Snip from result of my MTR test including comments

CREATE TABLE t1 (

col1 VARCHAR(7961) , col2 CHAR(10)

) ENGINE=InnoDB ;

ALTER TABLE `t1` ADD IF NOT EXISTS ( `col1` SERIAL ) ;

ERROR 42000: Specified key was too long; max key length is 3072 bytes

       ### Why that?

       ### It was asked to add a column of data type SERIAL which is according the manual

       ### BIGINT UNSIGNED NOT NULL AUTO_INCREMENT UNIQUE only if it does not already exist.

       ### And when making a KEY at all than it should be on a SERIAL --> BIGINT

       ### and not VARCHAR(7961)!

DROP TABLE t1;

CREATE TABLE t1 (

col1 VARCHAR(30) , col2 CHAR(10)

) ENGINE=InnoDB ;

ALTER TABLE `t1` ADD IF NOT EXISTS ( `col1` SERIAL ) ;

Warnings:

Note    1060    Duplicate column name 'col1'

             ### Looks nice.

layout_before and layout_after are not equal

layout_before : -->t1   CREATE TABLE `t1` (

  `col1` varchar(30) DEFAULT NULL,

  `col2` char(10) DEFAULT NULL

) ENGINE=InnoDB DEFAULT CHARSET=latin1<--

layout_after  : -->t1   CREATE TABLE `t1` (

  `col1` varchar(30) DEFAULT NULL,

  `col2` char(10) DEFAULT NULL,

  UNIQUE KEY `col1` (`col1`)

             ### So a failing DDL has just silent

             ### - created a KEY (consumes resources)

             ### - including a UNIQUE constraint which changes the properties of the column

             ###   'col1' when being affected by SQL.               

) ENGINE=InnoDB DEFAULT CHARSET=latin1<--

DROP TABLE t1;

CREATE TABLE t1 (

col1 VARCHAR(30) , col2 CHAR(10)

) ENGINE=InnoDB ;

INSERT INTO t1 (col1, col2) VALUES ('ã','ã'),('ã','ã');

SELECT * FROM t1;

col1    col2

ã   ã

ã   ã

ALTER IGNORE TABLE `t1` ADD IF NOT EXISTS ( `col1` SERIAL ) ;

Warnings:

Note    1060    Duplicate column name 'col1'

layout_before and layout_after are not equal

layout_before : -->t1   CREATE TABLE `t1` (

  `col1` varchar(30) DEFAULT NULL,

  `col2` char(10) DEFAULT NULL

) ENGINE=InnoDB DEFAULT CHARSET=latin1<--

layout_after  : -->t1   CREATE TABLE `t1` (

  `col1` varchar(30) DEFAULT NULL,

  `col2` char(10) DEFAULT NULL,

  UNIQUE KEY `col1` (`col1`)

) ENGINE=InnoDB DEFAULT CHARSET=latin1<--

SELECT * FROM t1;

col1    col2

ã   ã

             ### So a failing DDL with IGNORE has just removed a row.

DROP TABLE t1;

CREATE TABLE t1 (

col1 VARCHAR(7961) , col2 CHAR(10)

) ENGINE=InnoDB ;

ALTER TABLE `t1` ADD IF NOT EXISTS `col1` SERIAL ;

Warnings:

Note    1060    Duplicate column name 'col1'

             ### Looks nice and the table definition is correct too.

DROP TABLE t1;

The problem above is NOT InnoDB specific. I get it with MyISAM too.

10.2.23 commit d3afdb1e8f0ab5d2d5257ee931fe5d4d2ed82ced 2019-03-14

10.3.14 commit 51e48b9f8981986257a1cfbdf75e4fc29a5959c1 2018-12-24

                 source was pulled 2019-03-14

10.4.4 commit e3ebeebe3a4c14293a48971e0a062008a1473140 2019-03-14

         Here I get some a bit different but not sufficient better result.