了解将部分函数应用程序或 curry 与 overload 和 std：：visit 结合使用时的错误

问：

TL的;博士

我想了解下面的第一个代码有什么问题，即错误告诉我什么。

MRE系列

我已经能够将示例缩短为以下内容，这会生成与下面的原始代码相同的错误：

#include <boost/hana/functional/overload.hpp>

auto l1 = [](int){};    using L1 = decltype(l1);
auto l2 = [](double){}; using L2 = decltype(l2);
auto l3 = [](float){};  using L3 = decltype(l3);

using Ovl = boost::hana::overload_t<L1, L2, L3>
//const // uncomment to prevent error
;

Ovl ovl{l1,l2,l3};

Ovl ovl2{ovl}; // this triggers the compilation error

原始问题

当我尝试部分应用于通过 .（我知道我不能传递模板函数的名称，所以我不能真正部分应用，所以我把它包装在一个名为 just 的通用 lambda 中;我没有费心去做完美的转发，因为我几乎不相信它与问题有关。std::visitboost::hana::overloadstd::visitvisit

#include <boost/hana/functional/overload.hpp>
#include <boost/hana/functional/partial.hpp>
#include <boost/hana/functional/curry.hpp>
#include <iomanip>
#include <iostream>
#include <variant>
#include <vector>

using var_t = std::variant<int, long, double, std::string>;

int main() {
    std::vector<var_t> vec = {10, 15l, 1.5, "hello"};

    auto visitor = boost::hana::overload([](auto arg) { std::cout << arg << ' '; },
                    [](double arg) { std::cout << std::fixed << arg << ' '; },
                    [](const std::string& arg) { std::cout << std::quoted(arg) << ' '; }
                    );
    auto visit = [](auto const& visitor_, auto const&... visited_){
        return std::visit(visitor_, visited_...);
    };
    for (auto& v: vec) {
        boost::hana::partial(visit, visitor)(v);  // the error is essentially
        boost::hana::curry<2>(visit)(visitor)(v); // the same on these 2 lines
    }
    std::cout << std::endl;
}

