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

From N4527 (I presume it's still in the standard):

§ 20.4.2.6 (8):

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

The program above is ill-formed, according to the standard.

End of discussion.