C++17类模板部分推导

My understanding about the Template argument deduction for class templates proposal was to homogenize the behaviour of template functions and template classes in deduction contexts. But I think that I have misunderstood something.

如果我们有这个模板对象:

If we have this template object:

template <std::size_t S, typename T>
struct test
{
    static constexpr auto size = S;
    using type_t = T;

    test(type_t (&input)[size]) : data(input) {}
    type_t (&data)[size]{};
};

我倾向于使用辅助函数作为语法糖来创建test对象:

I tend to use a helper function as syntactic sugar for creating test objects:

template <std::size_t S, typename T>
test<S, T> helper(T (&input)[S]) { return input; }

如下图所示:

int main()
{
    int buffer[5];

    auto a = helper<5, int>(buffer); // No deduction
    auto b = helper<5>(buffer);      // Type deduced
    auto c = helper(buffer);         // Type and size deduced

    std::cout << a.size << b.size << c.size;

    return 0;
}

上面的代码按预期输出 555.我使用较新的编译器设置在 Wandbox 中尝试了相同的操作¹:

The code above outputs 555 as expected. I've tried the same in Wandbox using the newer compiler setup¹:

int main()
{
    int buffer[5];

    test<5, int> a(buffer); // No deduction: Ok.
    test<5> b(buffer);      // Type deduced: FAILS.
    test c(buffer);         // Type and size deduced: Ok.

    std::cout << a.size << b.size << c.size;

    return 0;
}

看起来类模板的模板参数推导仅适用于推导所有参数，我期望两种行为(辅助函数和类模板)是相同的，我是否误解了什么?

It looks like template argument deduction for class templates works only deducing all the parameters, I was expecting both behaviours (helper function and class template) to be the same, did I misunderstood something?