document that Packages can be nested in <empty> package

[cache-nez]

Example code:

import scala.mutable.List

package apackage {
    type AType
}

Corresponding .tasty file:

 0: PACKAGE(124)
 2:   TERMREFpkg 1 [<empty>]
 4:   IMPORT(6)
 6:     SELECT 2 [collection]
 8:       TERMREFpkg 3 [scala]
10:     IMPORTED 4 [Seq]
12:   PACKAGE(112)
14:     TERMREFpkg 5 [apackage]
16:     VALDEF(18) 6 [EmptyPackage$package]
19:       IDENTtpt 7 [EmptyPackage$package[ModuleClass]]
21:         TYPEREFsymbol 36
23:           TERMREFpkg 5 [apackage]
25:       APPLY(8)
27:         SELECTin(6) 11 [<init>[Signed Signature(List(),apackage.EmptyPackage$package$/T) @<init>]]
30:           NEW
31:             SHAREDterm 19
33:           SHAREDtype 21
35:       OBJECT
36:     TYPEDEF(88) 7 [EmptyPackage$package[ModuleClass]]
39:     ....

i.e. apackage is nested in <empty> package, as indicated by indentation and the addresses: <empty> package continues until address 124.

[dwijnand]

This isn't specific to TASTy, and I'm not sure if it's a bug or not. @odersky, wdty?

I looked at this in #13113 where I left some notes. Also, @som-snytt's spent time on this, see also his #7891 and #13114.

[odersky]

I think it's a bug. Will take a look.

[dwijnand]

I think the change, or at least the start of the change, is making parser return Thicket(pkgP, pkgQ) instead of the empty package with packages p and q inside.

I remember reading about this in gkossakowski/kentuckymule#6 (comment).

[odersky]

It's a representation choice. With the current representation, we need the additional rule that normal packages are never members of the empty package, even if they are defined in it. So you could say that empty package on the outside of a tree signals "no package was given, therefore, nested packages go to the root package and all other nested definitions go to the empty package". I think that's a defensible representation choice, and it's not worth breaking Tasty compatibility in order to change it. But we might want to update the Tasty documentation to mention this fact.