Imported From: https://issues.scala-lang.org/browse/SI-4820?orig=1
Reporter: @gkossakowski
See #4023

@gkossakowski said:
More interesting findings here. It turns out that it's broken for object (not case object) too. Thus the minimal test-case would be:

package test
abstract class Test[A] {
  object Y  
}

You'll get following errors:

mac-grek:badsigs grek$ rm -rf classes/* && scalac -d classes/ Test.scala && ~/scala/badsigs/badsigs.sh `pwd`/classes
Cleaning up /Users/grek/scala/badsigs/badsigs_working_dir
Checking /Users/grek/tmp/badsigs/classes:
Running Main app (will generate Java files and run ecj)
----------
1. ERROR in /Users/grek/scala/badsigs/badsigs_working_dir/src/C0.java (at line 0)
	import test.Test$Y$;
	^
Inconsistent classfile encountered: The undefined type parameter A is referenced from within Test$Y$
----------
1 problem (1 error)

----------
1. ERROR in /Users/grek/scala/badsigs/badsigs_working_dir/src/C1.java (at line 0)
	import test.Test;
	^
The class file Test<A> contains a signature '()Ltest/Test<TA;>.Y;' ill-formed at position 18
----------
1 problem (1 error)

Found 2 errors

However, if I use lazy val and inner class:

package test
abstract class Test[A] {  
  class Y
  lazy val Y = new Y
}

badsigs.sh doesn't report any problems. It turns out that signatures of accessor method Y are in both cases the same (and correct). It's InnerClasses attribute that's broken in first case (object Y is not reported as an inner class).

Diff of javap for both cases proves the point:

mac-grek:badsigs grek$ diff object_sig lazyval_sig 
[...]
>   InnerClass: 
>    public #3= #17 of #13; //Y=class test/Test$Y of class test/Test
[...]

@paulp said:
Trivia for 4820 fans: this ticket number is also the number of iulian's thesis.

http://library.epfl.ch/en/theses/?nr=4820

It's a sign from the heavens!

@gkossakowski said:
:-)

Do we have any progress on it?

@paulp said:
Yeah, I burned the entirety of wednesday on it. It is the name mangling chicken coming home to roost. I knew he was coming someday. I should have the situation summarized by our 8AM meeting.

@soc said:

"object Y is not reported as an inner class"

This sounds pretty much like the problem previously uncovered with java reflection and objects.

Commit Message Bot (anonymous) said:
(grek in r25639) Fix various InnerClasses bugs.

This commit fixes two major problems:

1. InnerClasses table missed entries
   that would close the chain between
   nested and top-level class.

2. In some situations, classes
   corresponding to objects would be
   not be reported in the InnerClasses
   table.

For details it's the best to check #4819, #4820 and
#4983.

First problem mentioned above was straightforward to
fix so I won't be going into details.

The second one deserves more attention. From now, classes
corresponding to objects are properly reported as inner
classes. Also, members (classes, objects) of objects are
reported as inner classes of classes corresponding to
objects.

There's one caveat though: top level objects get two
classes (regular and mirror). Members of top-level
objects are declared as inner classes of mirror class
and not regular one. The reason for that is to allow
importing them from Java. For example:

object A { class B }

will be compiled into following classes: A, A$, A$B.
If we declared A$B as inner class of A$ (regular class
for objects) then it would be impossible to import
B using "import A.B" or "import A$.B" constructs. The
reason for that is that Java compiler seems to blindly
put dollars instead of looking at InnerClasses attribute.

Since non-top-level objects don't have a mirror class
it's impossible to use the same solution. Thus, in case
like this:

object A { object B { class C } }

it's impossible to import C from Java. That's the tradeoff
for fixing other (more serious) problems. It's never been
possible to do that in a clean way so we are not making
situation worse.

As a nice consequence of this change, we get better way to
refer to inner members of top-level objects. It's been
reflected in one of test-cases that is updated by this
change.

Fixes #4789 #4819 #4820 #4983 and possibly some
other tickets related to reflection.

Review by extempore, dragos.

@soc said:
Judging from the last commit message it seems like this one should be closed, but it isn't.

I ran the badsigs tool and it doesn't report any errors:

Root is /home/soc/Entwicklung/badsigs
Working dir is /tmp/badsigs.VG0Ed
Checking /home/soc/Entwicklung/badsigs/./classes/:
Running Main app (will generate Java files and run ecj)

I guess this can be closed?

@heathermiller said:
Judging by the conversation, the resulting commit, and the fact that all seems to build and run:

Heathers-iMac:scala-heather hmiller$ ../badsigs/badsigs.sh ../scala-heather/classes
Root is /Users/hmiller/Dropbox/git-shared/badsigs
Working dir is /tmp/badsigs.vsQMC
Badsigs JAR is /Users/hmiller/Dropbox/git-shared/badsigs/target/badsigs-assembly-0.1-SNAPSHOT.jar
Checking /Users/hmiller/Dropbox/git-shared/badsigs/../scala-heather/classes:
Running Main app (will generate Java files and run ecj)
Heathers-iMac:scala-heather hmiller$ 

All seems resolved, so I'll be closing this.

@gkossakowski said:
Yes, it's fixed. I forgot to close it. Thanks Heather!