ij> create table t (c1 int generated always as identity, c2 generated always as (c1));
0 rows inserted/updated/deleted
ij> insert into t values (default, default);
ERROR 23502: Column 'C2' cannot accept a NULL value.
Preferably, this should work. Alternatively, if it is not supposed to work, it should fail when the table is created and the error message should state that a generated column is not allowed to reference an identity column.