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ambiguity in first phase of implicit argument search prevents second phase unless the method call has an expected type #6667

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scabug opened this issue Nov 14, 2012 · 11 comments
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@scabug
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@scabug scabug commented Nov 14, 2012

class C 
object C {
  implicit def companion = new C
}

object Test {
  implicit val inScope1, inScope2 = new C
  implicitly[C]: Unit // C.companion used.
  implicitly[C]       // ambiguity reported, rather than falling back to C.companion
}

See:

// Typers.scala
if (original != EmptyTree && pt != WildcardType)
  typer1.silent(tpr => {
    val withImplicitArgs = tpr.applyImplicitArgs(tree)
    if (tpr.context.hasErrors) tree // silent will wrap it in SilentTypeError anyway
    else tpr.typed(withImplicitArgs, mode, pt)
  }) match {
    case SilentResultValue(result) =>
      result
    case _ =>
      debuglog("fallback on implicits: " + tree + "/" + resetAllAttrs(original))
      val tree1 = typed(resetAllAttrs(original), mode, WildcardType)
      tree1.tpe = addAnnotations(tree1, tree1.tpe)
      if (tree1.isEmpty) tree1 else adapt(tree1, mode, pt, EmptyTree)
  }
else
  typer1.typed(typer1.applyImplicitArgs(tree), mode, pt)
@scabug
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@scabug scabug commented Nov 14, 2012

Imported From: https://issues.scala-lang.org/browse/SI-6667?orig=1
Reporter: @retronym
Affected Versions: 2.10.0-RC2
See #6668

@scabug
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@scabug scabug commented Nov 14, 2012

@dcsobral said:
The problem originally reported did not need a companion relation between class and object.

@scabug
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@scabug scabug commented Nov 14, 2012

@retronym said:
It did in a roundabout way; the companion/package objects of enclosing types of a class are actually part of the implicit scope.

But, we'll test both in the fix for this bug.

object Test {
  abstract class Box {
    val value: Int
  }

  implicit val a: Box = new Box {
    val value= 1
  }

  def main(args: Array[String]) {
    implicit val b: Box= new Box {
      val value= 2
    }

    new Object {
      new Test()
    }
    // compare with:
    // new Test()
  }
}

@scabug
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@scabug scabug commented Nov 15, 2012

@dcsobral said:
That still has a companion. I've tested this at work:

class Test()(implicit x: Bop.Box) {
  println(x.value)
}

object Bop {
  abstract class Box {
    val value: Int
  }

  implicit val a: Box = new Box {
    val value= 1
  }

  def main(args: Array[String]) {
    implicit val b: Box= new Box {
      val value= 2
    }

    new Object {
      new Test()
    }
    // compare with:
    // new Test()
  }
}

@scabug
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@scabug scabug commented Nov 15, 2012

@retronym said:
See the comments in #6668 for a discussion about the incongruence between the spec and implementation.

@scabug
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@scabug scabug commented Nov 15, 2012

@retronym said:
@daniel: in your example, the implicit scope of Box includes members in the companion object of it's enclosing type, Bop. It doesn't matter where Test is defined.

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@scabug scabug commented Nov 15, 2012

@paulp said:
If it's any consolation daniel, I'm even worse at this.

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@scabug scabug commented Nov 21, 2012

@retronym said:
Martin agrees that ambiguity errors in the in-scope implicit search should always prevent the implicits-of-expected-type search. I'll prepare a patch to that effect.

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@scabug scabug commented Nov 22, 2012

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@scabug scabug commented Nov 26, 2012

@scabug scabug closed this Dec 10, 2012
@scabug
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@scabug scabug commented Feb 4, 2013

@retronym said:
Demoting to a lint warning in 2.10.1, still an error in 2.11.0: scala/scala#2063

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