diff --git a/libcxx/include/__fwd/memory_resource.h b/libcxx/include/__fwd/memory_resource.h
--- a/libcxx/include/__fwd/memory_resource.h
+++ b/libcxx/include/__fwd/memory_resource.h
@@ -15,13 +15,13 @@
 #  pragma GCC system_header
 #endif
 
-_LIBCPP_BEGIN_NAMESPACE_STD
+namespace std { // purposefully not using versioning namespace
 
 namespace pmr {
 template <class _ValueType>
 class _LIBCPP_TEMPLATE_VIS polymorphic_allocator;
 } // namespace pmr
 
-_LIBCPP_END_NAMESPACE_STD
+} // namespace std
 
 #endif // _LIBCPP___FWD_MEMORY_RESOURCE_H
diff --git a/libcxx/include/__fwd/string.h b/libcxx/include/__fwd/string.h
--- a/libcxx/include/__fwd/string.h
+++ b/libcxx/include/__fwd/string.h
@@ -59,7 +59,11 @@
 
 #if _LIBCPP_STD_VER >= 17
 
+_LIBCPP_END_NAMESPACE_STD
+
+namespace std {
 namespace pmr {
+
 template <class _CharT, class _Traits = char_traits<_CharT>>
 using basic_string = std::basic_string<_CharT, _Traits, polymorphic_allocator<_CharT>>;
 
@@ -77,6 +81,9 @@
 using u32string = basic_string<char32_t>;
 
 } // namespace pmr
+} // namespace std
+
+_LIBCPP_BEGIN_NAMESPACE_STD
 
 #endif // _LIBCPP_STD_VER >= 17
 
diff --git a/libcxx/include/__memory_resource/memory_resource.h b/libcxx/include/__memory_resource/memory_resource.h
--- a/libcxx/include/__memory_resource/memory_resource.h
+++ b/libcxx/include/__memory_resource/memory_resource.h
@@ -18,7 +18,7 @@
 
 #if _LIBCPP_STD_VER > 14
 
-_LIBCPP_BEGIN_NAMESPACE_STD
+namespace std { // purposefully not using versioning namespace
 
 namespace pmr {
 
@@ -68,7 +68,7 @@
 
 } // namespace pmr
 
-_LIBCPP_END_NAMESPACE_STD
+} // namespace std
 
 #endif // _LIBCPP_STD_VER > 14
 
diff --git a/libcxx/include/__memory_resource/monotonic_buffer_resource.h b/libcxx/include/__memory_resource/monotonic_buffer_resource.h
--- a/libcxx/include/__memory_resource/monotonic_buffer_resource.h
+++ b/libcxx/include/__memory_resource/monotonic_buffer_resource.h
@@ -20,7 +20,7 @@
 
 #if _LIBCPP_STD_VER > 14
 
-_LIBCPP_BEGIN_NAMESPACE_STD
+namespace std { // purposefully not using versioning namespace
 
 namespace pmr {
 
@@ -113,7 +113,7 @@
 
 } // namespace pmr
 
-_LIBCPP_END_NAMESPACE_STD
+} // namespace std
 
 #endif // _LIBCPP_STD_VER > 14
 
diff --git a/libcxx/include/__memory_resource/polymorphic_allocator.h b/libcxx/include/__memory_resource/polymorphic_allocator.h
--- a/libcxx/include/__memory_resource/polymorphic_allocator.h
+++ b/libcxx/include/__memory_resource/polymorphic_allocator.h
@@ -28,7 +28,7 @@
 
 #if _LIBCPP_STD_VER > 14
 
-_LIBCPP_BEGIN_NAMESPACE_STD
+namespace std { // purposefully not using versioning namespace
 
 namespace pmr {
 
@@ -215,7 +215,7 @@
 
 } // namespace pmr
 
-_LIBCPP_END_NAMESPACE_STD
+} // namespace std
 
 #endif // _LIBCPP_STD_VER > 14
 
diff --git a/libcxx/include/__memory_resource/pool_options.h b/libcxx/include/__memory_resource/pool_options.h
--- a/libcxx/include/__memory_resource/pool_options.h
+++ b/libcxx/include/__memory_resource/pool_options.h
@@ -18,7 +18,7 @@
 
 #if _LIBCPP_STD_VER > 14
 
-_LIBCPP_BEGIN_NAMESPACE_STD
+namespace std { // purposefully not using versioning namespace
 
 namespace pmr {
 
@@ -31,7 +31,7 @@
 
 } // namespace pmr
 
-_LIBCPP_END_NAMESPACE_STD
+} // namespace std
 
 #endif // _LIBCPP_STD_VER > 14
 
diff --git a/libcxx/include/__memory_resource/synchronized_pool_resource.h b/libcxx/include/__memory_resource/synchronized_pool_resource.h
--- a/libcxx/include/__memory_resource/synchronized_pool_resource.h
+++ b/libcxx/include/__memory_resource/synchronized_pool_resource.h
@@ -24,7 +24,7 @@
 
