inc is builtin, not a function, moreover you pass an integer to it, so your call matches inc(single,integer), while inc(integer,integer) and inc(single,single) are defined. I doubt that you can do things like this in Delphi anyway, for the same reason
The helper might be a better direction. Sysutils already has a helper for single, so best is to inherit from there.
And keep in mind it won't work for properties anyway.