As Amit Langot pointed out, the column_constraint definition is missing whereas it is used in ALTER TABLE synopsis. It can be easily found in the CREATE TABLE synopsis, but it's not very user friendly.
I simply copied/paste the column_constraint definition from the CREATE TABLE synopsis to the ALTER TABLE synopsis. I also had to change the first word "where" to "and" as it's not the first definition in that synopsis.
I choose to add it above table_constraint as column_constraint is used before table_constraint.
The patch should apply to MASTER (or I messed up with git). I built and tested it successfully on my laptop.