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Compiler allows to super-call an abstract method. This leads to an AbstractMethodError at runtime. #4989

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scabug opened this issue Sep 12, 2011 · 5 comments
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@scabug scabug commented Sep 12, 2011

The following program compiles with no error message. At runtime, a java.lang.AbstractMethodError is thrown.

The point is, that method print defined in class A is overwritten in class B with an abstract definition.
In class C, which extends B, this abstract method is invoked with a super call. I expected a compilation error
on this super call.

object Test {

  class A {
    def print() { println("A"); }
  }
  
  abstract class B extends A {
    def print() : Unit
  }
  
  class C extends B {
    override def print() { println("C"); super.print(); }
  }

  def main(args: Array[String]): Unit = {
    val c = new C
    c.print
  }
}
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@scabug scabug commented Sep 12, 2011

Imported From: https://issues.scala-lang.org/browse/SI-4989?orig=1
Reporter: @dgruntz
Affected Versions: 2.9.1
Other Milestones: 2.10.0

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@scabug scabug commented May 13, 2012

@retronym said (edited on May 13, 2012 8:35:05 PM UTC):
A bit more analysis:

scala> abstract class A { def a() }; abstract class B extends A { def a() }
defined class A
defined class B

scala> object C extends B { override def a() = super.a() }
<console>:9: error: method a in class B is accessed from super. It may not be abstract unless it is overridden by a member declared `abstract' and `override'
       object C extends B { override def a() = super.a() }
                                                     ^

scala> import reflect.{mirror => m}
import reflect.{mirror=>m}

scala> m.classToType(classOf[B]).member(m.newTermName("a")).modifiers
res1: Set[scala.reflect.api.Modifier] = Set(deferred)

scala> class A { def a() {} }; abstract class B extends A { def a() }
defined class A
defined class B

scala> object C extends B { override def a() = super.a() }
defined module C

scala> m.classToType(classOf[B]).member(m.newTermName("a")).modifiers
res2: Set[scala.reflect.api.Modifier] = Set()

scala> C.a()
java.lang.AbstractMethodError: A.a()V
	at C$.a(<console>:10)
	at .<init>(<console>:12)
	at .<clinit>(<console>)

Key question: When does the symbol for method B#a lose the DEFERRED flag? Without it, the check in SuperAccessors doesn't catch this problem.

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@scabug scabug commented May 14, 2012

@paulp said:

Key question: When does the symbol for method B#a lose the DEFERRED flag?

It doesn't. When you request the member of B called a, you get the symbol defined in A, not in B, because concrete always overrides abstract. If you want B#a, use decl (or I guess it's called declaration in the library.)

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@scabug scabug commented Jun 17, 2012

@scabug scabug closed this Jun 24, 2012
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@scabug scabug commented Jun 24, 2012

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