Allow Fn::Length in Fn::Select (#3633)

src/cfnlint/rules/functions/Select.py

@@ -25,7 +25,7 @@ def __init__(self) -> None:
         self.fn_select = self.validate
 
     def schema(self, validator: Validator, instance: Any) -> dict[str, Any]:
-        return {
+        schema = {
             "type": "array",
             "maxItems": 2,
             "minItems": 2,
@@ -63,3 +63,8 @@ def schema(self, validator: Validator, instance: Any) -> dict[str, Any]:
                 },
             ],
         }
+
+        if validator.context.transforms.has_language_extensions_transform():
+            schema["fn_items"][0]["functions"].append("Fn::Length")  # type: ignore
+
+        return schema

test/unit/rules/functions/test_select.py

@@ -55,12 +55,12 @@ def template():
             ],
         ),
         (
-            "Invalid Fn::Select using an invalid function for index",
-            {"Fn::Select": [{"Fn::GetAtt": "MyResource"}, ["bar"]]},
+            "Invalid Fn::Length using an invalid function for index",
+            {"Fn::Select": [{"Fn::Length": [1, 2]}, ["bar"]]},
             {"type": "string"},
             [
                 ValidationError(
-                    "{'Fn::GetAtt': 'MyResource'} is not of type 'integer'",
+                    "{'Fn::Length': [1, 2]} is not of type 'integer'",
                     path=deque(["Fn::Select", 0]),
                     schema_path=deque(["fn_items", "type"]),
                     validator="fn_select",

test/unit/rules/functions/test_select_language_extension.py

@@ -0,0 +1,53 @@
+"""
+Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+SPDX-License-Identifier: MIT-0
+"""
+
+from collections import deque
+
+import pytest
+
+from cfnlint.jsonschema import ValidationError
+from cfnlint.rules.functions.Select import Select
+from cfnlint.rules.functions.SelectResolved import SelectResolved
+
+
+@pytest.fixture(scope="module")
+def rule():
+    rule = Select()
+    rule.child_rules["W1035"] = SelectResolved()
+    yield rule
+
+
+@pytest.fixture
+def template():
+    return {"Transform": "AWS::LanguageExtensions"}
+
+
+@pytest.mark.parametrize(
+    "name,instance,schema,expected",
+    [
+        (
+            "Invalid Fn::Select using an invalid function for index",
+            {"Fn::Select": [{"Fn::GetAtt": "MyResource"}, ["bar"]]},
+            {"type": "string"},
+            [
+                ValidationError(
+                    "{'Fn::GetAtt': 'MyResource'} is not of type 'integer'",
+                    path=deque(["Fn::Select", 0]),
+                    schema_path=deque(["fn_items", "type"]),
+                    validator="fn_select",
+                ),
+            ],
+        ),
+        (
+            "Valid Fn::Length with language extension",
+            {"Fn::Select": [{"Fn::Length": [1, 2]}, ["A", "B", "C"]]},
+            {"type": "string"},
+            [],
+        ),
+    ],
+)
+def test_validate(name, instance, schema, expected, rule, validator):
+    errs = list(rule.fn_select(validator, schema, instance, {}))
+    assert errs == expected, f"Test {name!r} got {errs!r}"