How to make specialization of `boost::mpl::sequence_tag` when only trait available?

[denzor200]

I have no problems when i know template of type, that will be passed into boost::mpl::sequence_tag:

namespace boost { namespace mpl
{
    template <typename>
    struct sequence_tag;

    template <typename... Elements>
    struct sequence_tag<std::tuple<Elements...>>
    {
        typedef fusion::fusion_sequence_tag type;
    };
}}

But my case is when have no access for std::tuple template and i only have an is_tuple trait:

namespace boost { namespace mpl
{
    template <typename, typename=void>
    struct sequence_tag;

    template <typename T>
    struct sequence_tag<T, std::enable_if_t<is_tuple<T>::value>>  // won't work
    {
        typedef fusion::fusion_sequence_tag type;
    };
}}

SFINAE is not working on boost::mpl::sequence_tag, does it have an another way?

[ideepakchauhan7]

To define boost::mpl::sequence_tag for types that satisfy the is_tuple trait without relying on SFINAE (which isn't working in this context), you can use template specialization with a helper struct. This approach leverages the compiler's ability to select the correct specialization based on the boolean value of is_tuple<T>::value.

namespace boost { namespace mpl {

    // Helper template with specializations based on is_tuple<T>::value
    template <typename T, bool IsTuple>
    struct sequence_tag_impl;

    // Specialization for when is_tuple<T>::value is true
    template <typename T>
    struct sequence_tag_impl<T, true> {
        typedef fusion::fusion_sequence_tag type;
    };

    // Specialization for when is_tuple<T>::value is false
    template <typename T>
    struct sequence_tag_impl<T, false> {
        typedef void type;  // Adjust this to the appropriate default tag
    };

    // Primary sequence_tag forwards to the helper based on is_tuple<T>::value
    template <typename T>
    struct sequence_tag : sequence_tag_impl<T, is_tuple<T>::value> {};

}}

Note :
In the false specialization, the default tag is set to void. Replace void with the correct default tag as required by your specific use case or Boost.MPL's expectations.