Inferring SafeInt from HList arity triggers a stackoverflow error.

The following are 2 almost identical programs, the first one compiles successfully:

// just fixture
  object big {

    val nat = Nat(100)
  }

val op1 = implicitly[ToInt[big.nat.N]]

def getOps[D <: HList, T <: Nat](v: D)(
    implicit
    ofLength: hlist.Length.Aux[D, T],
    asInt: ToInt[T]
): ToInt[T] = {
  asInt
}

val filled = HList.fill[Double](100)(0.0)

val op2 = getOps(filled)

The second, while only replaces `ToInt` with `SafeInt`, triggers a stackoverflow error:

val op1 = implicitly[SafeInt[big.nat.N]]

def getOps[D <: HList, T <: Nat](v: D)(
    implicit
    ofLength: hlist.Length.Aux[D, T],
    asInt: SafeInt[T]
): SafeInt[T] = {
  asInt
}

val filled = HList.fill[Double](100)(0.0)

val op2 = getOps(filled)

error:

Exception when compiling 42 sources to /home/peng/git/scalaspike/common/build/classes/scala/test

java.lang.StackOverflowError scala.reflect.internal.tpe.TypeMaps$TypeMap.mapOver(TypeMaps.scala:114) scala.reflect.internal.tpe.TypeMaps$FilterTypeCollector.traverse(TypeMaps.scala:1135) scala.reflect.internal.tpe.TypeMaps$TypeTraverser.apply(TypeMaps.scala:323) scala.reflect.internal.tpe.TypeMaps$TypeTraverser.apply(TypeMaps.scala:321) scala.reflect.internal.tpe.TypeMaps$TypeMap.mapOver(TypeMaps.scala:123) scala.reflect.internal.tpe.TypeMaps$FilterTypeCollector.traverse(TypeMaps.scala:1135) scala.reflect.internal.tpe.TypeMaps$TypeTraverser.apply(TypeMaps.scala:323) scala.reflect.internal.tpe.TypeMaps$TypeTraverser.apply(TypeMaps.scala:321) scala.reflect.internal.tpe.TypeMaps$TypeMap.mapOver(TypeMaps.scala:123) scala.reflect.internal.tpe.TypeMaps$FilterTypeCollector.traverse(TypeMaps.scala:1135) scala.reflect.internal.tpe.TypeMaps$TypeTraverser.apply(TypeMaps.scala:323) scala.reflect.internal.tpe.TypeMaps$TypeTraverser.apply(TypeMaps.scala:321) scala.reflect.internal.tpe.TypeMaps$TypeMap.mapOver(TypeMaps.scala:123) scala.reflect.internal.tpe.TypeMaps$FilterTypeCollector.traverse(TypeMaps.scala:1135) scala.reflect.internal.tpe.TypeMaps$TypeTraverser.apply(TypeMaps.scala:323) scala.reflect.internal.tpe.TypeMaps$TypeTraverser.apply(TypeMaps.scala:321) scala.reflect.internal.tpe.TypeMaps$TypeMap.mapOver(TypeMaps.scala:123) scala.reflect.internal.tpe.TypeMaps$FilterTypeCollector.traverse(TypeMaps.scala:1135) scala.reflect.internal.tpe.TypeMaps$TypeTraverser.apply(TypeMaps.scala:323) scala.reflect.internal.tpe.TypeMaps$TypeTraverser.apply(TypeMaps.scala:321) scala.reflect.internal.tpe.TypeMaps$TypeMap.mapOver(TypeMaps.scala:123) scala.reflect.internal.tpe.TypeMaps$FilterTypeCollector.traverse(TypeMaps.scala:1135) scala.reflect.internal.tpe.TypeMaps$TypeTraverser.apply(TypeMaps.scala:323) scala.reflect.internal.tpe.TypeMaps$TypeTraverser.apply(TypeMaps.scala:321) scala.reflect.internal.tpe.TypeMaps$TypeMap.mapOver(TypeMaps.scala:123) scala.reflect.internal.tpe.TypeMaps$FilterTypeCollector.traverse(TypeMaps.scala:1135) scala.reflect.internal.tpe.TypeMaps$TypeTraverser.apply(TypeMaps.scala:323) scala.reflect.internal.tpe.TypeMaps$TypeTraverser.apply(TypeMaps.scala:321) scala.reflect.internal.tpe.TypeMaps$TypeMap.mapOver(TypeMaps.scala:123) scala.reflect.internal.tpe.TypeMaps$FilterTypeCollector.traverse(TypeMaps.scala:1135) scala.reflect.internal.tpe.TypeMaps$TypeTraverser.apply(TypeMaps.scala:323) scala.reflect.internal.tpe.TypeMaps$TypeTraverser.apply(TypeMaps.scala:321) scala.reflect.internal.tpe.TypeMaps$TypeMap.mapOver(TypeMaps.scala:123) …

This should be enough information for the inference, any advice why this could happen?

[soronpo]

ToInt is for conversion SafeInt is for having a .Out <: Int with Singleton

[tribbloid]

I see, Sorry for the misunderstanding. So it is a synonym for “Require Int”.

Case closed