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"object creation impossible" despite exact signature match #8011

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scabug opened this issue Nov 26, 2013 · 3 comments
Closed

"object creation impossible" despite exact signature match #8011

scabug opened this issue Nov 26, 2013 · 3 comments
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@scabug
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@scabug scabug commented Nov 26, 2013

You'd be hard pressed to reason your way to why this doesn't compile.

object Thing {
  implicit class ThingOps1(val x: String) extends AnyVal {
    def fn[A] : Ordering[A] = new Ordering[A] {
      def compare(x1: A, x2: A): Int = ???
    }
  }
  implicit class ThingOps2[A](val x: String) extends AnyVal {
    def fn: Ordering[A] = new Ordering[A] {
      def compare(x1: A, x2: A): Int = ???
    }
  }
}
// a.scala:3: error: object creation impossible, since method compare
// in trait Ordering of type (x: A, y: A)Int is not defined
//     def fn[A] : Ordering[A] = new Ordering[A] {
//                                   ^
// a.scala:8: error: object creation impossible, since method compare
// in trait Ordering of type (x: A, y: A)Int is not defined
//     def fn: Ordering[A] = new Ordering[A] {
//                               ^
// two errors found

Remove the "extends AnyVals" and both compile. It's specific to generics:. This compiles:

implicit class ThingOps1(val x: String) extends AnyVal {
  def fn: Ordering[Boolean] = new Ordering[Boolean] {
    def compare(x1: Boolean, x2: Boolean): Int = ???
  }
}

As a workaround you can do something like

  class Ord[A](f: (A, A) => Int) extends Ordering[A] { 
    def compare(x1: A, x2: A): Int = f(x1, x2)
  }
  implicit class ThingOps1(val x: String) extends AnyVal {
    def fn[A] : Ordering[A] = new Ord[A]((x1, x2) => ???)
  }

How this has not yet been reported I don't know.

@scabug
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@scabug scabug commented Nov 26, 2013

Imported From: https://issues.scala-lang.org/browse/SI-8011?orig=1
Reporter: @paulp
Affected Versions: 2.10.0

@scabug
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@scabug scabug commented Nov 26, 2013

@retronym said:
This one was fixed a few days back in cb37548ef85d471951867b9f8a97cb9b9820fc66. While peforming some housework in Uncurry, I found that we weren't substituting symbols in ClassInfoType#parents. I included a test case in that commit that showed the same bug in extension methods (with partial functions).

I've submitted your test case: scala/scala#3195

@scabug
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@scabug scabug commented Nov 26, 2013

@paulp said:
Man, my trunk build is four days old.

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