fix: generalize Index.ptr (#3206)

* fix: generalize 'Index.ptr'

* include a test that checks TypeTracer

src/awkward/index.py

@@ -136,10 +136,16 @@ def metadata(self) -> dict:
 
     @property
     def ptr(self):
-        if self._nplike == Numpy.instance():
+        if isinstance(self._nplike, Numpy):
             return self._data.ctypes.data
-        elif self._nplike == Cupy.instance():
+        elif isinstance(self._nplike, Cupy):
             return self._data.data.ptr
+        elif isinstance(self._nplike, TypeTracer):
+            return 0
+        else:
+            raise NotImplementedError(
+                f"this function hasn't been implemented for the {type(self._nplike).__name__} backend"
+            )
 
     @property
     def length(self) -> ShapeItem:

tests/test_3206_generalize_index_ptr.py

@@ -0,0 +1,22 @@
+# BSD 3-Clause License; see https://github.com/scikit-hep/awkward/blob/main/LICENSE
+
+from __future__ import annotations
+
+import pytest
+
+import awkward as ak
+
+
+def test_1():
+    arr = ak.Array([[1, 3, 4], 5])
+    tarr = arr.layout.to_typetracer()
+
+    with pytest.raises(ak.errors.AxisError, match="exceeds the depth of this array"):
+        ak.flatten(tarr)
+
+
+def test_2():
+    arr = ak.Array([[[1, 3, 4]], [5]])
+    tarr = arr.layout.to_typetracer()
+
+    assert ak.flatten(tarr).type == ak.flatten(arr).type