Polymorphic functions do not require a column definition list. The non-polymorphic function signature does require the column definition list. That we accept a column definition list in the polymorphic case is settled code but seems like it led to this bug.
Hit send too soon...
I suppose this entire query form is basically a hack around the fact that we have no syntax to directly assign names to the fields of an "anonymous record type" literal.
with a(b) as (values (row(1,2,3))) select (a.b).* from a;
ERROR: record type has not been registered
Though oddly this doesn't seem to be universal:
with a(b) as (values (row(1,2,3))) select (row(c.*)).* from a, coalesce(a.b) as c (d int, e int, f int);
f1, f2, and f3 end up being the output field names...
