diff --git a/libcxx/include/algorithm b/libcxx/include/algorithm
index 9ec146a6798e..d51e19c7337f 100644
--- a/libcxx/include/algorithm
+++ b/libcxx/include/algorithm
@@ -202,20 +202,20 @@ template <class InputIterator1, class InputIterator2, class OutputIterator, clas
               OutputIterator result, BinaryOperation binary_op);
 
 template <class ForwardIterator, class T>
-    void
+    constexpr void      // constexpr in C++20
     replace(ForwardIterator first, ForwardIterator last, const T& old_value, const T& new_value);
 
 template <class ForwardIterator, class Predicate, class T>
-    void
+    constexpr void      // constexpr in C++20
     replace_if(ForwardIterator first, ForwardIterator last, Predicate pred, const T& new_value);
 
 template <class InputIterator, class OutputIterator, class T>
-    OutputIterator
+    constexpr OutputIterator      // constexpr in C++20
     replace_copy(InputIterator first, InputIterator last, OutputIterator result,
                  const T& old_value, const T& new_value);
 
 template <class InputIterator, class OutputIterator, class Predicate, class T>
-    OutputIterator
+    constexpr OutputIterator      // constexpr in C++20
     replace_copy_if(InputIterator first, InputIterator last, OutputIterator result, Predicate pred, const T& new_value);
 
 template <class ForwardIterator, class T>
@@ -1461,9 +1461,9 @@ __is_permutation(_ForwardIterator1 __first1, _ForwardIterator1 __last1,
         if (!__pred(*__first1, *__first2))
             break;
     if (__first1 == __last1)
-    	return __first2 == __last2;
+        return __first2 == __last2;
     else if (__first2 == __last2)
-    	return false;
+        return false;
 
     typedef typename iterator_traits<_ForwardIterator1>::difference_type _D1;
     _D1 __l1 = _VSTD::distance(__first1, __last1);
@@ -1969,7 +1969,7 @@ transform(_InputIterator1 __first1, _InputIterator1 __last1, _InputIterator2 __f
 // replace
 
 template <class _ForwardIterator, class _Tp>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 void
 replace(_ForwardIterator __first, _ForwardIterator __last, const _Tp& __old_value, const _Tp& __new_value)
 {
@@ -1981,7 +1981,7 @@ replace(_ForwardIterator __first, _ForwardIterator __last, const _Tp& __old_valu
 // replace_if
 
 template <class _ForwardIterator, class _Predicate, class _Tp>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 void
 replace_if(_ForwardIterator __first, _ForwardIterator __last, _Predicate __pred, const _Tp& __new_value)
 {
@@ -1993,7 +1993,7 @@ replace_if(_ForwardIterator __first, _ForwardIterator __last, _Predicate __pred,
 // replace_copy
 
 template <class _InputIterator, class _OutputIterator, class _Tp>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 _OutputIterator
 replace_copy(_InputIterator __first, _InputIterator __last, _OutputIterator __result,
              const _Tp& __old_value, const _Tp& __new_value)
@@ -2009,7 +2009,7 @@ replace_copy(_InputIterator __first, _InputIterator __last, _OutputIterator __re
 // replace_copy_if
 
 template <class _InputIterator, class _OutputIterator, class _Predicate, class _Tp>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 _OutputIterator
 replace_copy_if(_InputIterator __first, _InputIterator __last, _OutputIterator __result,
                 _Predicate __pred, const _Tp& __new_value)
diff --git a/libcxx/test/std/algorithms/alg.modifying.operations/alg.replace/replace.pass.cpp b/libcxx/test/std/algorithms/alg.modifying.operations/alg.replace/replace.pass.cpp
index f6033351ffca..56a744b2c9d4 100644
--- a/libcxx/test/std/algorithms/alg.modifying.operations/alg.replace/replace.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.modifying.operations/alg.replace/replace.pass.cpp
@@ -13,14 +13,27 @@
 //   requires OutputIterator<Iter, Iter::reference>
 //         && OutputIterator<Iter, const T&>
 //         && HasEqualTo<Iter::value_type, T>
-//   void
+//   constexpr void      // constexpr after C++17
 //   replace(Iter first, Iter last, const T& old_value, const T& new_value);
 
 #include <algorithm>
 #include <cassert>
 
+#include "test_macros.h"
 #include "test_iterators.h"
 
+
+#if TEST_STD_VER > 17
+TEST_CONSTEXPR bool test_constexpr() {
+          int ia[]       = {0, 1, 2, 3, 4};
+    const int expected[] = {0, 1, 5, 3, 4};
+
+    std::replace(std::begin(ia), std::end(ia), 2, 5);
+    return std::equal(std::begin(ia), std::end(ia), std::begin(expected), std::end(expected))
+        ;
+    }
+#endif
+
 template <class Iter>
 void
 test()
@@ -41,4 +54,8 @@ int main()
     test<bidirectional_iterator<int*> >();
     test<random_access_iterator<int*> >();
     test<int*>();
+
+#if TEST_STD_VER > 17
+    static_assert(test_constexpr());
+#endif
 }
diff --git a/libcxx/test/std/algorithms/alg.modifying.operations/alg.replace/replace_copy.pass.cpp b/libcxx/test/std/algorithms/alg.modifying.operations/alg.replace/replace_copy.pass.cpp
index 3c4d0e50817b..aad7f9f921d6 100644
--- a/libcxx/test/std/algorithms/alg.modifying.operations/alg.replace/replace_copy.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.modifying.operations/alg.replace/replace_copy.pass.cpp
@@ -13,15 +13,32 @@
 //   requires OutputIterator<OutIter, InIter::reference>
 //         && OutputIterator<OutIter, const T&>
 //         && HasEqualTo<InIter::value_type, T>
-//   OutIter
+//   constexpr OutIter      // constexpr after C++17
 //   replace_copy(InIter first, InIter last, OutIter result, const T& old_value,
 //                                                           const T& new_value);
 
