The JSON mutators always blame argument 1 in a diagnostic and report a position inside an internal buffer

When a mutator fails part-way, the diagnostic it raises names argument 1
and gives a position, whatever argument actually caused the failure and
however short argument 1 is.

SET @d31 = CONCAT(REPEAT('[', 31), '1', REPEAT(']', 31));

SELECT JSON_INSERT('[1]', '$[1]', JSON_COMPACT(@d31)) AS v;

v

NULL

Warnings:

Warning  4040  Limit of 32 on JSON nested structures depth is

               reached in argument 1 to function 'json_insert'

               at position 35

Argument 1 is the three-byte literal `[1]`. It is a valid document, it
has no position 35, and it is not what exceeded the depth limit -
argument 3 is.

How to repeat

SET @d31 = CONCAT(REPEAT('[', 31), '1', REPEAT(']', 31));

SELECT JSON_INSERT('[1]', '$[1]', JSON_COMPACT(@d31))        AS v;

SELECT JSON_SET('{"a":1}', '$.b', JSON_COMPACT(@d31))        AS v;

SELECT JSON_REPLACE('{"a":1}', '$.a', JSON_COMPACT(@d31))    AS v;

SELECT JSON_ARRAY_APPEND('[1]', '$', JSON_COMPACT(@d31))     AS v;

-- each names argument 1 at a position argument 1 does not have

Root cause

Every mutator's error path reports against the buffer the function has
been composing rather than against the argument that failed, and passes
a literal zero as the argument index. The reporting helper then adds
one to that index and computes the position as an offset into the buffer
it was handed, so the message is always "argument 1" at an offset into
something the caller never wrote.

Note

This rests on the message being a false statement about a named
argument, which is measurable, and not on any documented sentence about
which argument a diagnostic should name.

Affected versions

The mutators were added in 10.2.3 and have reported this way
throughout.

Verified on 10.11 only (10.11.19-MariaDB-debug, commit 1dab253482d).
Other branches were not tested.

Note

Found while auditing the JSON functions against their documented
contracts. Pinned as recorded output in five or more places -
func_json_depth.result:139/144/149/154/298/312 all record a warning
naming argument 1 at a position that argument does not have.