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Type incompatibility combining inner classes and abstract overrides #5685

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scabug opened this issue Apr 19, 2012 · 2 comments
Closed

Type incompatibility combining inner classes and abstract overrides #5685

scabug opened this issue Apr 19, 2012 · 2 comments
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@paulp
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2.10.0-M3

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@scabug
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scabug commented Apr 19, 2012 

trait X[A] {
  def x: A
}

trait XPrint[A] extends X[A] {
  abstract override def x: A = {
    val a = super.x
    println(a)
    a
  }
}

trait F[A, B] { outer =>
  def apply(xv: X[A]): X[B]
  
  def andThen[C](f: F[B, C]): F[A, C] = new F[A, C] {
    def apply(xv: X[A]): X[C] = f(new XX(xv) with XPrint[B])
  }

  class XX(xv: X[A]) extends X[B] {
    def x = outer(xv).x
  }
}

This fails to compile with the following error:

Test.scala:18: error: type mismatch;
 found   : this.XX with XPrint[B]
 required: X[B]

    def apply(xv: X[A]): X[C] = f(new XX(xv) with XPrint[B])
                                  ^
one error found

I thought a variance issue might be to blame, but changing A to be covariant gives another interesting error:

trait X[+A] {
  def x: A
}

trait XPrint[+A] extends X[A] {
  abstract override def x: A = {
    val a = super.x
    println(a)
    a
  }
}

trait F[-A, +B] { outer =>
  def apply(xv: X[A]): X[B]
  
  def andThen[C](f: F[B, C]): F[A, C] = new F[A, C] {
    def apply(xv: X[A]): X[C] = f(new XX(xv) with XPrint[B])
  }

  class XX(xv: X[A]) extends X[B] {
    def x = outer(xv).x
  }
}

gives the following:

Test.scala:18: error: illegal inheritance;
 anonymous class $anon inherits different type instances of trait X:
X[B] and X[C]
    def apply(xv: X[A]): X[C] = f(new XX(xv) with XPrint[B])
                                      ^
one error found

This is similar to the error I encountered in the original, non-minimized case. In the original case, however, the type parameters were invariant throughout; also, there is a related crasher that I've been unable to reproduce in a minimized case; in the crash, I get the following error, which provides no indication of the original source of the error:

[info] Compiling 2 Scala sources to /home/workspace/kjn-storage/platform/yggdrasil/target/scala-2.9.1/classes...
[error] {file:/home/workspace/kjn-storage/platform/}yggdrasil/compile:compile: scala.tools.nsc.symtab.Types$TypeError: type mismatch;
[error]  found   : com.precog.yggdrasil.Column[Boolean]
[error]  required: com.precog.yggdrasil.Column[A]
[error] Total time: 3 s, completed Apr 19, 2012 10:01:17 PM

In that original error case, the XX class was declared inside of the anonymous F; in the minimized case, moving the XX class inside of the anonymous F eliminates the two errors first noted.
@scabug
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scabug commented Apr 19, 2012

Imported From: https://issues.scala-lang.org/browse/SI-5685?orig=1
Reporter: Kris Nuttycombe (nuttycom)
Affected Versions: 2.9.0, 2.10.0-M2

@scabug
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scabug commented May 5, 2012

@paulp said:
Well, it took me a long time to see it. No more letting me do your debugging for you, kris!

  new F[A, C] {
    def apply(xv: X[A]): X[C] = f(new XX(xv) with XPrint[B])
  }

Since you're inside an anonymous F, that new XX is in the F[A, C] being created. You want "new outer.XX(sv)" at which point it compiles as given (with or without variance.)

@scabug scabug closed this as completed May 5, 2012
@scabug scabug added this to the 2.10.0-M3 milestone Apr 7, 2017
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