diff --git a/libcxx/include/__config b/libcxx/include/__config
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -1010,6 +1010,12 @@
 #  define _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_IS_CONSTANT_EVALUATED
 #endif
 
+#if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_CXX14_CONSTEXPR) && defined(__cpp_constexpr_dynamic_alloc)
+#  define _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_CONSTEXPR_DYNAMIC_ALLOC constexpr
+#else
+#  define _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_CONSTEXPR_DYNAMIC_ALLOC
+#endif
+
 // The _LIBCPP_NODISCARD_ATTRIBUTE should only be used to define other
 // NODISCARD macros to the correct attribute.
 #if __has_cpp_attribute(nodiscard) || defined(_LIBCPP_COMPILER_MSVC)
diff --git a/libcxx/include/memory b/libcxx/include/memory
--- a/libcxx/include/memory
+++ b/libcxx/include/memory
@@ -129,11 +129,13 @@
     constexpr allocator(const allocator&) noexcept;      // constexpr in C++20
     template <class U>
       constexpr allocator(const allocator<U>&) noexcept; // constexpr in C++20
-    ~allocator();
+    constexpr ~allocator();                              // constexpr in C++20
     pointer address(reference x) const noexcept;
     const_pointer address(const_reference x) const noexcept;
-    pointer allocate(size_type, allocator<void>::const_pointer hint = 0);
-    void deallocate(pointer p, size_type n) noexcept;
+    constexpr pointer                                    // constexpr in C++20
+    allocate(size_type, allocator<void>::const_pointer hint = 0);
+    constexpr void                                       // constexpr in C++20
+    deallocate(pointer p, size_type n) noexcept;
     size_type max_size() const noexcept;
     template<class U, class... Args>
         void construct(U* p, Args&&... args);
@@ -142,10 +144,10 @@
 };
 
 template <class T, class U>
-bool operator==(const allocator<T>&, const allocator<U>&) noexcept;
+constexpr bool operator==(const allocator<T>&, const allocator<U>&) noexcept; // constexpr in C++20
 
 template <class T, class U>
-bool operator!=(const allocator<T>&, const allocator<U>&) noexcept;
+constexpr bool operator!=(const allocator<T>&, const allocator<U>&) noexcept; // constexpr in C++20
 
 template <class OutputIterator, class T>
 class raw_storage_iterator
@@ -1844,7 +1846,7 @@
         {return _VSTD::addressof(__x);}
     _LIBCPP_INLINE_VISIBILITY const_pointer address(const_reference __x) const _NOEXCEPT
         {return _VSTD::addressof(__x);}
-    _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_CONSTEXPR_DYNAMIC_ALLOC
     pointer allocate(size_type __n, allocator<void>::const_pointer = 0)
         {
         if (__n > max_size())
@@ -1852,9 +1854,10 @@
                                  " 'n' exceeds maximum supported size");
         return static_cast<pointer>(_VSTD::__libcpp_allocate(__n * sizeof(_Tp), _LIBCPP_ALIGNOF(_Tp)));
         }
-    _LIBCPP_INLINE_VISIBILITY void deallocate(pointer __p, size_type __n) _NOEXCEPT
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_CONSTEXPR_DYNAMIC_ALLOC
+    void deallocate(pointer __p, size_type __n) _NOEXCEPT
         {_VSTD::__libcpp_deallocate((void*)__p, __n * sizeof(_Tp), _LIBCPP_ALIGNOF(_Tp));}
-    _LIBCPP_INLINE_VISIBILITY size_type max_size() const _NOEXCEPT
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 size_type max_size() const _NOEXCEPT
         {return size_type(~0) / sizeof(_Tp);}
 #if !defined(_LIBCPP_HAS_NO_RVALUE_REFERENCES) && !defined(_LIBCPP_HAS_NO_VARIADICS)
     template <class _Up, class... _Args>
@@ -1946,16 +1949,18 @@
 
     _LIBCPP_INLINE_VISIBILITY const_pointer address(const_reference __x) const _NOEXCEPT
         {return _VSTD::addressof(__x);}
-    _LIBCPP_INLINE_VISIBILITY pointer allocate(size_type __n, allocator<void>::const_pointer = 0)
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_CONSTEXPR_DYNAMIC_ALLOC
+    pointer allocate(size_type __n, allocator<void>::const_pointer = 0)
     {
         if (__n > max_size())
             __throw_length_error("allocator<const T>::allocate(size_t n)"
                                  " 'n' exceeds maximum supported size");
         return static_cast<pointer>(_VSTD::__libcpp_allocate(__n * sizeof(_Tp), _LIBCPP_ALIGNOF(_Tp)));
     }
-    _LIBCPP_INLINE_VISIBILITY void deallocate(pointer __p, size_type __n) _NOEXCEPT
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_CONSTEXPR_DYNAMIC_ALLOC
+    void deallocate(pointer __p, size_type __n) _NOEXCEPT
         {_VSTD::__libcpp_deallocate((void*) const_cast<_Tp *>(__p), __n * sizeof(_Tp), _LIBCPP_ALIGNOF(_Tp));}
-    _LIBCPP_INLINE_VISIBILITY size_type max_size() const _NOEXCEPT
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 size_type max_size() const _NOEXCEPT
         {return size_type(~0) / sizeof(_Tp);}
 #if !defined(_LIBCPP_HAS_NO_RVALUE_REFERENCES) && !defined(_LIBCPP_HAS_NO_VARIADICS)
     template <class _Up, class... _Args>
@@ -2022,11 +2027,11 @@
 };
 
