Item_singlerow_subselect will be rewritten only if it contains no FROM clause, and it is not part of UNION, and it is a scalar subquery. For now, there will be no conversion of subqueries with field or reference on top of item list (on the one hand, we can't change the name of such items, but on the other hand, we should assign to it the name of the whole subquery which will be reduced).

The following will not be reduced:

SELECT a;
SELECT 1 UNION SELECT 2;
SELECT 1 FROM t1;

The following select will be reduced:

SELECT 1;
SELECT a+2;

Such a subquery will be completely replaced by its expression from item list and its SELECT_LEX and SELECT_LEX_UNIT will be removed from SELECT_LEX's tree.

But every Item_field and Item_ref of that expression will be marked for processing by a special fix_fields() procedure. The fix_fields() procedures for such Items will be performed in the same way as for items of an inner subquery. Also, if this expression is Item_fields or Item_ref, then the name of this new item will be the same as the name of this item (but not (SELECT ...)). This is done to prevent broken references on such items from more inner subqueries.