Multiple evaluation of single reference to function with out parameters

Hi,

If I create a function with more than one out parameter, and then refer to
it inside parentheses as a record, e.g. select (function()).*, the function
appears to be evaluated multiple times, once for every column returned.
This seems to be true regardless of whether it's defined as volatile or
immutable.

Here's an example:

# create or replace function foobar(out foo integer, out bar integer)
volatile language plpgsql as $$ begin raise notice 'Called'; foo := 1; bar
:= 2; end; $$;
CREATE FUNCTION

If I call it the usual way, it's only evaluated once:

# select * from foobar();
NOTICE: 00000: Called
LOCATION: exec_stmt_raise, pl_exec.c:3165
foo | bar
-----+-----
1 | 2
(1 row)

Here the function was called once, and the results are returned correctly.
However, if I call it this way,

# select (foobar()).*;
NOTICE: 00000: Called
LOCATION: exec_stmt_raise, pl_exec.c:3165
NOTICE: 00000: Called
LOCATION: exec_stmt_raise, pl_exec.c:3165
foo | bar
-----+-----
1 | 2
(1 row)

This way the function seems to be called separately for each column it
returns, but the results are only returned once. If I define it with three
out parameters, it's called three times.

As far as I can tell, this behavior has been the same since at least
version 8.2 and up through 10 beta 4, but I can't find any references to it
and it seems very surprising. It could certainly cause unexpected results
if the function has side effects. Is this a bug?

Joel