ADL-based SFINAE

adl_sfinae.cpp

 namespace bitmasks_lib
 {
    std::false_type enableBitMaskOperators(type_tag<T>);

    // use the result of this function as SFINAE predicate
    template<typename T>
    constexpr bool bitmaskOperatorsEnabled()
    {
        using bitmasks_lib::enableBitMaskOperators;
        return decltype(enableBitMaskOperators(type_tag<T>()))::value;
    }
}

#define ENABLE_BITMASK_OPERATORS(T) std::true_type enableBitMaskOperators(type_tag<T>);

// usage:

namespace mynamespace
{
    enum class MyBitmap
    {
        ...
    };

    ENABLE_BITMASK_OPERATORS(MyBitmap)
}

Hello Manu,
where does the 'T' come from in
std::false_type enableBitMaskOperators(type_tag<T>);
Also, where is type_tag defined and why is it needed?