Function make_distinct_op

src/backend/parser/parse_expr.c:3114–3140

* make the node for an IS DISTINCT FROM operator */

 * make the node for an IS DISTINCT FROM operator
 */
static Expr *
make_distinct_op(ParseState *pstate, List *opname, Node *ltree, Node *rtree,
				 int location)
{
	Expr	   *result;

	result = make_op(pstate, opname, ltree, rtree,
					 pstate->p_last_srf, location);
	if (((OpExpr *) result)->opresulttype != BOOLOID)
		ereport(ERROR,
				(errcode(ERRCODE_DATATYPE_MISMATCH),
				 errmsg("IS DISTINCT FROM requires = operator to yield boolean"),
				 parser_errposition(pstate, location)));
	if (((OpExpr *) result)->opretset)
		ereport(ERROR,
				(errcode(ERRCODE_DATATYPE_MISMATCH),
		/* translator: %s is name of a SQL construct, eg NULLIF */
				 errmsg("%s must not return a set", "IS DISTINCT FROM"),
				 parser_errposition(pstate, location)));

	/*
	 * We rely on DistinctExpr and OpExpr being same struct
	 */
	NodeSetTag(result, T_DistinctExpr);

	return result;
}

/*
 * Produce a NullTest node from an IS [NOT] DISTINCT FROM NULL construct