Function main

example/optional/value.cpp:14–20

int main() {
    static_assert(hana::just('x').value() == 'x', "");
    BOOST_HANA_CONSTANT_CHECK(*hana::just(hana::type_c<int>) == hana::type_c<int>);
    BOOST_HANA_RUNTIME_CHECK(hana::just(std::string{"abcd"})->size() == 4);

    // hana::nothing.value(); // compile-time error
}