[unnecessary-list-index-lookup] Fix crashes for uninferrable 'start' …

…value in 'enumerate' (#9704)

---------

Co-authored-by: Daniël van Noord <[email protected]>

doc/whatsnew/fragments/9078.bugfix

@@ -0,0 +1,4 @@
+Fixed a crash when the ``start`` value in an ``enumerate`` was non-constant and impossible to infer
+(like in``enumerate(apples, start=int(random_apple_index)``) for ``unnecessary-list-index-lookup``.
+
+Closes #9078

pylint/checkers/refactoring/refactoring_checker.py

@@ -2454,7 +2454,9 @@ def _get_start_value(self, node: nodes.NodeNG) -> tuple[int | None, Confidence]:
             and isinstance(node.operand, (nodes.Attribute, nodes.Name))
         ):
             inferred = utils.safe_infer(node)
-            start_val = inferred.value if inferred else None
+            # inferred can be an astroid.base.Instance as in 'enumerate(x, int(y))' or
+            # not correctly inferred (None)
+            start_val = inferred.value if isinstance(inferred, nodes.Const) else None
             return start_val, INFERENCE
         if isinstance(node, nodes.UnaryOp):
             return node.operand.value, HIGH

tests/functional/u/unnecessary/unnecessary_list_index_lookup.py

@@ -156,3 +156,14 @@ def _get_extra_attrs(self, extra_columns):
 for idx, val in enumerate(my_list):
     if (val := 42) and my_list[idx] == 'b':
         print(1)
+
+def regression_9078(apples, cant_infer_this):
+    """Regression test for https://github.com/pylint-dev/pylint/issues/9078."""
+    for _, _ in enumerate(apples, int(cant_infer_this)):
+        ...
+
+def random_uninferrable_start(pears):
+    import random # pylint: disable=import-outside-toplevel
+
+    for _, _ in enumerate(pears, random.choice([5, 42])):
+        ...