fix: don't project categorical in ak._v2.packed (#1689)

* test: test packing of categoricals

* fix: don't project categoricals

FIXME: in future we should be able to specify this at the behaviour level

* test: improve clarity

src/awkward/_v2/contents/indexedarray.py

@@ -1244,7 +1244,16 @@ def continuation():
             raise ak._v2._util.error(AssertionError(result))
 
     def packed(self):
-        return self.project().packed()
+        if self.parameter("__array__") == "categorical":
+            return IndexedArray(
+                self._index,
+                self._content.packed(),
+                identifier=self._identifier,
+                parameters=self._parameters,
+                nplike=self._nplike,
+            )
+        else:
+            return self.project().packed()
 
     def _to_list(self, behavior, json_conversions):
         out = self._to_list_custom(behavior, json_conversions)
@@ -1261,8 +1270,8 @@ def _to_nplike(self, nplike):
         return IndexedArray(
             index,
             content,
-            identifier=self.identifier,
-            parameters=self.parameters,
+            identifier=self._identifier,
+            parameters=self._parameters,
             nplike=nplike,
         )
 

src/awkward/_v2/contents/indexedoptionarray.py

@@ -1684,10 +1684,20 @@ def continuation():
 
     def packed(self):
         original_index = self._index.raw(self._nplike)
-
         is_none = original_index < 0
         num_none = self._nplike.index_nplike.count_nonzero(is_none)
-        if self._content.length > len(original_index) - num_none:
+        if self.parameter("__array__") == "categorical" or self._content.length <= (
+            len(original_index) - num_none
+        ):
+            return ak._v2.contents.IndexedOptionArray(
+                self._index,
+                self._content.packed(),
+                self._identifier,
+                self._parameters,
+                self._nplike,
+            )
+
+        else:
             new_index = self._nplike.index_nplike.empty(
                 len(original_index), dtype=original_index.dtype
             )
@@ -1704,15 +1714,6 @@ def packed(self):
                 self._nplike,
             )
 
-        else:
-            return ak._v2.contents.IndexedOptionArray(
-                self._index,
-                self._content.packed(),
-                self._identifier,
-                self._parameters,
-                self._nplike,
-            )
-
     def _to_list(self, behavior, json_conversions):
         out = self._to_list_custom(behavior, json_conversions)
         if out is not None:

tests/v2/test_1688-pack-categorical.py

@@ -0,0 +1,24 @@
+# BSD 3-Clause License; see https://github.com/scikit-hep/awkward-1.0/blob/main/LICENSE
+
+import pytest  # noqa: F401
+import awkward as ak  # noqa: F401
+
+
+numpy = ak.nplike.Numpy.instance()
+
+
+def test():
+    this = ak._v2.to_categorical(["one", "two", "one", "three", "one", "four"])
+    assert ak._v2.is_categorical(this)
+    # Ensure packing by itself doesn't change the type
+    this_packed = ak._v2.packed(this)
+    assert this_packed.type == this.type
+    # Ensure the categories match between the two
+    assert ak._v2.all(ak._v2.categories(this_packed) == ak._v2.categories(this))
+
+    # Ensure the inner types match (ignoring the length change)
+    this_subset_packed = ak._v2.packed(this[:-1])
+    assert ak._v2.is_categorical(this_subset_packed)
+    assert this_subset_packed.type.content == this.type.content
+    # Ensure the categories match between the two
+    assert ak._v2.all(ak._v2.categories(this_subset_packed) == ak._v2.categories(this))