问分析编译错误:没有匹配的函数调用'std::pair<，>::pair()‘

EN

#include <bits/stdc++.h>
using namespace std;

struct foo {
  int n;
  foo(int n): n(n) {};
  // foo(): n(0) {};
};
int main(){
  unordered_map<int, pair<foo,foo>> m;
  m[3] = make_pair(foo(1),foo(2));
}

g++ -std=c++17 -Weffc++ -Wall -Wextra -Wsign-conversion  pairs.cpp -o ../build/pairs.bin
In file included from /usr/include/c++/8/functional:54,
                 from /usr/include/x86_64-linux-gnu/c++/8/bits/stdc++.h:71,
                 from pairs.cpp:1:
/usr/include/c++/8/tuple: In instantiation of ‘std::pair<_T1, _T2>::pair(std::tuple<_Args1 ...>&, std::tuple<_Args2 ...>&, std::_Index_tuple<_Indexes1 ...>, std::_Index_tuple<_Indexes2 ...>) [with _Args1 = {int&&}; long unsigned int ..._Indexes1 = {0}; _Args2 = {}; long unsigned int ..._Indexes2 = {}; _T1 = const int; _T2 = std::pair<foo, foo>]’:
/usr/include/c++/8/tuple:1657:63:   required from ‘std::pair<_T1, _T2>::pair(std::piecewise_construct_t, std::tuple<_Args1 ...>, std::tuple<_Args2 ...>) [with _Args1 = {int&&}; _Args2 = {}; _T1 = const int; _T2 = std::pair<foo, foo>]’
/usr/include/c++/8/ext/new_allocator.h:136:4:   required from ‘void __gnu_cxx::new_allocator<_Tp>::construct(_Up*, _Args&& ...) [with _Up = std::pair<const int, std::pair<foo, foo> >; _Args = {const std::piecewise_construct_t&, std::tuple<int&&>, std::tuple<>}; _Tp = std::__detail::_Hash_node<std::pair<const int, std::pair<foo, foo> >, false>]’
/usr/include/c++/8/bits/alloc_traits.h:475:4:   required from ‘static void std::allocator_traits<std::allocator<_CharT> >::construct(std::allocator_traits<std::allocator<_CharT> >::allocator_type&, _Up*, _Args&& ...) [with _Up = std::pair<const int, std::pair<foo, foo> >; _Args = {const std::piecewise_construct_t&, std::tuple<int&&>, std::tuple<>}; _Tp = std::__detail::_Hash_node<std::pair<const int, std::pair<foo, foo> >, false>; std::allocator_traits<std::allocator<_CharT> >::allocator_type = std::allocator<std::__detail::_Hash_node<std::pair<const int, std::pair<foo, foo> >, false> >]’
/usr/include/c++/8/bits/hashtable_policy.h:2093:36:   required from ‘std::__detail::_Hashtable_alloc<_NodeAlloc>::__node_type* std::__detail::_Hashtable_alloc<_NodeAlloc>::_M_allocate_node(_Args&& ...) [with _Args = {const std::piecewise_construct_t&, std::tuple<int&&>, std::tuple<>}; _NodeAlloc = std::allocator<std::__detail::_Hash_node<std::pair<const int, std::pair<foo, foo> >, false> >; std::__detail::_Hashtable_alloc<_NodeAlloc>::__node_type = std::__detail::_Hash_node<std::pair<const int, std::pair<foo, foo> >, false>]’
/usr/include/c++/8/bits/hashtable_policy.h:736:8:   required from ‘std::__detail::_Map_base<_Key, _Pair, _Alloc, std::__detail::_Select1st, _Equal, _H1, _H2, _Hash, _RehashPolicy, _Traits, true>::mapped_type& std::__detail::_Map_base<_Key, _Pair, _Alloc, std::__detail::_Select1st, _Equal, _H1, _H2, _Hash, _RehashPolicy, _Traits, true>::operator[](std::__detail::_Map_base<_Key, _Pair, _Alloc, std::__detail::_Select1st, _Equal, _H1, _H2, _Hash, _RehashPolicy, _Traits, true>::key_type&&) [with _Key = int; _Pair = std::pair<const int, std::pair<foo, foo> >; _Alloc = std::allocator<std::pair<const int, std::pair<foo, foo> > >; _Equal = std::equal_to<int>; _H1 = std::hash<int>; _H2 = std::__detail::_Mod_range_hashing; _Hash = std::__detail::_Default_ranged_hash; _RehashPolicy = std::__detail::_Prime_rehash_policy; _Traits = std::__detail::_Hashtable_traits<false, false, true>; std::__detail::_Map_base<_Key, _Pair, _Alloc, std::__detail::_Select1st, _Equal, _H1, _H2, _Hash, _RehashPolicy, _Traits, true>::mapped_type = std::pair<foo, foo>; std::__detail::_Map_base<_Key, _Pair, _Alloc, std::__detail::_Select1st, _Equal, _H1, _H2, _Hash, _RehashPolicy, _Traits, true>::key_type = int]’
/usr/include/c++/8/bits/unordered_map.h:978:20:   required from ‘std::unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::mapped_type& std::unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::operator[](std::unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::key_type&&) [with _Key = int; _Tp = std::pair<foo, foo>; _Hash = std::hash<int>; _Pred = std::equal_to<int>; _Alloc = std::allocator<std::pair<const int, std::pair<foo, foo> > >; std::unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::mapped_type = std::pair<foo, foo>; std::unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::key_type = int]’
pairs.cpp:11:6:   required from here
/usr/include/c++/8/tuple:1668:70: error: no matching function for call to ‘std::pair<foo, foo>::pair()’
         second(std::forward<_Args2>(std::get<_Indexes2>(__tuple2))...)