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#include <iostream>
#include <vector>
#include <type_traits>
 
template<
  typename Iter,
  typename T = typename std::enable_if<
    std::is_arithmetic<typename std::iterator_traits<Iter>::value_type>::value,
    typename std::iterator_traits<Iter>::value_type
  >::type
>
T summer(Iter start, Iter end)
{
    T sum{};
    for (auto it = start; it != end; it++)
    {
        sum += *it;
    }
    return sum;
}
 
int main()
{
    std::vector<double> vec {0.1, 0.2, 0.3};  // these are OK
    auto sum = summer(vec.begin(), vec.end());
    std::cout << sum << std::endl;
 
    int arr[] = {1, 2, 3};  // these are OK
    auto sum2 = summer(arr, arr+3);
    std::cout << sum2 << std::endl;
 
    std::vector<std::string> vec3 {"a", "b"};  // these are rejected
    auto sum3 = summer(vec3.begin(), vec3.end());
    std::cout << sum3 << std::endl;
 
    return 0;
}

prog.cpp: In function ‘int main()’:
prog.cpp:33:48: error: no matching function for call to ‘summer(std::vector<std::__cxx11::basic_string<char> >::iterator, std::vector<std::__cxx11::basic_string<char> >::iterator)’
     auto sum3 = summer(vec3.begin(), vec3.end());
                                                ^
prog.cpp:12:3: note: candidate: ‘template<class Iter, class T> T summer(Iter, Iter)’
 T summer(Iter start, Iter end)
   ^~~~~~
prog.cpp:12:3: note:   template argument deduction/substitution failed:
prog.cpp:7:3: error: no type named ‘type’ in ‘struct std::enable_if<false, std::__cxx11::basic_string<char> >’
   typename T = typename std::enable_if<
   ^~~~~~~~

Standard output is empty