diff --git a/include/memory b/include/memory
index bc5c4c653..ea52ba2cb 100644
--- a/include/memory
+++ b/include/memory
@@ -126,9 +126,10 @@ public:
 
     template <class U> struct rebind {typedef allocator<U> other;};
 
-    allocator() noexcept;
-    allocator(const allocator&) noexcept;
-    template <class U> allocator(const allocator<U>&) noexcept;
+    constexpr allocator() noexcept;                      // constexpr in C++20
+    constexpr allocator(const allocator&) noexcept;      // constexpr in C++20
+    template <class U>
+      constexpr allocator(const allocator<U>&) noexcept; // constexpr in C++20
     ~allocator();
     pointer address(reference x) const noexcept;
     const_pointer address(const_reference x) const noexcept;
@@ -1778,8 +1779,13 @@ public:
 
     template <class _Up> struct rebind {typedef allocator<_Up> other;};
 
-    _LIBCPP_INLINE_VISIBILITY allocator() _NOEXCEPT {}
-    template <class _Up> _LIBCPP_INLINE_VISIBILITY allocator(const allocator<_Up>&) _NOEXCEPT {}
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    allocator() _NOEXCEPT {}
+
+    template <class _Up> 
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    allocator(const allocator<_Up>&) _NOEXCEPT {}
+
     _LIBCPP_INLINE_VISIBILITY pointer address(reference __x) const _NOEXCEPT
         {return _VSTD::addressof(__x);}
     _LIBCPP_INLINE_VISIBILITY const_pointer address(const_reference __x) const _NOEXCEPT
@@ -1877,8 +1883,13 @@ public:
 
     template <class _Up> struct rebind {typedef allocator<_Up> other;};
 
-    _LIBCPP_INLINE_VISIBILITY allocator() _NOEXCEPT {}
-    template <class _Up> _LIBCPP_INLINE_VISIBILITY allocator(const allocator<_Up>&) _NOEXCEPT {}
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    allocator() _NOEXCEPT {}
+
+    template <class _Up>
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 
+    allocator(const allocator<_Up>&) _NOEXCEPT {}
+
     _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)
diff --git a/test/std/utilities/memory/default.allocator/allocator.ctor.pass.cpp b/test/std/utilities/memory/default.allocator/allocator.ctor.pass.cpp
new file mode 100644
index 000000000..3841cb533
--- /dev/null
+++ b/test/std/utilities/memory/default.allocator/allocator.ctor.pass.cpp
@@ -0,0 +1,50 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+// <memory>
+// UNSUPPORTED: c++98, c++03, c++11, c++14, c++17
+//
+// template <class T>
+// class allocator
+// {
+// public: // All of these are constexpr after C++17
+//  constexpr allocator() noexcept;
+//  constexpr allocator(const allocator&) noexcept;
+//  template<class U> constexpr allocator(const allocator<U>&) noexcept;
+// ...
+// };
+
+#include <memory>
+#include <cstddef>
+
+#include "test_macros.h"
+
+
+int main()
+{
+    {
+    typedef std::allocator<char> AC;
+    typedef std::allocator<long> AL;
+
+    constexpr AC a1;
+    constexpr AC a2{a1};
+    constexpr AL a3{a2};
+    (void) a3;
+    }
+    {
+    typedef std::allocator<const char> AC;
+    typedef std::allocator<const long> AL;
+
+    constexpr AC a1;
+    constexpr AC a2{a1};
+    constexpr AL a3{a2};
+    (void) a3;
+    }
+    
+}