Please note that SQL-99 is not a MariaDB manual, it's the standard, of which MariaDB only implements a subset. Here is the documentation of the feature syntax in current MariaDB: https://mariadb.com/kb/en/mariadb/constraint/

You can either have CHECK(expression) as a part of a column definition, or CONSTRAINT [constraint_name] CHECK (expression). The latter goes outside the column definition; the page doesn't say that, it should probably be added as an explicit note; it is indeed not implemented as a part of a column definition, which the example in the description is about. [NOT] DEFERRABLE is not implemented either.

I've converted it into a task.

I don't see any reason why "b INT CHECK (b>2)" is valid column definition but "b INT constraint bc CHECK (b>2)" is invalid , except that this path was overlooked during implementation of linked task; sorry that my description doesn't make explicit stress on that.

I think the main reason is that there were no actual grammar changes here, the existing syntax is the legacy from the times where people cared less about complying with the standard. It's just the constraints were ignored earlier, and now they are actually used; but both variants that work now were grammatically acceptable before, e.g. you can do in previous versions

CREATE TABLE t1 (b INT CHECK (b>2));

or

CREATE TABLE t1 (b INT, CONSTRAINT bc CHECK (b>2));

it won't complain. But it will complain about

CREATE TABLE t1 (b INT CONSTRAINT bc CHECK (b>2));

I am not sure what are we discussing here - if there was any reason to not implement discussed variant of syntax during mentioned MDEV-7563 , it probably should be noted here or inside MDEV-7563, otherwise - we definitely should implement that.

For the reference, the standard syntax is

<column definition> ::=

<column name> [ <data type or domain name> ]

[ <default clause> | <identity column specification> | <generation clause>

| <system time period start column specification>

| <system time period end column specification> ]

[ <column constraint definition>... ]

[ <collate clause> ]

<column constraint definition> ::=

[ <constraint name definition> ] <column constraint> [ <constraint characteristics> ]

<column constraint> ::=

NOT NULL

| <unique specification>

| <references specification>

| <check constraint definition>

So "CONSTRAINT name" can be specified for any constraint, also for UNIQUE and NOT NULL, not only for CHECK