fix: incorrect passing of Index inside nplike function in to_IndexedOptionArray64 (#3727)

* do not pass index into np like function

* Use to64() shortcut

Co-authored-by: Peter Fackeldey <fackeldey.peter@gmail.com>

* fix typo

* add test

* remve whitespace

---------

Co-authored-by: Peter Fackeldey <fackeldey.peter@gmail.com>

Author: ikrommyd

src/awkward/contents/indexedoptionarray.py

@@ -269,7 +269,7 @@ def to_IndexedOptionArray64(self) -> IndexedOptionArray:
             return self
         else:
             return IndexedOptionArray(
-                self._backend.nplike.astype(self._index, dtype=np.int64),
+                self._index.to64(),
                 self._content,
                 parameters=self._parameters,
             )

tests/test_3727_to_indexedoptionarray64_with_int32_index.py

@@ -0,0 +1,18 @@
+# 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
+
+
+def test():
+    index32 = ak.index.Index(np.array([0, 1, -1], dtype=np.int32))
+    content = ak.contents.NumpyArray(np.array([10, 20, 30]))
+    arr = ak.contents.IndexedOptionArray(index32, content)
+    new_arr = arr.to_IndexedOptionArray64()
+
+    assert arr.index.dtype == np.dtype("int32")
+    assert new_arr.index.dtype == np.dtype("int64")
+    assert arr.to_list() == new_arr.to_list()