 template <class _Tp, class _Up>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 bool operator==(const allocator<_Tp>&, const allocator<_Up>&) _NOEXCEPT {return true;}
 
 template <class _Tp, class _Up>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 bool operator!=(const allocator<_Tp>&, const allocator<_Up>&) _NOEXCEPT {return false;}
 
 template <class _OutputIterator, class _Tp>
diff --git a/libcxx/include/new b/libcxx/include/new
--- a/libcxx/include/new
+++ b/libcxx/include/new
@@ -234,7 +234,8 @@
 #endif
 }
 
-inline _LIBCPP_INLINE_VISIBILITY void *__libcpp_allocate(size_t __size, size_t __align) {
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_CONSTEXPR_DYNAMIC_ALLOC void*
+__libcpp_allocate(size_t __size, size_t __align) {
 #ifndef _LIBCPP_HAS_NO_ALIGNED_ALLOCATION
   if (__is_overaligned_for_new(__align)) {
     const align_val_t __align_val = static_cast<align_val_t>(__align);
@@ -255,7 +256,40 @@
 }
 
 struct _DeallocateCaller {
-  static inline _LIBCPP_INLINE_VISIBILITY
+ private:
+  template <class _A1, class _A2>
+  static inline _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_CONSTEXPR_DYNAMIC_ALLOC
+  void __do_call(void *__ptr, _A1 __a1, _A2 __a2) {
+#if defined(_LIBCPP_HAS_NO_BUILTIN_OPERATOR_NEW_DELETE) || \
+    defined(_LIBCPP_HAS_NO_BUILTIN_OVERLOADED_OPERATOR_NEW_DELETE)
+    return ::operator delete(__ptr, __a1, __a2);
+#else
+    return __builtin_operator_delete(__ptr, __a1, __a2);
+#endif
+  }
+
+  template <class _A1>
+  static inline _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_CONSTEXPR_DYNAMIC_ALLOC
+  void __do_call(void *__ptr, _A1 __a1) {
+#if defined(_LIBCPP_HAS_NO_BUILTIN_OPERATOR_NEW_DELETE) || \
+    defined(_LIBCPP_HAS_NO_BUILTIN_OVERLOADED_OPERATOR_NEW_DELETE)
+    return ::operator delete(__ptr, __a1);
+#else
+    return __builtin_operator_delete(__ptr, __a1);
+#endif
+  }
+
+  static inline _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_CONSTEXPR_DYNAMIC_ALLOC
+  void __do_call(void *__ptr) {
+#ifdef _LIBCPP_HAS_NO_BUILTIN_OPERATOR_NEW_DELETE
+    return ::operator delete(__ptr);
+#else
+    return __builtin_operator_delete(__ptr);
+#endif
+  }
+
+ public:
+  static inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_CONSTEXPR_DYNAMIC_ALLOC
   void __do_deallocate_handle_size_align(void *__ptr, size_t __size, size_t __align) {
 #if defined(_LIBCPP_HAS_NO_ALIGNED_ALLOCATION)
     ((void)__align);
@@ -286,7 +320,8 @@
   }
 
  private:
-  static inline void __do_deallocate_handle_size(void *__ptr, size_t __size) {
+  static inline _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_CONSTEXPR_DYNAMIC_ALLOC void
+  __do_deallocate_handle_size(void* __ptr, size_t __size) {
 #ifdef _LIBCPP_HAS_NO_SIZED_DEALLOCATION
     ((void)__size);
     return __do_call(__ptr);
@@ -296,7 +331,8 @@
   }
 
