The following code example is taken from the book
C++ Templates - The Complete Guide, 2nd Edition
by David Vandevoorde, Nicolai M. Josuttis, and Douglas Gregor,
Addison-Wesley, 2017
© Copyright David Vandevoorde, Nicolai M. Josuttis, Douglas Gregor 2017
#include <utility> // for declval
#include <type_traits> // for true_type, false_type, and void_t
// primary template:
template<typename, typename, typename = std::void_t<>>
struct HasPlusT : std::false_type
{
};
// partial specialization (may be SFINAE'd away):
template<typename T1, typename T2>
struct HasPlusT<T1, T2, std::void_t<decltype(std::declval<T1>()
+ std::declval<T2>())>>
: std::true_type
{
};