This is a part of MDEV-6017.

This MDEV covers supporting creation of index on expression directly, without explicitly creating a column. It should use syntax like:

alter table t1 add index (a+b);

SHOW CREATE TABLE should display the created index.
INFORMATION_SCHEMA tables should display the expression.

Q: So, should this create a table column? (I guess yes). Should the added column be visible to the user?
A: Yes, no. Like for a long unique — a "system invisible" virtual column is created automatically

Q: which name should the created index have? (for columns, it's first column's name, with _N suffix if there's a naming conflict).
A: Like all other auto-created columns, there's a function to create a unique name for it, it checks for collisions