What is the difference between the 2 versions of std::enable_if

I don't understand why the first (good) version of the code compiles, but the second doesn't

I have read this, this , this, and of course this but i still do not understand why for one version it compiles, while for the other it doesn't. If somebody could please explain it (like for total dummies), I would really appreciate it.

GOOD version

template <typename As, typename std::enable_if<
std::is_arithmetic<As>::value, As>::type* = nullptr   > 
As getStringAs(const std::string& arg_name)
{
    std::istringstream istr(arg_name);
    As val;
    istr >> val;
    if (istr.fail())
        throw std::invalid_argument(arg_name);
    return val;
}

BAD version

template <typename As, typename std::enable_if_t<
std::is_arithmetic<As>::value, As> = 0   > 
As getStringAs(const std::string& arg_name)
{
    std::istringstream istr(arg_name);
    As val;
    istr >> val;
    if (istr.fail())
        throw std::invalid_argument(arg_name);
    return val;
}

Intended Usage:

int main()
{
   return getStringAs<float>("2.f");
}

Thank you very much!