Domain casting still doesn't work right

Peter Eisentraut <peter_e@gmx.net> writes:

Here's another example of domain casting not working right:
create domain foo as varchar;
select cast(x.y as foo) from (select 'foo') as x(y);
ERROR: coerce_type: no conversion function from "unknown" to foo

Not the domain's fault. You get the same error without it:

regression=# select cast(x.y as varchar) from (select 'foo') as x(y);
ERROR: coerce_type: no conversion function from "unknown" to character varying

The problem, if it is one, is that the subselect's output column is
irrevocably assigned the datatype "unknown" when we form the subselect
result list. We could make this particular problem go away by coercing
the subselect result to text, but that would create problems in other
areas, notably UNIONs. In a UNION you need to leave the individual
subselect's outputs typed as unknown as long as possible, so that they
don't screw up a type assignment derived from another subselect where
the column isn't an untyped literal.

regards, tom lane