[LIBC++]std::reduce(begin, end, init) does not accept init with only move constructor

[kamleshbhalui]

Summary

As per the c++ when the user-declare move-constructor compiler deletes the copy constructor.
so modified the std::reduce(begin, end, init) so it does not require copy constructor and behavior matches with
std::reduce(begin, end, init,op).

It fixes this https://bugs.llvm.org/show_bug.cgi?id=43013

[mclow.lists]

I don't think that https://bugs.llvm.org/show_bug.cgi?id=43013 is actually a bug.

[kamleshbhalui]

This comment was removed by kamleshbhalui.

libcxx/include/numeric

reduce(_InputIterator __first, _InputIterator __last, _Tp __init, _BinaryOp __b)
     return __init;
 }
 
 template <class _InputIterator, class _Tp>
 inline _LIBCPP_INLINE_VISIBILITY
 _Tp
 reduce(_InputIterator __first, _InputIterator __last, _Tp __init)
 {
-    return _VSTD::reduce(__first, __last, __init, _VSTD::plus<>());
+    for (; __first != __last; ++__first)
+        __init = _VSTD::plus<>()(__init, *__first);
+    return __init;
 }
 
 template <class _InputIterator>
 inline _LIBCPP_INLINE_VISIBILITY
 typename iterator_traits<_InputIterator>::value_type
 reduce(_InputIterator __first, _InputIterator __last)
 {
     return _VSTD::reduce(__first, __last,

libcxx/test/std/numerics/numeric.ops/reduce/reduce_init.pass.cpp

int main(int, char**)
     test_return_type<char, double>();
 
     test<input_iterator<const int*> >();
     test<forward_iterator<const int*> >();
     test<bidirectional_iterator<const int*> >();
     test<random_access_iterator<const int*> >();
     test<const int*>();
 
+    //test for checking std::reduce should not require copy constructor for initial object
+    {
+    struct T {
+     T(int){}
+     T(T &&){}
+     T &operator=(T &&){}
+     T operator+(const T &) const{}
+    };
+    T t[1]{1};
+    std::reduce(t, t + 1, T(0));
+    }
   return 0;
 }