Fix bound method optional index

trans/transpiler/phases/typing/types.py

@@ -279,7 +279,7 @@ class FunctionType(TypeOperator):
     def remove_self(self):
         res = FunctionType(self.parameters[1:], self.return_type)
         res.variadic = self.variadic
-        res.optional_at = self.optional_at
+        res.optional_at = self.optional_at - 1 if self.optional_at is not None else None
         return res