Don't crash on reversible dunder methods with extra arguments (#5348)

Closes #5337.

Author: srittau

mypy/checker.py

@@ -1003,7 +1003,7 @@ def check_reverse_op_method(self, defn: FuncItem,
             reverse_type = reverse_type.copy_modified(arg_types=[reverse_type.arg_types[0]] * 2,
                                                       arg_kinds=[ARG_POS] * 2,
                                                       arg_names=[reverse_type.arg_names[0], "_"])
-        assert len(reverse_type.arg_types) == 2
+        assert len(reverse_type.arg_types) >= 2
 
         forward_name = nodes.normal_from_reverse_op[reverse_name]
         forward_inst = reverse_type.arg_types[1]

test-data/unit/check-expressions.test

@@ -674,6 +674,10 @@ s = A() < B()
 n = A() < B() # E: Incompatible types in assignment (expression has type "str", variable has type "int")
 s = object() < B() # E: Unsupported operand types for > ("B" and "object")
 
+[case testReversibleComparisonWithExtraArgument]
+class C:
+    def __lt__(self, o: object, x: str = "") -> int: ...
+
 [case testErrorContextAndBinaryOperators]
 import typing
 class A: