Refinements of value types based on Java primitives erase to Object when used as parameters of a method returning a generic

[scabug]

So, this is fun:

First, a test file:

class Test {
  type T = Long {type Tag = Nothing}
  type S = String {type Tag = Nothing}
  def foo(t: T): List[Int] = Nil
  def bar(t: T): Int = 0
  def baz(s: S): List[Int] = Nil
}

Now the output of javap:

public class Test {
  public scala.collection.immutable.List<java.lang.Object> foo(java.lang.Object);
  public int bar(long);
  public scala.collection.immutable.List<java.lang.Object> baz(java.lang.String);
  public Test();
}

Note the bytecode parameter types of each method. Weird.

What's weirder is:

$ scalac -Xprint:erasure Test.scala
[[syntax trees at end of                   erasure]] // Test.scala
package <empty> {
  class Test extends Object {
    def <init>(): Test = {
      Test.super.<init>();
      ()
    };
    def foo(t: Long): List = immutable.this.Nil;
    def bar(t: Long): Int = 0;
    def baz(s: String): List = immutable.this.Nil
  }
}

Anyway, actually fixing this seems like it would break binary compatibility, so I don't think this can be fixed in 2.11.x.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-8756?orig=1
Reporter: @clhodapp
Affected Versions: 2.11.2
See #4214

[scabug]

@SethTisue said:
This is certainly surprising, but it is necessary. For all the gory details, see #4214 and https://groups.google.com/d/msg/scala-internals/K2dELqajQbg/gV0tbjRHJ4UJ.

[scabug]

@paulp said (edited on Jul 9, 2015 8:50:30 PM UTC):
That's not the same problem. This is a fixable bug.

Play "one of these kids is not like the others" with the six bytecode signatures. That kid is the bug.

trait Bippy[A]

class Test {
  type T1 = Long
  type T2 = Long { type Tag = Nothing }

  def f1(t: T1): Bippy[Object] = ???
  def f2(t: T2): Bippy[Object] = ???
  def g1(t: T1): Int           = ???
  def g2(t: T2): Int           = ???
  def h1(t: T1): Object        = ???
  def h2(t: T2): Object        = ???

  // public Bippy<java.lang.Object> f1(long);
  // public Bippy<java.lang.Object> f2(java.lang.Object);
  // public int g1(long);
  // public int g2(long);
  // public java.lang.Object h1(long);
  // public java.lang.Object h2(long);
}

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@SethTisue said:
I see now. Thanks for the correction. Reopening.

[scabug]

@retronym said:
scala/scala#5174