Making `std::get` play nice with SFINAE

Question

std::get does not seem to be SFINAE-friendly, as shown by the following test case:

template 
auto foo(C &c) -> declty         


        

Answer 1

std::get is explicitly not SFINAE-friendly, as per [tuple.elem]:

template 
  constexpr T& get(tuple& t) noexcept;
// and the other like overloads

Requires: The type T occurs exactly once in Types.... Otherwise, the program is ill-formed.

std::get is also explicitly not SFINAE-friendly.

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As far as the other questions:

Is there a reason for std::get not to be SFINAE-friendly?

Don't know. Typically, this isn't a point that needs to be SFINAE-ed on. So I guess it wasn't considered something that needed to be done. Hard errors are a lot easier to understand than scrolling through a bunch of non-viable candidate options. If you believe there to be compelling reason for std::get to be SFINAE-friendly, you could submit an LWG issue about it.

Is there a better workaround than what is outlined above?

Sure. You could write your own SFINAE-friendly verison of get (please note, it uses C++17 fold expression):

template ::value + ...) == 1, int> = 0>
constexpr T& my_get(tuple& t) noexcept {
    return std::get(t);
}

And then do with that as you wish.