Java: incompatible types compile error when type that is reused is used in a type that is unreachable

[Kh4n]

Here is a buggy .ksy:

meta:
  id: bug
  file-extension: na
  endian: le
seq:
  - id: body
    type: type_inner
# uncomment to fix bug
#  - id: body2
#    type: type_unused
types:
  type_reused:
    seq:
      - id: count
        type: s4
  type_inner:
    seq:
      - id: file_name
        type: type_reused
  type_unused:
    seq:
      - id: source_code_loc
        type: type_reused

The compile error is:

/home/kh4n/Documents/jt-loader/target/generated-sources/kaitai/name/kh4n/jtloader/Bug.java:[110,59] incompatible types: name.kh4n.jtloader.Bug.TypeUnused cannot be converted to name.kh4n.jtloader.Bug.TypeInner

It is caused because type_reused (generated as TypeReused) has its _parent set to Bug.TypeInner:

        public TypeReused(KaitaiStream _io, Bug.TypeInner _parent, Bug _root) {
            super(_io);
            this._parent = _parent;
            this._root = _root;
            _read();
        }
...

If you uncomment this becomes:

        public TypeReused(KaitaiStream _io, KaitaiStruct _parent, Bug _root) {
            super(_io);
            this._parent = _parent;
            this._root = _root;
            _read();
        }

as it should. Manually changing the generated file also works. Not a huge issue because you can just use the type somewhere, but worth mentioning nonetheless as it is pretty common to have a type defined and then used somewhere that is currently unreachable as you are prototyping, which can be very confusing to debug.

I think it might be caused by an optimization which notices that type_unused is unreachable so it makes type_reused's parent type_inner (the only place it is technically used) but then the type_unused code is still emitted. I don't know the code base but if someone points me to possible file(s) I can try to fix the issue :). I will try my best to solve it on my own.

[generalmimon]

Nice catch. My hypothesis is that when KSC derives types of _parent parameters, the entry point is the top-level seq and instances and then it proceeds recursively to all the types used there. Therefore, types that are not reachable by traversing the dependency tree with the top-level seq+instances as the root node are not processed at all by the type derivation module. It's as if they don't even exist.

This perfectly matches another related problems in unused types (it seems that no issues have been created for these, because I've only noted them on my personal list of KS bugs without associated issues):

• unused types sometimes trigger a compile-type exception due to unability to deduce _parent type (this reminded me of it: https://github.com/sonusz/PhasorToolBox/blob/ca9c5a6d35409a369ac73738b0c5e7621fec3bea/kaitai/data.ksy#L5-L11)

• unused types are not checked for param mismatch

  meta:
    id: params_mismatch_unused_type
  types:
    uses_foo:
      seq:
        - id: t
          type: foo  # expected to pass a boolean argument and doesn't do so -> this is not reported
    foo:
      params:
        - id: a
          type: bool

[Kh4n]

Looking through JavaCompiler and NoNeedForFullClassPath, these seem relevant?

[Mingun]

This is duplicate of #414.