Infer the __len__ result of a subclass of an integer

Rather than failing the inference altogether, we can infer the default
int value for subclasses of ints.

1 files changed, 11 insertions, 2 deletions

diff --git a/astroid/helpers.py b/astroid/helpers.py
index be133b38..d94c9542 100644
--- a/astroid/helpers.py
+++ b/astroid/helpers.py
@@ -254,12 +254,16 @@ def object_len(node, context=None):
         return len(inferred_node.elts)
     if isinstance(inferred_node, nodes.Dict):
         return len(inferred_node.items)
+
+    node_type = object_type(inferred_node, context=context)
+    if not node_type:
+        raise exceptions.InferenceError(node=node)
+
     try:
-        node_type = object_type(inferred_node, context=context)
         len_call = next(node_type.igetattr("__len__", context=context))
     except exceptions.AttributeInferenceError:
         raise exceptions.AstroidTypeError(
-            "object of type '{}' has no len()".format(len_call.pytype())
+            "object of type '{}' has no len()".format(node_type.pytype())
         )
 
     result_of_len = next(len_call.infer_call_result(node, context))
@@ -268,6 +272,11 @@ def object_len(node, context=None):
         and result_of_len.pytype() == "builtins.int"
     ):
         return result_of_len.value
+    if isinstance(result_of_len, bases.Instance) and result_of_len.is_subtype_of(
+        "builtins.int"
+    ):
+        # Fake a result as we don't know the arguments of the instance call.
+        return 0
     raise exceptions.AstroidTypeError(
         "'{}' object cannot be interpreted as an integer".format(result_of_len)
     )