Drew Adams <drew.adams@oracle.com> schrieb am Di., 12. Juli 2016 um
18:14 Uhr:

> It is not backward compatible, since `fboundp' returns t if
> there is a function value and `symbol-function' returns that
> function value.  This will break any code that depends on
> getting t or nil.
>

And the docstring of `fboundp' does specify that it returns t (not just
non-nil), so this is indeed a breaking chance.

I'd suggest to simply remove the FIXME instead.