> For example:
>
> > > (defun f (a b) (list a b))
> > >
> > > (defalias 'g (apply-partially #'f 1))
> > >
> > > what would (func-arity 'g) return?
> >
> > Ideally, it should return (1 . 1).
>
> This signature can be interpreted as "accepts any number of arguments",
> whereby it doesn't.  The condition-case solution with the wrong number
> of args handler at least also catches this case.

But Emacs itself clearly _knows_ that only one argument is acceptable.
So a function that replicates the steps made by the Lisp interpreter
to arrive at this conclusion will be able to reach the same
conclusion.  So I don't see any insoluble problems here.