overloaded function question

I am working on the regression tests for a couple of new 'cube' functions
for build cubes from float8s and I am seeing an unexpected failure to
implicitly cast int's to float8 in some, but not all cases.
For example:
area=# select cube(2::float);
 cube
------
 (2)
(1 row)

area=# select cube(2::int);
ERROR:  Function cube(integer) does not exist
        Unable to identify a function that satisfies the given argument types
        You may need to add explicit typecasts

There are both cube(text) and cube(float8) functions defined.

When I use the two argument version I don't have a similar problem.
There are cube(cube, float8) and cube(float8,float8) functions defined.
I do not get a error message when calling cube(1,2).

Is this a bug? Is there some other 'cube' function I am not remembering that
causes a conflict?