2.9 regression: can't use scala.collection.immutable.List from Java

[scabug]

as of r24330:

  ~> cat S.scala                                                                                                
class S { val b = List("foo") }
  ~> cat J.java
class J { String s = (new S()).b().apply(0); }
  ~> rm *.class                                                                              
  ~> /usr/local/scala-2.8.1.final/bin/scalac S.scala                                         
  ~> javac -classpath .:/usr/local/scala-2.8.1.final/lib/scala-library.jar J.java            
  ~> rm *.class
  ~> ~/scala/dists/scala-2.9.0.r24330-b20110222144825/bin/scalac S.scala                     
  ~> javac -classpath .:/Users/tisue/scala/dists/scala-2.9.0.r24330-b20110222144825/lib/scala-library.jar J.java
J.java:1: incompatible types
found   : java.lang.Object
required: java.lang.String
class J { String s = (new S()).b().apply(0); }
                                        ^
1 error

possibly related: #4238

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-4291?orig=1
Reporter: @SethTisue

[scabug]

@paulp said:
Regression, concretely:

% scala29 -nocompdaemon -e 'println(classOf[List[_]].getMethods filter (_.getName == "apply") map (_.getGenericReturnType) mkString ", ")'
class java.lang.Object, class java.lang.Object
% scala281 -nocompdaemon -e 'println(classOf[List[_]].getMethods filter (_.getName == "apply") map (_.getGenericReturnType) mkString ", ")'
A, class java.lang.Object  

"A" of course being what you want, List's type parameter.

[scabug]

@paulp said:
Must be caused by r24319 I think. Still working in r24314, busted in r24323. I'm supposed to be looking at that and making sure it's not breaking anything anyway.

[scabug]

@SethTisue said:
this appears to be another manifestation of the same issue:

  ~> cat S.scala
trait T { def f: Option[String] = None }
class S extends T
  ~> cat J.java
class J extends S { }
  ~> /usr/local/scala-2.8.1.final/bin/scalac S.scala                             
  ~> javac -classpath .:/usr/local/scala-2.8.1.final/lib/scala-library.jar J.java
  ~> rm *.class
  ~> ~/scala/dists/scala-2.9.0.r24330-b20110222144825/bin/scalac S.scala
  ~> javac -Xlint:unchecked -classpath :/Users/tisue/scala/dists/scala-2.9.0.r24330-b20110222144825/lib/scala-library.jar J.java
J.java:1: warning: f() in S implements f() in T; return type requires unchecked conversion
found   : scala.Option
required: scala.Option<java.lang.String>
class J extends S { }
^
1 warning

[scabug]

@paulp said:
Thanks for catching this so quickly: it makes a huge difference.

[scabug]

@paulp said:
(In r24344) Reversion of r24319 for causing #4291. Test case to fence out #4291
in future attempts. Closes #4291, references #4214, no review.