presto-main/src/main/java/io/prestosql/operator/scalar/annotations/OperatorValidator.java/OperatorValidator/validateComparisonOperatorSignature
class OperatorValidator:    private static void validateComparisonOperatorSignature(OperatorType operatorType, TypeSignature returnType, List<TypeSignature> argumentTypes, int expectedArgumentCount)
    {
        validateOperatorSignature(operatorType, returnType, argumentTypes, expectedArgumentCount);
        checkArgument(returnType.getBase().equals(StandardTypes.BOOLEAN), "%s operator must return a BOOLEAN: %s", operatorType, formatSignature(operatorType, returnType, argumentTypes));
    }