C++ SFINAE enable_if_t in member function, how to disambiguate?

Suppose we have some SFINAE member function:

class foo{
    template <class S, class = std::enable_if_t<std::is_integral<S>, S>
    void bar(S&& s);
    template <class S, class = std::enable_if_t<!std::is_integral<S>, S>
    void bar(S&& s);
}

If we declared it as above, then how can we define them? Both of their function signatures would look like:

template <class S, class>
inline void foo::bar(S&& s){ ... do something ... }

I have seen examples where one returns an std::enable_if_t<...> like:

template <class S, class>
decltype(auto) bar(S&& s) -> std::enable_if_t<!std::is_integral<S>, S>(...){
    ... do something ...
}

To disambiguate based off of the return type. But I don't want to return anything.