Why isn't pg_stat_get_subscription() marked as proretset?

The code in pg_stat_get_subscription() appears to believe that it
can return a set of rows, but its pg_proc entry does not have
proretset set. It may be that this somehow accidentally fails
to malfunction when the function is used via the system views,
but if you try to call it directly it falls over:

regression=# select pg_stat_get_subscription(0);
ERROR: set-valued function called in context that cannot accept a set

regards, tom lane