presto-main/src/main/java/io/prestosql/operator/scalar/annotations/OperatorValidator.java/OperatorValidator/validateOperatorSignature
class OperatorValidator:    private static void validateOperatorSignature(OperatorType operatorType, TypeSignature returnType, List<TypeSignature> argumentTypes, int expectedArgumentCount)
    {
        String signature = formatSignature(operatorType, returnType, argumentTypes);
        checkArgument(!returnType.getBase().equals(UnknownType.NAME), "%s operator return type can not be NULL: %s", operatorType, signature);
        checkArgument(argumentTypes.size() == expectedArgumentCount, "%s operator must have exactly %s argument: %s", operatorType, expectedArgumentCount, signature);
    }