LETTING or HAVING clause should only reference aggregates/group or constant

LETTING, HAVING clauses are processed after GROUP BY.
These clauses can't reference any field that are not part of GROUP BY. (part of aggregate argument is okay).
Otherwise it is consider as MISSING

The follwing statements should raise error.

CREATE PRIMARY INDEX ON default;
EXPLAIN SELECT p, SUM FROM default GROUP BY p LETTING l1 = a ;
EXPLAIN SELECT p, SUM FROM default GROUP BY p LETTING l1 = a HAVING b > 2;