Nested methods capture outer instance when lifted

[scabug]

In this unit test, methodLift fails, while asFunction passes.

@Test
def methodLift {
    def isPrime(c: Int) = BigInt(c).isProbablePrime(1)
  assertNoOuter(isPrime)
}
@Test
def asFunction {
  val isPrime = (c: Int) => BigInt(c).isProbablePrime(1)
  assertNoOuter(isPrime)
}
private def assertNoOuter(f: Int => Boolean) {
  assertFalse(f.getClass.getDeclaredFields.exists(_.getType == this.getClass))
}

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-9390?orig=1
Reporter: @nilskp
Affected Versions: 2.11.7
See #4581, #4440, #9408, #1419

[scabug]

@nilskp said:
I haven't checked if this is also present in 2.12.

[scabug]

@lrytz said (edited on Jul 24, 2015 8:14:42 AM UTC):
This is actually not a bug. The reason is that the lambda body of the eta-expansion of isPrime invokes the isPrime method, which is an instance method, so the lambda needs to capture the C instance.

The class

class C {
  def methodLift = {
    def isPrime(c: Int) = BigInt(c).isProbablePrime(1)
    val f: Int => Boolean = isPrime
    f(0)
  }
}

compiles to (with delambdafy:method, indylambda)

class C {
  def anonfun$1(i: Int) = this.isPrime$1(i)
  def isPrime$1(i: Int) = BigInt(c).isProbablePrime(1)
  def methodLift = {
    // lambda captures `this`, in order to be able to invoke the instance method `anonfun$1`
    val f = indyLambda(this, MethodHandle(anonfun$1))
    f.apply(0)
  }
}

[scabug]

@lrytz said:
Maybe the optimizer could fix this in some cases: if we inline isPrime$1 into anonfun$1 then we can get rid of the captured value. Logged here: https://github.com/scala-opt/scala/issues/34

[scabug]

@lrytz said:
In fact, the lifted isPrime$1 method could be made static, or moved to the companion object, that would also fix the issue. Somewhat related to #4581.

[scabug]

@nilskp said:
I just tried to @inline the def, but unfortunately that didn't work either. Not sure if it should or not.

[scabug]

@lrytz said (edited on Jul 25, 2015 5:45:38 PM UTC):
at the optimizer phase, where inlining is performed, the outer reference is already there. so even if the method is inlined, we need an additional optimization to eliminate unused outer references, which is not implemented, neither in the 2.11 optimizer nor in the new one for 2.12.

[scabug]

@retronym said:
This might be something we need to fix to be serialization-compatible in 2.12, see https://gist.github.com/JoshRosen/8aacdee0162da430868e7f73247d45d8

[scabug]

@retronym said:
The Delambdafy phase already implements a similar virtual -> static translation for lambda implementation methods, perhaps we can generalize that to do the same thing for lambda-lifted local defs.

[scabug]

@retronym said:
Here's an attempt to generalize delambdafy to STATIC-ify lifted methods: scala/scala#5099

[scabug]

@Blaisorblade said:
Fixed by scala/scala#5099 as mentioned in the release notes for 2.12.0-M5: https://github.com/scala/scala/releases/tag/v2.12.0-M5. I've reopened this just to change from "Not a bug" to "Fixed".