The parser that scans the sketch does not understand c++ so can't make sense of the default arguments.
It actually doesn't understand C either, which is also the problem. Well, that and it's using regular expressions and is not a "true" parser which is why it's causing problems.
I also don't think it's smart enough to recognise existing function prototypes in a sketch.
--Phil.
P.S. Oh, although now that I think of it, if it doesn't recognise it as a function declaration then you might be able to supply your own prototype and not have a clash.