ctai_basic_tricks_final.cpp

namespace details
{
    template <typename>
    struct starts_with_123_impl;

    template <size_t ...values>
    struct starts_with_123_impl<values_container<1, 2, 3, values...>>
    {
         static constexpr bool value = true;
    };

    template <size_t ...values>
    struct starts_with_123_impl<values_container<values...>>
    {
         static constexpr bool value = false;
    };
}

template <typename container>
constexpr auto starts_with_123 = details::starts_with_123_impl<container>::value;