(2^63 - 1)::bigint => out of range? (because of the double precision)

Hi there,

I've decided to run some tests to see how my tables' ids would survive
when their yielding sequences would start hitting their MAXVALUE's, by
doing some "SELECT setval('foo_id_seq', ~maxbigint)". As I don't like
to hardcode numbers (esp. huge numbers, because sequences are always[*]
bigint's), I've tried to use (2^63 - 1)::bigint as "maxbigint", to no
avail, in contrast to (2^31 - 1)::int (-> below is short mnemonic for
"returns"):

select (2^31 - 1)::int -> 2147483647 (correct)

select (2^63 - 1)::bigint -> bigint out of range (???)
select (9223372036854775807)::bigint -> 9223372036854775807 (correct)

Apparently, this is because the type of 2^63 is double precision, which
is inexact; if I explicitly cast any of 2 or 63 to ::numeric, it behaves
as expected:

select (2::numeric^63 - 1)::bigint -> 9223372036854775807 (ok)
select (2^63::numeric - 1)::bigint -> 9223372036854775807 (ditto)

What is the rationale for (int ^ int) to return double precision rather
than numeric? I am missing something obvious here?

./danfe

P.S. On a tangentally related note, why is "NO CYCLE" is the default
for sequences?

[*] Per documentation, "The [SQL] standard's AS <data type> expression
is not supported." Another "why is it so?" question, btw. ;-)