fix: UnmaskedArray was missing its _nextcarry_outindex (#3097)

* fix: UnmaskedArray was missing its _nextcarry_outindex

* add BitMaskedArray and ByteMaskedArray tests, too

---------

Co-authored-by: Ianna Osborne <ianna.osborne@cern.ch>

src/awkward/contents/unmaskedarray.py

@@ -268,6 +268,12 @@ def _carry(self, carry: Index, allow_lazy: bool) -> Content:
             self._content._carry(carry, allow_lazy), parameters=self._parameters
         )
 
+    def _nextcarry_outindex(self) -> tuple[int, ak.index.Index64, ak.index.Index64]:
+        counting = self._backend.index_nplike.arange(self._content.length)
+        nextcarry = ak.index.Index64(counting, nplike=self._backend.index_nplike)
+        outindex = ak.index.Index64(counting, nplike=self._backend.index_nplike)
+        return 0, nextcarry, outindex
+
     def _getitem_next_jagged(
         self, slicestarts: Index, slicestops: Index, slicecontent: Content, tail
     ) -> Content:

tests/test_3097_UnmaskedArray_missing_nextcarry_outindex.py

@@ -0,0 +1,52 @@
+# BSD 3-Clause License; see https://github.com/scikit-hep/awkward/blob/main/LICENSE
+
+from __future__ import annotations
+
+import numpy as np
+
+import awkward as ak
+from awkward.contents import (
+    BitMaskedArray,
+    ByteMaskedArray,
+    ListOffsetArray,
+    NumpyArray,
+    UnmaskedArray,
+)
+from awkward.index import Index8, Index64, IndexU8
+
+
+def test_UnmaskedArray():
+    layout = ListOffsetArray(
+        Index64(np.array([0, 3, 3, 5])),
+        UnmaskedArray(NumpyArray(np.array([1.1, 2.2, 3.3, 4.4, 5.5]))),
+    )
+    out = ak.drop_none(ak.Array(layout), axis=1)
+    assert out.tolist() == [[1.1, 2.2, 3.3], [], [4.4, 5.5]]
+
+
+def test_ByteMaskedArray():
+    layout = ListOffsetArray(
+        Index64(np.array([0, 3, 3, 5])),
+        ByteMaskedArray(
+            Index8(np.array([0, 1, 1, 0, 1])),
+            NumpyArray(np.array([1.1, 2.2, 3.3, 4.4, 5.5])),
+            True,
+        ),
+    )
+    out = ak.drop_none(ak.Array(layout), axis=1)
+    assert out.tolist() == [[2.2, 3.3], [], [5.5]]
+
+
+def test_BitMaskedArray():
+    layout = ListOffsetArray(
+        Index64(np.array([0, 3, 3, 5])),
+        BitMaskedArray(
+            IndexU8(np.array([15])),
+            NumpyArray(np.array([1.1, 2.2, 3.3, 4.4, 5.5])),
+            True,
+            5,
+            True,
+        ),
+    )
+    out = ak.drop_none(ak.Array(layout), axis=1)
+    assert out.tolist() == [[1.1, 2.2, 3.3], [], [4.4]]