Fix incorrect type usage; nice catch by Sebastian

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@181569 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/algorithm b/include/algorithm
index f7e26cf..703a9b3 100644
--- a/include/algorithm
+++ b/include/algorithm
@@ -1301,7 +1301,7 @@
     _D1 __l1 = _VSTD::distance(__first1, __last1);
 
     typedef typename iterator_traits<_ForwardIterator2>::difference_type _D2;
-    _D1 __l2 = _VSTD::distance(__first2, __last2);
+    _D2 __l2 = _VSTD::distance(__first2, __last2);
     if (__l1 != __l2)
         return false;