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- #ifndef BOOST_HANA_FWD_EMPTY_HPP
- #define BOOST_HANA_FWD_EMPTY_HPP
- #include <boost/hana/config.hpp>
- #include <boost/hana/core/when.hpp>
- BOOST_HANA_NAMESPACE_BEGIN
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- #ifdef BOOST_HANA_DOXYGEN_INVOKED
- template <typename M>
- constexpr auto empty = []() {
- return tag-dispatched;
- };
- #else
- template <typename M, typename = void>
- struct empty_impl : empty_impl<M, when<true>> { };
- template <typename M>
- struct empty_t {
- constexpr auto operator()() const;
- };
- template <typename M>
- constexpr empty_t<M> empty{};
- #endif
- BOOST_HANA_NAMESPACE_END
- #endif
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