Type-level lambda expressions for template metaprogramming in C++20

meta-lambdas.hpp

#include <boost/preprocessor/punctuation/is_begin_parens.hpp>
#include <boost/preprocessor/punctuation/remove_parens.hpp>

#include <boost/preprocessor/control/if.hpp>

#include <boost/preprocessor/seq/for_each.hpp>
#include <boost/preprocessor/seq/variadic_seq_to_seq.hpp>

namespace metalambda {
    template <auto T>
    struct template_funcall_adapter {
        template <class... Ts>
        using f = decltype(T.template operator()<Ts...>());
    };
}

#define METALAMBDA_RETURN(...) return __VA_ARGS__(); }>
#define METALAMBDA_RETURN_THUNK METALAMBDA_RETURN

#define METALAMBDA_GENERATE_LOCAL(r, data, elem) using BOOST_PP_REMOVE_PARENS(elem);
#define METALAMBDA_LOCALS(seq) BOOST_PP_SEQ_FOR_EACH(METALAMBDA_GENERATE_LOCAL, _, BOOST_PP_VARIADIC_SEQ_TO_SEQ(seq)) METALAMBDA_RETURN

#define METALAMBDA_DISPATCH_LOCALS_OR_PARAMS_IMPL(continuation, ...) continuation(__VA_ARGS__)
#define METALAMBDA_DISPATCH_LOCALS_OR_PARAMS(...) METALAMBDA_DISPATCH_LOCALS_OR_PARAMS_IMPL(BOOST_PP_IF(BOOST_PP_IS_BEGIN_PARENS(__VA_ARGS__), METALAMBDA_LOCALS, METALAMBDA_RETURN_THUNK), __VA_ARGS__)

#define METALAMBDA_PARAMS(...) ::metalambda::template_funcall_adapter<[]<__VA_ARGS__>(){ METALAMBDA_DISPATCH_LOCALS_OR_PARAMS

#define METALAMBDA(...) METALAMBDA_PARAMS(__VA_ARGS__)

#define II(...) METALAMBDA(__VA_ARGS__)


#define METALAMBDA_RETURN_AND_EVAL(...) return __VA_ARGS__(); }>::template f<>

#define LET(...) typename BOOST_PP_CAT(METALAMBDA(class = void)(__VA_ARGS__), _AND_EVAL)


#include <kvasir/mpl/mpl.hpp>

using size_lt = II(class T1, class T2)(kvasir::mpl::bool_<sizeof(T1) < sizeof(T2)>);

using int_long_list =
    LET((x = int)(y = long))
        (kvasir::mpl::list<x, y>);