Regression when accessing protected Java methods

[szeiger]

There are already several tickets related to Java interop with protected methods. This may have the same root cause as one of them but it seems worth reporting separately because it's a regression (which I didn't see in the other tickets) and doesn't require any casts.

Given Java source file

package foo;

public class Foo {
  protected void foo() {}
}

and a Scala source file:

package bar

import foo.Foo

trait BarMixin { self: Foo =>
  protected def foo: Unit
  def bar = foo
}

class Bar extends Foo with BarMixin

object Bar extends App {
  (new Bar).foo // 1
  (new Bar).bar // 2
}

Both call sites compile in 2.12 and 2.13. Call site 1 always fails with

[error] Exception in thread "main" java.lang.IllegalAccessError: class bar.Bar$ tried to access protected method 'void foo.Foo.foo()' (bar.Bar$ and foo.Foo are in unnamed module of loader 'app')
[error] 	at bar.Bar$.delayedEndpoint$bar$Bar$1(Bar.scala:13)
[error] 	at bar.Bar$delayedInit$body.apply(Bar.scala:12)
[error] 	at scala.Function0.apply$mcV$sp(Function0.scala:39)
[error] 	at scala.Function0.apply$mcV$sp$(Function0.scala:39)
[error] 	at scala.runtime.AbstractFunction0.apply$mcV$sp(AbstractFunction0.scala:17)
[error] 	at scala.App.$anonfun$main$1(App.scala:76)
[error] 	at scala.App.$anonfun$main$1$adapted(App.scala:76)
[error] 	at scala.collection.IterableOnceOps.foreach(IterableOnce.scala:563)
[error] 	at scala.collection.IterableOnceOps.foreach$(IterableOnce.scala:561)
[error] 	at scala.collection.AbstractIterable.foreach(Iterable.scala:919)
[error] 	at scala.App.main(App.scala:76)
[error] 	at scala.App.main$(App.scala:74)
[error] 	at bar.Bar$.main(Bar.scala:12)
[error] 	at bar.Bar.main(Bar.scala)

Call site 2 works as expected up to 2.13.6 but fails in 2.13.7 and higher:

[error] Exception in thread "main" java.lang.IllegalAccessError: class bar.BarMixin tried to access protected method 'void foo.Foo.foo()' (bar.BarMixin and foo.Foo are in unnamed module of loader 'app')
[error] 	at bar.BarMixin.bar(Bar.scala:7)
[error] 	at bar.BarMixin.bar$(Bar.scala:7)
[error] 	at bar.Bar.bar(Bar.scala:10)
[error] 	at bar.Bar$.delayedEndpoint$bar$Bar$1(Bar.scala:14)
[error] 	at bar.Bar$delayedInit$body.apply(Bar.scala:12)
[error] 	at scala.Function0.apply$mcV$sp(Function0.scala:39)
[error] 	at scala.Function0.apply$mcV$sp$(Function0.scala:39)
[error] 	at scala.runtime.AbstractFunction0.apply$mcV$sp(AbstractFunction0.scala:17)
[error] 	at scala.App.$anonfun$main$1(App.scala:76)
[error] 	at scala.App.$anonfun$main$1$adapted(App.scala:76)
[error] 	at scala.collection.IterableOnceOps.foreach(IterableOnce.scala:563)
[error] 	at scala.collection.IterableOnceOps.foreach$(IterableOnce.scala:561)
[error] 	at scala.collection.AbstractIterable.foreach(Iterable.scala:926)
[error] 	at scala.App.main(App.scala:76)
[error] 	at scala.App.main$(App.scala:74)
[error] 	at bar.Bar$.main(Bar.scala:12)
[error] 	at bar.Bar.main(Bar.scala)

[lrytz]

This is an endless struggle.. Do you have links to the other issues you came across?

[szeiger]

The candidates I looked at were #12926, #10249 , #10656 (leading to similar runtime errors) and #12589 (not a runtime error but also caused by misinterpreting Java access modifiers)

[szeiger]

Most workarounds that I tried are hitting the same issue. What works is either making the Java methods public (which is what we are going to do) or adding a Scala class that extends the Java class and exposes the method under a new name (important; overriding the existing method does not help because scalac will always generate a call against the base type).

[som-snytt]

-- [E049] Reference Error: Bar.scala:8:12 ------------------------------------------------------------------------------
8 |  def bar = foo
  |            ^^^
  |            Reference to foo is ambiguous.
  |            It is both defined in trait BarMixin
  |            and inherited subsequently in trait BarMixin
  |
  | longer explanation available when compiling with `-explain`
-- [E173] Reference Error: Bar.scala:14:12 -----------------------------------------------------------------------------
14 |  (new Bar).foo // 1
   |  ^^^^^^^^^^^^^
   |method foo in class Foo cannot be accessed as a member of bar.Bar from object Bar.
   |  protected[foo] method foo can only be accessed from package foo, or class Foo in package foo or one of its subclasses.
2 errors found