std::pair substitution failed with std::size_t

Question

#include <utility>
#include <vector>

using namespace std;

std::pair<std::size_t, std::size_t> func(const std::vector<int>& numbers, int target) {
    for(std::size_t i =0; i < numbers.size(); i++)
    {   
      for(std::size_t j = i; j < numbers.size(); j++)
      {   
        if(numbers[i] + numbers[j] == target)
          return std::make_pair<std::size_t, std::size_t>(i,j);
      }   
    }   
    return std::make_pair<std::size_t, std::size_t>(0,0);
}

Error:

test.cpp: In function ‘std::pair<long unsigned int, long unsigned int> func(const std::vector<int>&, int)’:
test.cpp:12:62: error: no matching function for call to ‘make_pair<std::size_t, std::size_t>(std::size_t&, std::size_t&)’
           return std::make_pair<std::size_t, std::size_t>(i,j);
                                                              ^
In file included from /usr/include/c++/7/utility:70:0,
                 from test.cpp:1:
/usr/include/c++/7/bits/stl_pair.h:524:5: note: candidate: template<class _T1, class _T2> constexpr std::pair<typename std::__decay_and_strip<_Tp>::__type, typename std::__decay_and_strip<_T2>::__type> std::make_pair(_T1&&, _T2&&)
     make_pair(_T1&& __x, _T2&& __y)
     ^~~~~~~~~
/usr/include/c++/7/bits/stl_pair.h:524:5: note:   template argument deduction/substitution failed:
test.cpp:12:62: note:   cannot convert ‘i’ (type ‘std::size_t {aka long unsigned int}’) to type ‘long unsigned int&&’
           return std::make_pair<std::size_t, std::size_t>(i,j);
                                                          ^

Why does it try to substitute with unsigned long int when we have clearly mentioned the type in templated pair and also the datatype of i and j?

Answer 1

When you specifying template argument as std::size_t, the type of function parameter of std::make_pair becomes std::size_t&&, i.e. an rvalue-reference; i and j are lvalues and can't be bound to rvalue-reference.

Just letting std::make_pair do the template argument deduction like std::make_pair(i,j) (template parameter is std::size_t&, function parameter is std::size_t&) would be fine. std::make_pair takes forwarding references, it's supposed to accept both lvalues and rvalues (for std::make_pair(0,0), template parameter is std::size_t, function parameter is std::size_t&&).