parameter_type_id might have been created after old_function_type. This piece of code will then make old_function_type reference id of an instruction that is defined below the old_function_type which causes a segmentation fault in the type manager.

There is a bigger concern here, though. If this behaviour is invalid, then the validator should've raised an error. If it is valid, then the type manager shouldn't have caused a segmentation fault. Either way, segfault should've never occurred.