Scala runtime reflections get all the members of a specific type even for inner classes

Question

With scala 2.12.10

Suppose I want to implicitly convert at runtime a case class, in this case Special to a case class SpecialString. The implicit conversion is provided by a trait External. The name for SpecialString should be the declaration name of the class Special.

import scala.reflect.runtime.universe.{runtimeMirror, typeOf}
import scala.reflect.runtime.universe


case class Special(i: Int)
case class SpecialString(s: String)

trait External {
  val rm = runtimeMirror(getClass.getClassLoader)
  val im = rm.reflect(this)
  val members = im.symbol.typeSignature.members
  def specials: Iterable[universe.Symbol] = members.filter(_.typeSignature <:< typeOf[Special] )
  implicit def SpecialIntToString(s: Special): SpecialString = {
    val name = im.reflectField(specials.filter(x => im.reflectField(x.asTerm).get.asInstanceOf[Special] == s).head.asTerm).symbol.toString.replace("value ", "")
    SpecialString(s"name = $name")
  }
}

Currently I'm able to implicitly convert convert members declared inside a class extending the External trait.

class MyClass extends External {
  val firstSpecial = Special(1)
  val two = 2
  val specialS: SpecialString = firstSpecial
}

class MySecondClass extends MyClass {
  val specialS2: SpecialString = firstSpecial
}
val myClass = new MyClass
print(myClass.specialS) // SpecialString(name = firstSpecial)

But I'm not able to convert members that are declared in a super class

class MyClass {
  val firstSpecial = Special(1)
  val two = 2
  val specialS: SpecialString = firstSpecial
}

class MySecondClass extends MyClass with External {
  val specialS2: SpecialString = firstSpecial
}
val myClass = new MyClass
print(myClass.specialS)
val mySecondClass = new MySecondClass
print(mySecondClass.specialS2) // java.util.NoSuchElementException: next on empty iterator

Any help?

Answer 1

If you find necessary member by name rather than typeSignature (and it's actually found then) and print specials.head.typeSignature and typeOf[Special] you'll see why one is not a subtype of another

trait External {
  ...
  def specials: Iterable[universe.Symbol] =
    members.filter(_.name == universe.TermName("firstSpecial") )
    //members.filter(_.typeSignature.resultType <:< typeOf[Special] )
  println(s"specials.head.typeSignature=${specials.head.typeSignature}=${universe.showRaw(specials.head.typeSignature)}")
  println(s"typeOf[Special]=${typeOf[Special]}=${universe.showRaw(typeOf[Special])}")
  println(s"specials.head.typeSignature <:< typeOf[Special]=${specials.head.typeSignature <:< typeOf[Special]}")
  ...
}

//specials.head.typeSignature=pckg.App.Special=NullaryMethodType(TypeRef(ThisType(pckg.App), pckg.App.Special, List()))
//typeOf[Special]            =pckg.App.Special=TypeRef(ThisType(pckg.App), pckg.App.Special, List())
//specials.head.typeSignature <:< typeOf[Special]=false

Type of a nullary method returning Special is not a subtype of Special.

You should add resultType. Replace

trait External {
  ...
  def specials: Iterable[universe.Symbol] =
    members.filter(_.typeSignature <:< typeOf[Special] )

with

trait External {
  ...
  def specials: Iterable[universe.Symbol] =
    members.filter(_.typeSignature.resultType <:< typeOf[Special])

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