 #if _LIBCPP_STD_VER > 14
 
-_LIBCPP_BEGIN_NAMESPACE_STD
+namespace std { // purposefully not using versioning namespace
 
 namespace pmr {
 
@@ -87,7 +87,7 @@
 
 } // namespace pmr
 
-_LIBCPP_END_NAMESPACE_STD
+} // namespace std
 
 #endif // _LIBCPP_STD_VER > 14
 
diff --git a/libcxx/include/__memory_resource/unsynchronized_pool_resource.h b/libcxx/include/__memory_resource/unsynchronized_pool_resource.h
--- a/libcxx/include/__memory_resource/unsynchronized_pool_resource.h
+++ b/libcxx/include/__memory_resource/unsynchronized_pool_resource.h
@@ -21,7 +21,7 @@
 
 #if _LIBCPP_STD_VER > 14
 
-_LIBCPP_BEGIN_NAMESPACE_STD
+namespace std { // purposefully not using versioning namespace
 
 namespace pmr {
 
@@ -99,7 +99,7 @@
 
 } // namespace pmr
 
-_LIBCPP_END_NAMESPACE_STD
+} // namespace std
 
 #endif // _LIBCPP_STD_VER > 14
 
diff --git a/libcxx/include/deque b/libcxx/include/deque
--- a/libcxx/include/deque
+++ b/libcxx/include/deque
@@ -2420,12 +2420,15 @@
 _LIBCPP_END_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER > 14
-_LIBCPP_BEGIN_NAMESPACE_STD
+namespace std {
+
 namespace pmr {
 template <class _ValueT>
 using deque = std::deque<_ValueT, polymorphic_allocator<_ValueT>>;
 } // namespace pmr
-_LIBCPP_END_NAMESPACE_STD
+
+} // namespace std
+
 #endif
 
 _LIBCPP_POP_MACROS
diff --git a/libcxx/include/forward_list b/libcxx/include/forward_list
--- a/libcxx/include/forward_list
+++ b/libcxx/include/forward_list
@@ -1780,12 +1780,16 @@
 _LIBCPP_END_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER > 14
-_LIBCPP_BEGIN_NAMESPACE_STD
+
+namespace std {
+
 namespace pmr {
 template <class _ValueT>
 using forward_list = std::forward_list<_ValueT, polymorphic_allocator<_ValueT>>;
 } // namespace pmr
-_LIBCPP_END_NAMESPACE_STD
+
+} // namespace std
+
 #endif
 
 _LIBCPP_POP_MACROS
diff --git a/libcxx/include/list b/libcxx/include/list
--- a/libcxx/include/list
+++ b/libcxx/include/list
@@ -2363,12 +2363,16 @@
 _LIBCPP_END_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER > 14
-_LIBCPP_BEGIN_NAMESPACE_STD
+
+namespace std {
+
 namespace pmr {
 template <class _ValueT>
 using list = std::list<_ValueT, polymorphic_allocator<_ValueT>>;
 } // namespace pmr
-_LIBCPP_END_NAMESPACE_STD
+
+} // namespace std
+
 #endif
 
 _LIBCPP_POP_MACROS
diff --git a/libcxx/include/map b/libcxx/include/map
--- a/libcxx/include/map
+++ b/libcxx/include/map
@@ -2340,7 +2340,9 @@
 _LIBCPP_END_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER > 14
-_LIBCPP_BEGIN_NAMESPACE_STD
+
+namespace std {
+
 namespace pmr {
 template <class _KeyT, class _ValueT, class _CompareT = std::less<_KeyT>>
 using map = std::map<_KeyT, _ValueT, _CompareT, polymorphic_allocator<std::pair<const _KeyT, _ValueT>>>;
@@ -2348,7 +2350,9 @@
 template <class _KeyT, class _ValueT, class _CompareT = std::less<_KeyT>>
 using multimap = std::multimap<_KeyT, _ValueT, _CompareT, polymorphic_allocator<std::pair<const _KeyT, _ValueT>>>;
 } // namespace pmr
-_LIBCPP_END_NAMESPACE_STD
+
+} // namespace std
+
 #endif
 
 #if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
diff --git a/libcxx/include/regex b/libcxx/include/regex
--- a/libcxx/include/regex
+++ b/libcxx/include/regex
@@ -6845,7 +6845,9 @@
 _LIBCPP_END_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER > 14
-_LIBCPP_BEGIN_NAMESPACE_STD
+
+namespace std {
+
 namespace pmr {
 template <class _BidirT>
 using match_results = std::match_results<_BidirT, polymorphic_allocator<std::sub_match<_BidirT>>>;
@@ -6858,7 +6860,9 @@
 using wsmatch = match_results<std::pmr::wstring::const_iterator>;
 #endif
 } // namespace pmr
-_LIBCPP_END_NAMESPACE_STD
+
+} // namespace std
+
 #endif
 
