CREATE DATABASE tmp_1;

use tmp_1;

CREATE TABLE users

(

    u_id INT NOT NULL,

    UNIQUE INDEX UNIQ_1483A5E967CB5338 (u_id),

    PRIMARY KEY (u_id)

) ENGINE = InnoDB;

CREATE TABLE contracts

(

    id      INT AUTO_INCREMENT NOT NULL,

    user_id INT                NOT NULL,

    INDEX IDX_950A973BE43E81 (user_id),

    PRIMARY KEY (id)

) ENGINE = InnoDB;

ALTER TABLE contracts ADD CONSTRAINT FK_950A973BE43E81 FOREIGN KEY (user_id) REFERENCES users (u_id);

ALTER TABLE contracts ADD status ENUM ('a', 'b', 'c');

ALTER TABLE users

    CHANGE u_id id INT NOT NULL,

    ADD name VARCHAR(255) DEFAULT NULL;

ALTER TABLE users CHANGE id u_id INT NOT NULL;

ALTER TABLE contracts CHANGE status status VARCHAR(20) DEFAULT NULL;

Expected outcome:
Column status in contracts table is changed to type VARCHAR(20) successfully.

Actual outcome:
The queries (the last line) fail with this error:

ERROR 1025 (HY000): Error on rename of './tmp_1/#sql-1_8' to './tmp_1/contracts' (errno: 150 "Foreign key constraint is incorrectly formed")

After the error occurs, the table contracts does not exist anymore (in the filesystem it is replaced with a file #sql-1_8).

Reproduction and details:
This seems to be a very specific bug which I cannot reproduce in any other way. Some points that I've noticed while trying to reproduce this:

• This is reproducable on all version from mariadb:10.3.2 and upwards (probably introduced with https://mariadb.com/kb/en/library/instant-add-column-for-innodb/)
• The last line must be an ALTER TABLE statement which changes a columns type from enum to varchar or vice versa, no other types work as far as I know.
• Before changing a column type which triggers the error, there must be a new column added to the table which is the owning side of a foreign key in a single SQL query. i.e. if we changed

  ALTER TABLE users CHANGE u_id id INT NOT NULL, ADD name VARCHAR(255) DEFAULT NULL;

  to

  ALTER TABLE users CHANGE u_id id INT NOT NULL;

  ALTER TABLE users ADD name VARCHAR(255) DEFAULT NULL;

  There would not be an error anymore.