 #ifndef _LIBCPP_HAS_NO_ALIGNED_ALLOCATION
-  static inline void __do_deallocate_handle_size(void *__ptr, size_t __size, align_val_t __align) {
+  static inline _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_CONSTEXPR_DYNAMIC_ALLOC void
+  __do_deallocate_handle_size(void *__ptr, size_t __size, align_val_t __align) {
 #ifdef _LIBCPP_HAS_NO_SIZED_DEALLOCATION
     ((void)__size);
     return __do_call(__ptr, __align);
@@ -305,42 +341,15 @@
 #endif
   }
 #endif
-
-private:
-  template <class _A1, class _A2>
-  static inline void __do_call(void *__ptr, _A1 __a1, _A2 __a2) {
-#if defined(_LIBCPP_HAS_NO_BUILTIN_OPERATOR_NEW_DELETE) || \
-    defined(_LIBCPP_HAS_NO_BUILTIN_OVERLOADED_OPERATOR_NEW_DELETE)
-    return ::operator delete(__ptr, __a1, __a2);
-#else
-    return __builtin_operator_delete(__ptr, __a1, __a2);
-#endif
-  }
-
-  template <class _A1>
-  static inline void __do_call(void *__ptr, _A1 __a1) {
-#if defined(_LIBCPP_HAS_NO_BUILTIN_OPERATOR_NEW_DELETE) || \
-    defined(_LIBCPP_HAS_NO_BUILTIN_OVERLOADED_OPERATOR_NEW_DELETE)
-    return ::operator delete(__ptr, __a1);
-#else
-    return __builtin_operator_delete(__ptr, __a1);
-#endif
-  }
-
-  static inline void __do_call(void *__ptr) {
-#ifdef _LIBCPP_HAS_NO_BUILTIN_OPERATOR_NEW_DELETE
-    return ::operator delete(__ptr);
-#else
-    return __builtin_operator_delete(__ptr);
-#endif
-  }
 };
 
-inline _LIBCPP_INLINE_VISIBILITY void __libcpp_deallocate(void* __ptr, size_t __size, size_t __align) {
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_CONSTEXPR_DYNAMIC_ALLOC void
+__libcpp_deallocate(void* __ptr, size_t __size, size_t __align) {
   _DeallocateCaller::__do_deallocate_handle_size_align(__ptr, __size, __align);
 }
 
-inline _LIBCPP_INLINE_VISIBILITY void __libcpp_deallocate_unsized(void* __ptr, size_t __align) {
+inline _LIBCPP_INLINE_VISIBILITY void
+__libcpp_deallocate_unsized(void* __ptr, size_t __align) {
   _DeallocateCaller::__do_deallocate_handle_align(__ptr, __align);
 }
 
diff --git a/libcxx/test/std/utilities/memory/default.allocator/allocator.globals/eq.pass.cpp b/libcxx/test/std/utilities/memory/default.allocator/allocator.globals/eq.pass.cpp
--- a/libcxx/test/std/utilities/memory/default.allocator/allocator.globals/eq.pass.cpp
+++ b/libcxx/test/std/utilities/memory/default.allocator/allocator.globals/eq.pass.cpp
@@ -23,12 +23,24 @@
 
 #include "test_macros.h"
 
-int main(int, char**)
+TEST_CONSTEXPR_CXX20 bool
+test()
 {
     std::allocator<int> a1;
     std::allocator<int> a2;
     assert(a1 == a2);
     assert(!(a1 != a2));
 
+  return true;
+}
+
+int main(int, char**)
+{
+  test();
+
+#if TEST_STD_VER > 17
+  static_assert(test());
+#endif
+
   return 0;
 }
diff --git a/libcxx/test/std/utilities/memory/default.allocator/allocator.members/allocate.fail.cpp b/libcxx/test/std/utilities/memory/default.allocator/allocator.members/allocate.fail.cpp
--- a/libcxx/test/std/utilities/memory/default.allocator/allocator.members/allocate.fail.cpp
+++ b/libcxx/test/std/utilities/memory/default.allocator/allocator.members/allocate.fail.cpp
@@ -25,5 +25,14 @@
     a.allocate(3);          // expected-error {{ignoring return value of function declared with 'nodiscard' attribute}}
     a.allocate(3, nullptr); // expected-error {{ignoring return value of function declared with 'nodiscard' attribute}}
 
+#if TEST_STD_VER > 17 && defined(__cpp_constexpr_dynamic_alloc)
+    static_assert([]() constexpr {  // expected-error {{static_assert expression is not an integral constant expression}}
+        std::allocator<int> a;
+        TEST_IGNORE_NODISCARD a.allocate(3);
+
+      return true;
+    }());
+#endif
+
   return 0;
 }
diff --git a/libcxx/test/std/utilities/memory/default.allocator/allocator.members/allocate.pass.cpp b/libcxx/test/std/utilities/memory/default.allocator/allocator.members/allocate.pass.cpp
--- a/libcxx/test/std/utilities/memory/default.allocator/allocator.members/allocate.pass.cpp
+++ b/libcxx/test/std/utilities/memory/default.allocator/allocator.members/allocate.pass.cpp
@@ -98,6 +98,18 @@
   }
 }
 
