Mathematical Operators Behavior with Strings Is Not Documented

let $l1 = $('a' || 1);

--echo ('a' || 1) -> $l1;

mysqltest: At line 1: Type error: operator '||' requires integer operands

MariaDB [(none)]> select 'a' || 1;

+----------+

| 'a' || 1 |

+----------+

|        1 |

+----------+

let $m60 = $(5 + '5');

-- echo 5 + '5' -> $m60;                            # 55 or error (depends on coercion rules

mysqltest: At line 178: Type error: operator '+' requires integer operands

MariaDB [(none)]> select 5 + '5';

+---------+

| 5 + '5' |

+---------+

|      10 |

+---------+

1 row in set (0.001 sec)