Implementation of a lazy conjunction based on SFINAE (see DR 1227: http://www.open-std.org/JTC1/SC22/WG21/docs/cwg_defects.html#1227)

lazy_and.cpp

// Note: This original idea was shown to me by Eric Fiselier. All credits where due.

#include <type_traits>


template <typename Condition, typename T = void>
struct enable_if
    : std::enable_if<Condition::value, T>
{ };

std::true_type expand(...);

template <typename ...T>
decltype(expand(typename enable_if<T, void*>::type{}...)) and_impl(int);

template <typename ...T>
std::false_type and_impl(...);

template <typename ...T>
struct and_
    : decltype(and_impl<T...>(int{}))
{ };



template <typename ...T>
struct Fails {
    static_assert(sizeof...(T) && false, "do not instantiate");
    static constexpr bool value = true;
};

static_assert(and_<std::true_type, std::true_type>::value, "");
static_assert(!and_<std::false_type, Fails<>>::value, "");
static_assert(!and_<std::true_type, std::false_type, Fails<>>::value, "");


int main() { }