 _LIBCPP_POP_MACROS
diff --git a/libcxx/include/set b/libcxx/include/set
--- a/libcxx/include/set
+++ b/libcxx/include/set
@@ -1585,7 +1585,9 @@
 _LIBCPP_END_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER > 14
-_LIBCPP_BEGIN_NAMESPACE_STD
+
+namespace std {
+
 namespace pmr {
 template <class _KeyT, class _CompareT = std::less<_KeyT>>
 using set = std::set<_KeyT, _CompareT, polymorphic_allocator<_KeyT>>;
@@ -1593,7 +1595,9 @@
 template <class _KeyT, class _CompareT = std::less<_KeyT>>
 using multiset = std::multiset<_KeyT, _CompareT, polymorphic_allocator<_KeyT>>;
 } // namespace pmr
-_LIBCPP_END_NAMESPACE_STD
+
+} // namespace std
+
 #endif
 
 #if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
diff --git a/libcxx/include/unordered_map b/libcxx/include/unordered_map
--- a/libcxx/include/unordered_map
+++ b/libcxx/include/unordered_map
@@ -2630,7 +2630,9 @@
 _LIBCPP_END_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER > 14
-_LIBCPP_BEGIN_NAMESPACE_STD
+
+namespace std {
+
 namespace pmr {
 template <class _KeyT, class _ValueT, class _HashT = std::hash<_KeyT>, class _PredT = std::equal_to<_KeyT>>
 using unordered_map =
@@ -2640,7 +2642,9 @@
 using unordered_multimap =
     std::unordered_multimap<_KeyT, _ValueT, _HashT, _PredT, polymorphic_allocator<std::pair<const _KeyT, _ValueT>>>;
 } // namespace pmr
-_LIBCPP_END_NAMESPACE_STD
+
+} // namespace std
+
 #endif
 
 #if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
diff --git a/libcxx/include/unordered_set b/libcxx/include/unordered_set
--- a/libcxx/include/unordered_set
+++ b/libcxx/include/unordered_set
@@ -1804,7 +1804,9 @@
 _LIBCPP_END_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER > 14
-_LIBCPP_BEGIN_NAMESPACE_STD
+
+namespace std {
+
 namespace pmr {
 template <class _KeyT, class _HashT = std::hash<_KeyT>, class _PredT = std::equal_to<_KeyT>>
 using unordered_set = std::unordered_set<_KeyT, _HashT, _PredT, polymorphic_allocator<_KeyT>>;
@@ -1812,7 +1814,9 @@
 template <class _KeyT, class _HashT = std::hash<_KeyT>, class _PredT = std::equal_to<_KeyT>>
 using unordered_multiset = std::unordered_multiset<_KeyT, _HashT, _PredT, polymorphic_allocator<_KeyT>>;
 } // namespace pmr
-_LIBCPP_END_NAMESPACE_STD
+
+} // namespace std
+
 #endif
 
 #if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
diff --git a/libcxx/include/vector b/libcxx/include/vector
--- a/libcxx/include/vector
+++ b/libcxx/include/vector
@@ -3345,12 +3345,15 @@
 _LIBCPP_END_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER > 14
-_LIBCPP_BEGIN_NAMESPACE_STD
+namespace std {
+
 namespace pmr {
 template <class _ValueT>
 using vector = std::vector<_ValueT, polymorphic_allocator<_ValueT>>;
 } // namespace pmr
-_LIBCPP_END_NAMESPACE_STD
+
+} // namespace std
+
 #endif
 
 _LIBCPP_POP_MACROS
diff --git a/libcxx/src/memory_resource.cpp b/libcxx/src/memory_resource.cpp
--- a/libcxx/src/memory_resource.cpp
+++ b/libcxx/src/memory_resource.cpp
@@ -9,6 +9,12 @@
 #include <memory>
 #include <memory_resource>
 
+#if defined(_LIBCPP_ABI_MICROSOFT)
+#  include "support/runtime/memory_resource_msvc.ipp"
+#else
+#  include "support/runtime/memory_resource_fallback.ipp"
+#endif
+
 #ifndef _LIBCPP_HAS_NO_ATOMIC_HEADER
 #  include <atomic>
 #elif !defined(_LIBCPP_HAS_NO_THREADS)
@@ -18,7 +24,7 @@
 #  endif
 #endif
 
-_LIBCPP_BEGIN_NAMESPACE_STD
+namespace std {
 
 namespace pmr {
 
@@ -26,112 +32,6 @@
 
 memory_resource::~memory_resource() = default;
 
