test: ensure typetracer isn't seen by users

tests/test_1940-ak-backend.py

@@ -0,0 +1,22 @@
+# BSD 3-Clause License; see https://github.com/scikit-hep/awkward-1.0/blob/main/LICENSE
+
+import numpy as np  # noqa: F401
+import pytest  # noqa: F401
+
+import awkward as ak
+
+
+def test_typetracer():
+    array = ak.Array([[0, 1, 2, 3], [8, 9, 10, 11]])
+    typetracer = ak.Array(array.layout.typetracer)
+
+    with pytest.raises(ValueError, match="internal backend"):
+        ak.backend(typetracer)
+
+
+def test_typetracer_mixed():
+    array = ak.Array([[0, 1, 2, 3], [8, 9, 10, 11]])
+    typetracer = ak.Array(array.layout.typetracer)
+
+    with pytest.raises(ValueError, match="internal backend"):
+        ak.backend(typetracer, array)