JSON Path computation doesn't do lax mode unboxing

(Filing this not because I expect we will fix this [soon], but rather to document this)

MariaDB supports "lax mode" for JSON Path evaluation (there is also "strict mode" which is not supported).

The SQL Standard specifies that in this mode the JSON Path evaluator does "boxing" and "unboxing":
"boxing" means treat a JSON value "foo" as if it was in a one-element array: ["foo"].
"unboxing" means the reverse: treat ["foo"\, ... ] as if it was just "foo".

MySQL doesn't do "boxing" or "unboxing".
(EDIT: "boxing" is done, see the comments. "unboxing" is not done)

MariaDB follows MySQL and also doesn't do that.

Example:

SELECT JSON_VALUE('{"user": [{"name": "Alice"}]}', '$.user.name') AS extracted_name

FROM DUAL;

This returns NULL in both MariaDB and MySQL. And also in MS SQL Server (sqlfiiddle).
In OracleDB, it returns "Alice" (sqlfiddle)
In PostgreSQL,

SELECT jsonb_path_query('{"user": [{"name": "Alice"}]}', '$.user.name');

also returns "Alice".

This was also discussed, especially w.r.t. mongodb, in https://jira.mariadb.org/browse/MDEV-37288?focusedCommentId=310846&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-310846

More specifically, the unboxing could happen anywhere along the path, as https://www.mongodb.com/docs/manual/core/indexes/index-types/index-multikey/create-multikey-index-embedded/#std-label-index-create-multikey-embedded shows. This means for

set @j = '{

  "item": "t-shirt",

  "stock": [

    {

      "size": "small",

      "quantity": 8

    },

    {

      "size": "large",

      "quantity": 10

    }

   ]

}';

one could get something like json_extract(@j, '$.stock.quantity') = 8 to evaluate to true.