbort(s"${tpe.show} is not an intersection type")
      }
    type Elems

    given Type[Elems] = tupled

    Apply(
      TypeApply(
        Select.unique(
          New(
            Applied(
              TypeTree.of[IntersectionTypeMirrorImpl],
              List(
                TypeTree.of[A],
                TypeTree.of[Elems]
              )
            )
          ),
          "<init>"
        ),
        List(
          TypeTree.of[A],
          TypeTree.of[Elems]
        )
      ),
      Nil
    ).asExprOf[IntersectionTypeMirror[A]]
  }
}

Output

[error] ./main.scala:22:20
[error] cannot reduce inline match with
[error]  scrutinee:  scala.compiletime.erasedValue[mirror$proxy1.ElementTypes] : mirror$proxy1.ElementTypes
[error]  patterns :  case _:EmptyTuple
[error]              case _:*:[head @ _, tail @ _]
[error] @main def main() = validatorForAnd[Int, Greater[1] & Less[3]]
[error]                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Expectation

I think it should compile. The code used in tapir actually uses non-transparent inlines in code that calls transparent inlines, which I don't think should have worked, but did (probably because of the previous match types). But even after correcting that, the code errors with what seems to be an abstract type member.

[WojciechMazur]

I think this issue origined in the SIP-56 and the new match types https://github.com/lampepfl/dotty/pull/18262#issuecomment-1848377927 @sjrd can you can a look at this issue?

[sjrd]

This is an inline match, not a match type. SIP-56 did not touch those.

[nicolasstucki]

Minimized

macro.scala // same

main.scala

import scala.compiletime._

inline def summonValidators[A <: Tuple]: Int =
  inline erasedValue[A] match
    case _: EmptyTuple => 1
    case _: (head *: tail) => 2

def main() =
  val mirror = IntersectionTypeMirror.derived[Some[Any] & Option[Int]]
  summonValidators[mirror.ElementTypes]

[nicolasstucki]

For the minimized example, the type that we get from rec(and)

        case and: AndType => rec(and).asType.asInstanceOf[Type[Elems]]

is

scala.Tuple.Concat[
  scala.*:[_ >: scala.Nothing <: scala.Any, _ >: scala.Nothing <: scala.Any][scala.Some[scala.Any], scala.Tuple$package.EmptyTuple],
  scala.*:[_ >: scala.Nothing <: scala.Any, _ >: scala.Nothing <: scala.Any][scala.Option[scala.Int], scala.Tuple$package.EmptyTuple]]

This is a nonsensical type. I suspect there is a bug in the macro implementation. My guess is in the use of

    val tplPrependType = TypeRepr.of[? *: ?]

[nicolasstucki]

This is a way to fix prependTypes

    def prependTypes(head: TypeRepr, tail: TypeRepr): TypeRepr =
      (head.asType, tail.asType) match {
        case ('[h], '[type t <: Tuple; t]) => TypeRepr.of[h *: t]
      }

[jchyb]

Fixed and merged in tapir