Fix PR#35119 : set_union misbehaves with move_iterators. Thanks to Denis Yaroshevskiy for both the bug report and the fix.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@316914 91177308-0d34-0410-b5e6-96231b3b80d8

include/algorithm

@@ -5547,9 +5547,9 @@ __set_union(_InputIterator1 __first1, _InputIterator1 __last1,
         }
         else
         {
-            *__result = *__first1;
             if (!__comp(*__first1, *__first2))
                 ++__first2;
+            *__result = *__first1;
             ++__first1;
         }
     }

test/std/algorithms/alg.sorting/alg.set.operations/set.union/set_union_move.pass.cpp

@@ -0,0 +1,44 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <algorithm>
+
+// template<InputIterator InIter1, InputIterator InIter2, typename OutIter,
+//          CopyConstructible Compare>
+//   requires OutputIterator<OutIter, InIter1::reference>
+//         && OutputIterator<OutIter, InIter2::reference>
+//         && Predicate<Compare, InIter1::value_type, InIter2::value_type>
+//         && Predicate<Compare, InIter2::value_type, InIter1::value_type>
+//   OutIter
+//   set_union(InIter1 first1, InIter1 last1, InIter2 first2, InIter2 last2,
+//             OutIter result, Compare comp);
+
+#include <algorithm>
+#include <cassert>
+#include <iterator>
+#include <vector>
+
+#include "MoveOnly.h"
+
+
+int main()
+{
+    std::vector<MoveOnly> lhs, rhs;
+    lhs.push_back(MoveOnly(2));
+    rhs.push_back(MoveOnly(2));
+
+    std::vector<MoveOnly> res;
+    std::set_union(std::make_move_iterator(lhs.begin()),
+                   std::make_move_iterator(lhs.end()),
+                   std::make_move_iterator(rhs.begin()),
+                   std::make_move_iterator(rhs.end()), std::back_inserter(res));
+
+    assert(res.size() == 1);
+    assert(res[0].get() == 2);
+}