diff --git a/libcxx/include/algorithm b/libcxx/include/algorithm
--- a/libcxx/include/algorithm
+++ b/libcxx/include/algorithm
@@ -2505,7 +2505,7 @@
 // rotate_copy
 
 template <class _ForwardIterator, class _OutputIterator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 _OutputIterator
 rotate_copy(_ForwardIterator __first, _ForwardIterator __middle, _ForwardIterator __last, _OutputIterator __result)
 {
@@ -4393,7 +4393,7 @@
 }
 
 template <class _InputIterator1, class _InputIterator2, class _OutputIterator, class _Compare>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 _OutputIterator
 merge(_InputIterator1 __first1, _InputIterator1 __last1,
       _InputIterator2 __first2, _InputIterator2 __last2, _OutputIterator __result, _Compare __comp)
@@ -4403,7 +4403,7 @@
 }
 
 template <class _InputIterator1, class _InputIterator2, class _OutputIterator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 _OutputIterator
 merge(_InputIterator1 __first1, _InputIterator1 __last1,
       _InputIterator2 __first2, _InputIterator2 __last2, _OutputIterator __result)
diff --git a/libcxx/test/std/algorithms/alg.modifying.operations/alg.rotate/rotate_copy.pass.cpp b/libcxx/test/std/algorithms/alg.modifying.operations/alg.rotate/rotate_copy.pass.cpp
--- a/libcxx/test/std/algorithms/alg.modifying.operations/alg.rotate/rotate_copy.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.modifying.operations/alg.rotate/rotate_copy.pass.cpp
@@ -18,21 +18,22 @@
 #include "test_macros.h"
 #include "test_iterators.h"
 
-// #if TEST_STD_VER > 17
-// TEST_CONSTEXPR bool test_constexpr() {
-//     int ia[] = {1, 3, 5, 2, 5, 6};
-//     int ib[std::size(ia)] = {0};
-//
-//     const size_t N = 2;
-//     const auto middle = std::begin(ia) + N;
-//     auto it = std::rotate_copy(std::begin(ia), middle, std::end(ia), std::begin(ib));
-//
-//     return std::distance(std::begin(ib), it) == std::size(ia)
-//         && std::equal   (std::begin(ia), middle,       std::begin(ib) + std::size(ia) - N)
-//         && std::equal   (middle,         std::end(ia), std::begin(ib))
-//            ;
-//     }
-// #endif
+#if TEST_STD_VER > 17
+TEST_CONSTEXPR bool test_constexpr() {
+  int ia[] = {1, 3, 5, 2, 5, 6};
+  int ib[std::size(ia)] = {0};
+
+  const size_t N = 2;
+  const auto middle = std::begin(ia) + N;
+  auto it =
+      std::rotate_copy(std::begin(ia), middle, std::end(ia), std::begin(ib));
+
+  return std::distance(std::begin(ib), it) == std::size(ia) &&
+         std::equal(std::begin(ia), middle,
+                    std::begin(ib) + std::size(ia) - N) &&
+         std::equal(middle, std::end(ia), std::begin(ib));
+}
+#endif
 
 template <class InIter, class OutIter>
 void
@@ -148,9 +149,9 @@
     test<const int*, random_access_iterator<int*> >();
     test<const int*, int*>();
 
-// #if TEST_STD_VER > 17
-//    static_assert(test_constexpr());
-// #endif
+#if TEST_STD_VER > 17
+    static_assert(test_constexpr());
+#endif
 
-  return 0;
+    return 0;
 }