错误是 （godbolt）：

In file included from deleteme.cpp:1:
/usr/include/boost/hana/functional/overload.hpp: In instantiation of ‘constexpr boost::hana::overload_t<F, G>::overload_t(F_&&, G_&& ...) [with F_ = boost::hana::overload_t<main()::<lambda(auto:22)>, main()::<lambda(double)>, main()::<lambda(const string&)> >&; G_ = {}; F = main()::<lambda(auto:22)>; G = {main()::<lambda(double)>, main()::<lambda(const string&)>}]’:
/usr/include/boost/hana/detail/ebo.hpp:62:36:   required from ‘constexpr _hana::ebo<K, V, true>::ebo(T&&) [with T = boost::hana::overload_t<main()::<lambda(auto:22)>, main()::<lambda(double)>, main()::<lambda(const string&)> >&; K = boost::hana::detail::bti<1>; V = boost::hana::overload_t<main()::<lambda(auto:22)>, main()::<lambda(double)>, main()::<lambda(const string&)> >]’
/usr/include/boost/hana/basic_tuple.hpp:70:65:   required from ‘constexpr boost::hana::detail::basic_tuple_impl<std::integer_sequence<long unsigned int, _Idx ...>, Xn ...>::basic_tuple_impl(Yn&& ...) [with Yn = {main()::<lambda(const auto:25&, const auto:26& ...)>, boost::hana::overload_t<main()::<lambda(auto:22)>, main()::<lambda(double)>, main()::<lambda(const string&)> >&}; long unsigned int ...n = {0, 1}; Xn = {main()::<lambda(const auto:25&, const auto:26& ...)>, boost::hana::overload_t<main()::<lambda(auto:22)>, main()::<lambda(double)>, main()::<lambda(const string&)> >}]’
/usr/include/boost/hana/basic_tuple.hpp:97:44:   required from ‘constexpr boost::hana::basic_tuple<Xs>::basic_tuple(Yn&& ...) [with Yn = {main()::<lambda(const auto:25&, const auto:26& ...)>, boost::hana::overload_t<main()::<lambda(auto:22)>, main()::<lambda(double)>, main()::<lambda(const string&)> >&}; Xn = {main()::<lambda(const auto:25&, const auto:26& ...)>, boost::hana::overload_t<main()::<lambda(auto:22)>, main()::<lambda(double)>, main()::<lambda(const string&)> >}]’
/usr/include/boost/hana/functional/partial.hpp:71:15:   required from ‘constexpr boost::hana::partial_t<std::integer_sequence<long unsigned int, _Idx ...>, F, X ...>::partial_t(boost::hana::make_partial_t::secret, T&& ...) [with T = {main()::<lambda(const auto:25&, const auto:26& ...)>, boost::hana::overload_t<main()::<lambda(auto:22)>, main()::<lambda(double)>, main()::<lambda(const string&)> >&}; long unsigned int ...n = {0}; F = main()::<lambda(const auto:25&, const auto:26& ...)>; X = {boost::hana::overload_t<main()::<lambda(auto:22)>, main()::<lambda(double)>, main()::<lambda(const string&)> >}]’
/usr/include/boost/hana/functional/partial.hpp:61:74:   required from ‘constexpr boost::hana::partial_t<std::integer_sequence<long unsigned int, __integer_pack()(sizeof ... (X ...))...>, typename boost::hana::detail::decay<Xs>::type, typename boost::hana::detail::decay<X, typename std::remove_reference<X>::type>::type ...> boost::hana::make_partial_t::operator()(F&&, X&& ...) const [with F = main()::<lambda(const auto:25&, const auto:26& ...)>; X = {boost::hana::overload_t<main()::<lambda(auto:22)>, main()::<lambda(double)>, main()::<lambda(const string&)> >&}; typename boost::hana::detail::decay<Xs>::type = main()::<lambda(const auto:25&, const auto:26& ...)>; typename std::remove_reference<_Tp>::type = main()::<lambda(const auto:25&, const auto:26& ...)>]’
/usr/include/boost/hana/functional/curry.hpp:149:24:   required from ‘constexpr decltype(auto) boost::hana::curry_t<n, F>::operator()(X&& ...) && [with X = {boost::hana::overload_t<main()::<lambda(auto:22)>, main()::<lambda(double)>, main()::<lambda(const string&)> >&}; long unsigned int n = 2; F = main()::<lambda(const auto:25&, const auto:26& ...)>]’
deleteme.cpp:31:37:   required from here
/usr/include/boost/hana/functional/overload.hpp:53:61: error: no matching function for call to ‘boost::hana::overload_t<main()::<lambda(double)>, main()::<lambda(const string&)> >::overload_t()’
   53 |             , overload_t<G...>::type(static_cast<G_&&>(g)...)
      |                                                             ^
/usr/include/boost/hana/functional/overload.hpp:51:28: note: candidate: ‘template<class F_, class ... G_> constexpr boost::hana::overload_t<F, G>::overload_t(F_&&, G_&& ...) [with F_ = F_; G_ = {G_ ...}; F = main()::<lambda(double)>; G = {main()::<lambda(const string&)>}]’
   51 |         constexpr explicit overload_t(F_&& f, G_&& ...g)
      |                            ^~~~~~~~~~
/usr/include/boost/hana/functional/overload.hpp:51:28: note:   template argument deduction/substitution failed:
/usr/include/boost/hana/functional/overload.hpp:53:61: note:   candidate expects at least 1 argument, 0 provided
   53 |             , overload_t<G...>::type(static_cast<G_&&>(g)...)
      |                                                             ^
/usr/include/boost/hana/functional/overload.hpp:42:12: note: candidate: ‘constexpr boost::hana::overload_t<main()::<lambda(double)>, main()::<lambda(const string&)> >::overload_t(const boost::hana::overload_t<main()::<lambda(double)>, main()::<lambda(const string&)> >&)’
   42 |     struct overload_t
      |            ^~~~~~~~~~
/usr/include/boost/hana/functional/overload.hpp:42:12: note:   candidate expects 1 argument, 0 provided
/usr/include/boost/hana/functional/overload.hpp:42:12: note: candidate: ‘constexpr boost::hana::overload_t<main()::<lambda(double)>, main()::<lambda(const string&)> >::overload_t(boost::hana::overload_t<main()::<lambda(double)>, main()::<lambda(const string&)> >&&)’
/usr/include/boost/hana/functional/overload.hpp:42:12: note:   candidate expects 1 argument, 0 provided

但是，如果我定义为“手动”咖喱，它的行为符合我的预期（godbolt）：visit

#include <boost/hana/functional/overload.hpp>
#include <boost/hana/functional/partial.hpp>
#include <boost/hana/functional/curry.hpp>
#include <iomanip>
#include <iostream>
#include <variant>
#include <vector>

// the variant to visit
using var_t = std::variant<int, long, double, std::string>;

int main() {
    std::vector<var_t> vec = {10, 15l, 1.5, "hello"};

    auto visitor = boost::hana::overload([](auto arg) { std::cout << arg << ' '; },
                    [](double arg) { std::cout << std::fixed << arg << ' '; },
                    [](const std::string& arg) { std::cout << std::quoted(arg) << ' '; }
                    );

    auto visit_c = [](auto const& visitor_){
        return [&visitor_](auto const&... visited_){
            return std::visit(visitor_, visited_...);
        };
    };
    for (auto& v: vec) {
        visit_c(visitor)(v);
    }
    std::cout << std::endl;
}

（示例改编自 cppreference。