Overriding checks in refchecks and bridging in erasure get confused by self type

[retronym]

reproduction steps

class C extends (Set[AnyRef] => Unit) { // error: class C needs to be abstract. Missing implementation for: def apply(v1: Set[_]): Unit // inherited from trait Function1
  self: (Set[_] => Unit) =>

  def apply(s: Set[AnyRef]): Unit = ()
}

abstract class C extends (Set[AnyRef] => Unit) {
  self: (Set[_] => Unit) =>

  def apply(s: Set[AnyRef]): Unit = () // No bridge method created!
}

[dwijnand]

Was the bridge method expected? Or is this just an "errors and warnings" issue?

[som-snytt]

"Self-types deemed confusing." A different interaction of self-type and override in #12161

I think the notation is like in chess, so that

// No bridge method created!

is different from

// No bridge method created?!

[dwijnand]

I like "# Expectations": please tell me, explicitly, your expectations so I don't need to ask. 😄 (within the team @eed3si9n and I have strongly been in favour, while Seth's deemed it unnatural/redundant - which it is sometimes, but also a useful reminder to be explicitly/crystal clear.)

[SethTisue]

fwiw, the current bug template does end with:

problem

(explain how the above behavior isn't what you expected)

[retronym]

I distilled this from some (closed source) user code.

I believe that its a bug that the bridge method is omitted. But I can't find a way to demonstrate the LinkageError as I can't subclass abstract class C from Scala.

Here's an example without the self type that does bridge apply(Object)Object to apply(Set)void:

scala> abstract class C2 extends scala.runtime.AbstractFunction1[Set[AnyRef], Unit] { def apply(s: Set[AnyRef]): Unit = () }
defined class C2

scala> :javap -v C2#apply
  public void apply(scala.collection.immutable.Set<java.lang.Object>);
    descriptor: (Lscala/collection/immutable/Set;)V
    flags: ACC_PUBLIC
    Code:
      stack=0, locals=2, args_size=2
         0: return
      LineNumberTable:
        line 1: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       1     0  this   L$line3/$read$$iw$C2;
            0       1     1     s   Lscala/collection/immutable/Set;
    Signature: #17                          // (Lscala/collection/immutable/Set<Ljava/lang/Object;>;)V
    MethodParameters:
      Name                           Flags
      s                              final
  public java.lang.Object apply(java.lang.Object);
    descriptor: (Ljava/lang/Object;)Ljava/lang/Object;
    flags: ACC_PUBLIC, ACC_BRIDGE, ACC_SYNTHETIC
    Code:
      stack=2, locals=2, args_size=2
         0: aload_0
         1: aload_1
         2: checkcast     #29                 // class scala/collection/immutable/Set
         5: invokevirtual #31                 // Method apply:(Lscala/collection/immutable/Set;)V
         8: getstatic     #37                 // Field scala/runtime/BoxedUnit.UNIT:Lscala/runtime/BoxedUnit;
        11: areturn
      LineNumberTable:
        line 1: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      12     0  this   L$line3/$read$$iw$C2;
            0      12     1    v1   Ljava/lang/Object;
    MethodParameters:
      Name                           Flags
      v1                             final

Compare with:

scala> abstract class C3 extends scala.runtime.AbstractFunction1[Set[AnyRef], Unit] { self: (Set[_] => Unit) =>  def apply(s: Set[AnyRef]): Unit = () }
defined class C3

scala> :javap -v C3#apply
  public void apply(scala.collection.immutable.Set<java.lang.Object>);
    descriptor: (Lscala/collection/immutable/Set;)V
    flags: ACC_PUBLIC
    Code:
      stack=0, locals=2, args_size=2
         0: return
      LineNumberTable:
        line 1: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       1     0  this   L$line4/$read$$iw$C3;
            0       1     1     s   Lscala/collection/immutable/Set;
    Signature: #17                          // (Lscala/collection/immutable/Set<Ljava/lang/Object;>;)V
    MethodParameters:
      Name                           Flags
      s                              final

[som-snytt]

scala> :javap -v C1#apply

Thanks for exploiting this little feature.