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
template<typename T, typename U>
auto addA(T t, U u) -> decltype(t+u)
{
return t + u;
}
void addA(...);
template<typename T, typename U>
auto addB(T t, U u) -> decltype(auto)
{
return t + u;
}
void addB(...);
struct X {
};
using AddResultA = decltype(addA(X(), X())); // OK: AddResultA is void
using AddResultB = decltype(addB(X(), X())); // ERROR: instantiation of addB<X>
// is ill-formed