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Reflection on inner classes fails when outer object is not fully reflected before #7567

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scabug opened this issue Jun 10, 2013 · 4 comments

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@scabug
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commented Jun 10, 2013

In the following example, I am trying to get the instance of Bar (copy from Stackoverflow - http://stackoverflow.com/questions/17012294/recovering-a-singleton-instance-via-reflection-from-sealed-super-trait-when-typ/17016268#17016268 ):

import reflect.runtime.universe._
import reflect.runtime.{currentMirror => cm}

object Foo {
  case object Bar extends Foo
}
sealed trait Foo

def getModule(tpe: Type): Any = {
  val classSymbol  = tpe.typeSymbol.asClass
  val compSymbol   = classSymbol.companionSymbol  // gives <none> !
  val moduleSymbol = compSymbol.asModule
  val moduleMirror = cm.reflectModule(moduleSymbol)
  moduleMirror.instance
}

val fooC     = typeOf[Foo].typeSymbol.asClass
val subs     = fooC.knownDirectSubclasses
// fooC.companionSymbol.typeSignature
val tpeOther = subs.last.asType.toType

val res = getModule(tpeOther)
println(res)

The companion symbol resolution in the getModule method fails here. If I uncomment the line fooC.companionSymbol.typeSignature, it works as expected.

@scabug

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commented Jun 10, 2013

Imported From: https://issues.scala-lang.org/browse/SI-7567?orig=1
Reporter: @Sciss
Affected Versions: 2.10.2-RC2

@scabug

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commented Sep 15, 2013

Lars Hupel (larsrh) said:
Related to: milessabin/shapeless#48

import scala.language.experimental.macros
import scala.reflect.macros.Context
def fooImpl[T: c.WeakTypeTag](c: Context): c.Expr[Unit] = { println(c.weakTypeOf[T].typeSymbol.asClass.companionSymbol); c.universe.reify {} }
def foo[T] = macro fooImpl[T]

scala> { case class Frob(x: Int); foo[Frob] }
<none>

However:

import scala.reflect.runtime.universe._

scala> { case class Frob(x: Int); weakTypeOf[Frob].typeSymbol.asClass.companionSymbol }
scala.ScalaReflectionException: free type Frob is not a class

So, run-time and compile-time reflection disagree about whether Frob is a "class" or a "free type".

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commented Sep 15, 2013

@xeno-by said:
That's a known issue caused by the fact that metadata of local classes is not pickled. Therefore local classes, when used in reify and weakTypeOf, are reified as dummy free types.

@SethTisue

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commented Mar 2, 2018

not clear to me there's anything actionable here?

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