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

Don't SFINAE on std::get; that is not permitted.

Here are two relatively sfinae friendly ways to test if you can using std::get; get(t):

template
using can_get=std::integral_constant::value>;

namespace helper{
  template
  struct can_get_type:std::false_type{};
  template
  struct can_get_type>:
    std::integral_constant+...)==1>
  {};
}
template
using can_get=typename helpers::can_get_type::type;

Then your code reads:

template {},int> =0>
decltype(auto) foo(C &c) {
  return std::get(c);
}