+#if TEST_STD_VER > 17 && defined(__cpp_constexpr_dynamic_alloc)
+template <size_t Align>
+constexpr bool test_aligned_constexpr() {
+    typedef AlignedType<Align> T;
+    std::allocator<T> a;
+    T* ap = a.allocate(3);
+    a.deallocate(ap, 3);
+
+  return true;
+}
+#endif
+
 int main(int, char**) {
     test_aligned<1>();
     test_aligned<2>();
@@ -108,5 +120,16 @@
     test_aligned<OverAligned>();
     test_aligned<OverAligned * 2>();
 
+#if TEST_STD_VER > 17 && defined(__cpp_constexpr_dynamic_alloc)
+    static_assert(test_aligned_constexpr<1>());
+    static_assert(test_aligned_constexpr<2>());
+    static_assert(test_aligned_constexpr<4>());
+    static_assert(test_aligned_constexpr<8>());
+    static_assert(test_aligned_constexpr<16>());
+    static_assert(test_aligned_constexpr<MaxAligned>());
+    static_assert(test_aligned_constexpr<OverAligned>());
+    static_assert(test_aligned_constexpr<OverAligned * 2>());
+#endif
+
   return 0;
 }
diff --git a/libcxx/test/std/utilities/memory/default.allocator/allocator.members/allocate.fail.cpp b/libcxx/test/std/utilities/memory/default.allocator/allocator.members/allocate.size.fail.cpp
copy from libcxx/test/std/utilities/memory/default.allocator/allocator.members/allocate.fail.cpp
copy to libcxx/test/std/utilities/memory/default.allocator/allocator.members/allocate.size.fail.cpp
--- a/libcxx/test/std/utilities/memory/default.allocator/allocator.members/allocate.fail.cpp
+++ b/libcxx/test/std/utilities/memory/default.allocator/allocator.members/allocate.size.fail.cpp
@@ -19,11 +19,20 @@
 
 #include "test_macros.h"
 
+template <typename T>
+constexpr bool test()
+{
+    std::allocator<T> a;
+    TEST_IGNORE_NODISCARD a.allocate(((size_t) -1) / sizeof(T) + 1);
+    TEST_IGNORE_NODISCARD a.allocate((size_t) -1);
+
+  return true;
+}
+
 int main(int, char**)
 {
-    std::allocator<int> a;
-    a.allocate(3);          // expected-error {{ignoring return value of function declared with 'nodiscard' attribute}}
-    a.allocate(3, nullptr); // expected-error {{ignoring return value of function declared with 'nodiscard' attribute}}
+    static_assert(test<double>());  // expected-error {{static_assert expression is not an integral constant expression}}
+    LIBCPP_STATIC_ASSERT(test<const double>());  // expected-error {{static_assert expression is not an integral constant expression}}
 
   return 0;
 }
diff --git a/libcxx/test/std/utilities/memory/default.allocator/allocator_types.pass.cpp b/libcxx/test/std/utilities/memory/default.allocator/allocator_types.pass.cpp
--- a/libcxx/test/std/utilities/memory/default.allocator/allocator_types.pass.cpp
+++ b/libcxx/test/std/utilities/memory/default.allocator/allocator_types.pass.cpp
@@ -33,7 +33,8 @@
 
 #include "test_macros.h"
 
-int main(int, char**)
+TEST_CONSTEXPR_CXX20 bool
+test()
 {
     static_assert((std::is_same<std::allocator<char>::size_type, std::size_t>::value), "");
     static_assert((std::is_same<std::allocator<char>::difference_type, std::ptrdiff_t>::value), "");
@@ -54,5 +55,16 @@
     std::allocator<int> a3 = a2;
     ((void)a3);
 
+  return true;
+}
+
+int main(int, char**)
+{
+    test();
+
+#if TEST_STD_VER > 17
+    static_assert(test());
+#endif
+
   return 0;
 }
diff --git a/libcxx/test/std/utilities/memory/default.allocator/allocator_void.pass.cpp b/libcxx/test/std/utilities/memory/default.allocator/allocator_void.pass.cpp
--- a/libcxx/test/std/utilities/memory/default.allocator/allocator_void.pass.cpp
+++ b/libcxx/test/std/utilities/memory/default.allocator/allocator_void.pass.cpp
@@ -24,7 +24,8 @@
 
 #include "test_macros.h"
 
-int main(int, char**)
+TEST_CONSTEXPR_CXX20 bool
+test()
 {
     static_assert((std::is_same<std::allocator<void>::pointer, void*>::value), "");
     static_assert((std::is_same<std::allocator<void>::const_pointer, const void*>::value), "");
@@ -35,5 +36,16 @@
     std::allocator<void> a2 = a;
     a2 = a;
 
+  return true;
+}
+
+int main(int, char**)
+{
+  test();
+
+#if TEST_STD_VER > 17
+  static_assert(test());
+#endif
+
   return 0;
 }