Throw ValueError when __len__() is a negative value

Author: ChJR

vm/src/obj/objbool.rs

@@ -1,3 +1,4 @@
+use num_bigint::Sign;
 use num_traits::Zero;
 
 use crate::function::PyFuncArgs;
@@ -51,7 +52,16 @@ pub fn boolval(vm: &VirtualMachine, obj: PyObjectRef) -> PyResult<bool> {
                 let method = method_or_err?;
                 let bool_obj = vm.invoke(&method, PyFuncArgs::default())?;
                 match bool_obj.payload::<PyInt>() {
-                    Some(int_obj) => !int_obj.as_bigint().is_zero(),
+                    Some(int_obj) => {
+                        let len_val = int_obj.as_bigint();
+                        if len_val.sign() == Sign::Minus {
+                            return Err(
+                                vm.new_value_error("__len__() should return >= 0".to_string())
+                            );
+                        }
+
+                        !len_val.is_zero()
+                    }
                     None => {
                         return Err(vm.new_type_error(format!(
                             "{} object cannot be interpreted as integer",