Looks like a bug to me. A shorter example:
(%i1) fpprec:500;
(%o1) 500
(%i2) float(0b0);
float: floating point overflow converting 0.0b0
Aside from bigfloat 0, I think float should return 0.0 for bigfloats
which are smaller than the smallest float (instead of triggering
an underflow error).
Thanks for this report. It would be a good idea to make a
bug report in the tracker too so we don't lose it.
Robert Dodier