factorial(float) was obviously intended to work the way it does, so I'd leave it alone in whatever changes are made to resolve _this_ issue.  I view it as a harmless-enough quirk, but, regardless, if people want to deprecate it that should be a different issue.

What the code ends up doing for Decimal and Fraction and god-only-knows-what-else is just as obviously confused.  So clean that up and declare victory ;-)