 #include <algorithm>
 #include <cassert>
 
+#include "test_macros.h"
 #include "test_iterators.h"
 
+
+#if TEST_STD_VER > 17
+TEST_CONSTEXPR bool test_constexpr() {
+    int ia[] = {0, 1, 2, 3, 4};
+    int ib[] = {0, 0, 0, 0, 0, 0}; // one bigger
+    const int expected[] = {0, 1, 5, 3, 4};
+
+    auto it = std::replace_copy(std::begin(ia), std::end(ia), std::begin(ib), 2, 5);
+    
+    return it == (std::begin(ib) + std::size(ia))
+        && *it == 0 // don't overwrite the last value in the output array
+        && std::equal(std::begin(ib), it, std::begin(expected), std::end(expected))
+        ;
+    }
+#endif
+
 template <class InIter, class OutIter>
 void
 test()
@@ -69,4 +86,8 @@ int main()
     test<const int*, bidirectional_iterator<int*> >();
     test<const int*, random_access_iterator<int*> >();
     test<const int*, int*>();
+
+#if TEST_STD_VER > 17
+    static_assert(test_constexpr());
+#endif
 }
diff --git a/libcxx/test/std/algorithms/alg.modifying.operations/alg.replace/replace_copy_if.pass.cpp b/libcxx/test/std/algorithms/alg.modifying.operations/alg.replace/replace_copy_if.pass.cpp
index f2ffece12e8b..6ba74ff07bd2 100644
--- a/libcxx/test/std/algorithms/alg.modifying.operations/alg.replace/replace_copy_if.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.modifying.operations/alg.replace/replace_copy_if.pass.cpp
@@ -14,16 +14,33 @@
 //   requires OutputIterator<OutIter, InIter::reference>
 //         && OutputIterator<OutIter, const T&>
 //         && CopyConstructible<Pred>
-//   OutIter
+//   constexpr OutIter      // constexpr after C++17
 //   replace_copy_if(InIter first, InIter last, OutIter result, Pred pred, const T& new_value);
 
 #include <algorithm>
 #include <functional>
 #include <cassert>
 
+#include "test_macros.h"
 #include "test_iterators.h"
 
-bool equalToTwo(int v) { return v == 2; }
+TEST_CONSTEXPR bool equalToTwo(int v) { return v == 2; }
+
+
+#if TEST_STD_VER > 17
+TEST_CONSTEXPR bool test_constexpr() {
+    int ia[] = {0, 1, 2, 3, 4};
+    int ib[] = {0, 0, 0, 0, 0, 0}; // one bigger
+    const int expected[] = {0, 1, 5, 3, 4};
+
+    auto it = std::replace_copy_if(std::begin(ia), std::end(ia), std::begin(ib), equalToTwo, 5);
+    
+    return it == (std::begin(ib) + std::size(ia))
+        && *it == 0 // don't overwrite the last value in the output array
+        && std::equal(std::begin(ib), it, std::begin(expected), std::end(expected))
+        ;
+    }
+#endif
 
 template <class InIter, class OutIter>
 void
@@ -73,4 +90,8 @@ int main()
     test<const int*, bidirectional_iterator<int*> >();
     test<const int*, random_access_iterator<int*> >();
     test<const int*, int*>();
+
+#if TEST_STD_VER > 17
+    static_assert(test_constexpr());
+#endif
 }
diff --git a/libcxx/test/std/algorithms/alg.modifying.operations/alg.replace/replace_if.pass.cpp b/libcxx/test/std/algorithms/alg.modifying.operations/alg.replace/replace_if.pass.cpp
index ebb2945d7c43..eeff4068740f 100644
--- a/libcxx/test/std/algorithms/alg.modifying.operations/alg.replace/replace_if.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.modifying.operations/alg.replace/replace_if.pass.cpp
@@ -13,16 +13,29 @@
 //   requires OutputIterator<Iter, Iter::reference>
 //         && OutputIterator<Iter, const T&>
 //         && CopyConstructible<Pred>
-//   void
+//   constexpr void      // constexpr after C++17
 //   replace_if(Iter first, Iter last, Pred pred, const T& new_value);
 
 #include <algorithm>
 #include <functional>
 #include <cassert>
 
+#include "test_macros.h"
 #include "test_iterators.h"
 
-bool equalToTwo(int v) { return v == 2; }
+TEST_CONSTEXPR bool equalToTwo(int v) { return v == 2; }
+
+#if TEST_STD_VER > 17
+TEST_CONSTEXPR bool test_constexpr() {
+          int ia[]       = {0, 1, 2, 3, 4};
+    const int expected[] = {0, 1, 5, 3, 4};
+
+    std::replace_if(std::begin(ia), std::end(ia), equalToTwo, 5);
+    return std::equal(std::begin(ia), std::end(ia), std::begin(expected), std::end(expected))
+        ;
+    }
+#endif
+
 
 template <class Iter>
 void
@@ -44,4 +57,8 @@ int main()
     test<bidirectional_iterator<int*> >();
     test<random_access_iterator<int*> >();
     test<int*>();
+
+#if TEST_STD_VER > 17
+    static_assert(test_constexpr());
+#endif
 }