Fix: Lengths of empty regular slices

src/awkward/_v2/contents/content.py

@@ -520,6 +520,13 @@ def _getitem(self, where):
                 self._nplike,
             )
 
+            if any(isinstance(x, list) and len(x) == 0 for x in items) and any(
+                isinstance(x, slice) for x in items
+            ):
+                attempt = next.maybe_toNumpyArray()
+                if attempt is not None:
+                    next = attempt
+
             out = next._getitem_next(nextwhere[0], nextwhere[1:], None)
 
             if out.length == 0:

tests/v2/test_1568-fix-lengths-empty-regular-slices.py

@@ -0,0 +1,114 @@
+# 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
+import numpy as np  # noqa: F401
+
+to_list = ak._v2.operations.to_list
+
+
+def test_lengths_empty_regular_slices():
+    d = np.arange(2 * 3).reshape(2, 3)
+    e = ak._v2.contents.NumpyArray(d)
+
+    assert ak._v2.to_list(d[:, []]) == ak._v2.to_list(e[:, []]) == [[], []]
+    assert ak._v2.to_list(d[1:, []]) == ak._v2.to_list(e[1:, []]) == [[]]
+
+    assert d[:, []].shape == ak._v2.to_numpy(e[:, []]).shape
+    assert d[1:, []].shape == ak._v2.to_numpy(e[1:, []]).shape
+
+    f = ak._v2.operations.to_regular(e, axis=1)
+    assert to_list(f[:, []]) == [[], []]
+
+    d = np.arange(5 * 7 * 11 * 13 * 17).reshape(5, 7, 11, 13, 17)
+    e = ak._v2.contents.NumpyArray(d)
+    f = ak._v2.operations.to_regular(e, axis=1)
+
+    assert (
+        ak._v2.to_list(d[-4:, -4:, []])
+        == ak._v2.to_list(e[-4:, -4:, []])
+        == ak._v2.to_list(f[-4:, -4:, []])
+        == [[[], [], [], []], [[], [], [], []], [[], [], [], []], [[], [], [], []]]
+    )
+    assert (
+        ak._v2.to_list(d[:, [], []])
+        == ak._v2.to_list(e[:, [], []])
+        == ak._v2.to_list(f[:, [], []])
+        == [[], [], [], [], []]
+    )
+    assert (
+        ak._v2.to_list(d[1:4, [], []])
+        == ak._v2.to_list(e[1:4, [], []])
+        == ak._v2.to_list(f[1:4, [], []])
+        == [[], [], []]
+    )
+    assert (
+        ak._v2.to_list(d[:, [], [], []])
+        == ak._v2.to_list(e[:, [], [], []])
+        == ak._v2.to_list(f[:, [], [], []])
+        == [[], [], [], [], []]
+    )
+    assert (
+        ak._v2.to_list(d[1:4, [], [], []])
+        == ak._v2.to_list(e[1:4, [], [], []])
+        == ak._v2.to_list(f[1:4, [], [], []])
+        == [[], [], []]
+    )
+    assert (
+        ak._v2.to_list(d[:, [], :, []])
+        == ak._v2.to_list(e[:, [], :, []])
+        == ak._v2.to_list(f[:, [], :, []])
+        == []
+    )
+    assert (
+        ak._v2.to_list(d[1:4, [], -3:, []])
+        == ak._v2.to_list(e[1:4, [], -3:, []])
+        == ak._v2.to_list(f[1:4, [], -3:, []])
+        == []
+    )
+
+    assert (
+        d[:, :, []].shape
+        == ak._v2.to_numpy(e[:, :, []]).shape
+        == ak._v2.to_numpy(f[:, :, []]).shape
+    )
+    assert (
+        d[:, [], []].shape
+        == ak._v2.to_numpy(e[:, [], []]).shape
+        == ak._v2.to_numpy(f[:, [], []]).shape
+    )
+    assert (
+        d[1:4, :, [], []].shape
+        == ak._v2.to_numpy(e[1:4, :, [], []]).shape
+        == ak._v2.to_numpy(f[1:4, :, [], []]).shape
+    )
+    assert (
+        d[:, [], [], []].shape
+        == ak._v2.to_numpy(e[:, [], [], []]).shape
+        == ak._v2.to_numpy(f[:, [], [], []]).shape
+    )
+    assert (
+        d[1:4, [], [], []].shape
+        == ak._v2.to_numpy(e[1:4, [], [], []]).shape
+        == ak._v2.to_numpy(f[1:4, [], [], []]).shape
+    )
+    assert (
+        d[:, [], :, []].shape
+        == ak._v2.to_numpy(e[:, [], :, []]).shape
+        == ak._v2.to_numpy(f[:, [], :, []]).shape
+    )
+    assert (
+        d[1:4, [], -3:, []].shape
+        == ak._v2.to_numpy(e[1:4, [], -3:, []]).shape
+        == ak._v2.to_numpy(f[1:4, [], -3:, []]).shape
+    )
+
+    np1 = np.ones((5, 7))
+    a = ak._v2.Array(np.ones((5, 7)))
+    assert (
+        ak._v2.to_list(np1[:, []]) == ak._v2.to_list(a[:, []]) == [[], [], [], [], []]
+    )
+    assert ak._v2.to_list(np1[[], :]) == ak._v2.to_list(a[[], :]) == []
+
+    assert np1[:, []].shape == ak._v2.to_numpy(a[:, []]).shape == (5, 0)
+    assert np1[[], :].shape == ak._v2.to_numpy(a[[], :]).shape == (0, 7)