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- // Copyright Louis Dionne 2013-2017
- // Distributed under the Boost Software License, Version 1.0.
- // (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
- #include <boost/hana/at.hpp>
- #include <boost/hana/back.hpp>
- #include <boost/hana/front.hpp>
- #include <boost/hana/integral_constant.hpp>
- #include <boost/hana/tuple.hpp>
- #include <boost/hana/type.hpp>
- #include <type_traits>
- namespace hana = boost::hana;
- // The fact that a reference to a `type<...>` is returned from `front(types)`
- // and friends used to break the `decltype(front(types))::type` pattern,
- // because we would be trying to fetch `::type` inside a reference. To work
- // around this, a unary `operator+` turning a lvalue `type` into a rvalue
- // `type` was added.
- struct T; struct U; struct V;
- int main() {
- auto check = [](auto types) {
- static_assert(std::is_same<
- typename decltype(+hana::front(types))::type, T
- >{}, "");
- static_assert(std::is_same<
- typename decltype(+hana::at(types, hana::size_c<1>))::type, U
- >{}, "");
- static_assert(std::is_same<
- typename decltype(+hana::back(types))::type, V
- >{}, "");
- };
- check(hana::make_tuple(hana::type_c<T>, hana::type_c<U>, hana::type_c<V>));
- check(hana::tuple_t<T, U, V>);
- }
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