defaultdb> SET experimental_enable_expression_indexes=true;
SET

defaultdb> CREATE TABLE t (a INT, b INT, UNIQUE INDEX ((a + b)));
CREATE TABLE

defaultdb> INSERT INTO t VALUES (1, 2) ON CONFLICT ((a + b)) DO NOTHING;
invalid syntax: statement ignored: at or near "(": syntax error
SQLSTATE: 42601
DETAIL: source SQL:
INSERT INTO t VALUES (1, 2) ON CONFLICT ((a + b)) DO NOTHING
                                         ^
HINT: try \h INSERT

defaultdb> INSERT INTO t VALUES (1, 2) ON CONFLICT ((a + b)) DO UPDATE SET a = 10;
invalid syntax: statement ignored: at or near "(": syntax error
SQLSTATE: 42601
DETAIL: source SQL:
INSERT INTO t VALUES (1, 2) ON CONFLICT ((a + b)) DO UPDATE SET a = 10
                                         ^
HINT: try \h INSERT