-// new_delete_resource()
-
-#ifdef _LIBCPP_HAS_NO_ALIGNED_ALLOCATION
-static bool is_aligned_to(void* ptr, size_t align) {
-  void* p2     = ptr;
-  size_t space = 1;
-  void* result = std::align(align, 1, p2, space);
-  return (result == ptr);
-}
-#endif
-
-class _LIBCPP_TYPE_VIS __new_delete_memory_resource_imp : public memory_resource {
-  void* do_allocate(size_t bytes, size_t align) override {
-#ifndef _LIBCPP_HAS_NO_ALIGNED_ALLOCATION
-    return std::__libcpp_allocate(bytes, align);
-#else
-    if (bytes == 0)
-      bytes = 1;
-    void* result = std::__libcpp_allocate(bytes, align);
-    if (!is_aligned_to(result, align)) {
-      std::__libcpp_deallocate(result, bytes, align);
-      __throw_bad_alloc();
-    }
-    return result;
-#endif
-  }
-
-  void do_deallocate(void* p, size_t bytes, size_t align) override { std::__libcpp_deallocate(p, bytes, align); }
-
-  bool do_is_equal(const memory_resource& other) const noexcept override { return &other == this; }
-};
-
-// null_memory_resource()
-
-class _LIBCPP_TYPE_VIS __null_memory_resource_imp : public memory_resource {
-  void* do_allocate(size_t, size_t) override { __throw_bad_alloc(); }
-  void do_deallocate(void*, size_t, size_t) override {}
-  bool do_is_equal(const memory_resource& other) const noexcept override { return &other == this; }
-};
-
-namespace {
-
-union ResourceInitHelper {
-  struct {
-    __new_delete_memory_resource_imp new_delete_res;
-    __null_memory_resource_imp null_res;
-  } resources;
-  char dummy;
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 ResourceInitHelper() : resources() {}
-  ~ResourceInitHelper() {}
-};
-
-// Pretend we're inside a system header so the compiler doesn't flag the use of the init_priority
-// attribute with a value that's reserved for the implementation (we're the implementation).
-#include "memory_resource_init_helper.h"
-
-} // end namespace
-
-memory_resource* new_delete_resource() noexcept { return &res_init.resources.new_delete_res; }
-
-memory_resource* null_memory_resource() noexcept { return &res_init.resources.null_res; }
-
-// default_memory_resource()
-
-static memory_resource* __default_memory_resource(bool set = false, memory_resource* new_res = nullptr) noexcept {
-#ifndef _LIBCPP_HAS_NO_ATOMIC_HEADER
-  static constinit atomic<memory_resource*> __res{&res_init.resources.new_delete_res};
-  if (set) {
-    new_res = new_res ? new_res : new_delete_resource();
-    // TODO: Can a weaker ordering be used?
-    return std::atomic_exchange_explicit(&__res, new_res, memory_order_acq_rel);
-  } else {
-    return std::atomic_load_explicit(&__res, memory_order_acquire);
-  }
-#elif !defined(_LIBCPP_HAS_NO_THREADS)
-  static constinit memory_resource* res = &res_init.resources.new_delete_res;
-  static mutex res_lock;
-  if (set) {
-    new_res = new_res ? new_res : new_delete_resource();
-    lock_guard<mutex> guard(res_lock);
-    memory_resource* old_res = res;
-    res = new_res;
-    return old_res;
-  } else {
-    lock_guard<mutex> guard(res_lock);
-    return res;
-  }
-#else
-  static constinit memory_resource* res = &res_init.resources.new_delete_res;
-  if (set) {
-    new_res                  = new_res ? new_res : new_delete_resource();
-    memory_resource* old_res = res;
-    res                      = new_res;
-    return old_res;
-  } else {
-    return res;
-  }
-#endif
-}
-
-memory_resource* get_default_resource() noexcept { return __default_memory_resource(); }
-
-memory_resource* set_default_resource(memory_resource* __new_res) noexcept {
-  return __default_memory_resource(true, __new_res);
-}
-
 // 23.12.5, mem.res.pool
 
 static size_t roundup(size_t count, size_t alignment) {
@@ -493,4 +393,4 @@
 
 } // namespace pmr
 
-_LIBCPP_END_NAMESPACE_STD
+} // namespace std
diff --git a/libcxx/src/support/runtime/memory_resource_fallback.ipp b/libcxx/src/support/runtime/memory_resource_fallback.ipp
new file mode 100644
--- /dev/null
+++ b/libcxx/src/support/runtime/memory_resource_fallback.ipp
@@ -0,0 +1,121 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+namespace std {
+
+namespace pmr {
+
+// new_delete_resource()
+
+#ifdef _LIBCPP_HAS_NO_ALIGNED_ALLOCATION
+static bool is_aligned_to(void* ptr, size_t align) {
+  void* p2     = ptr;
+  size_t space = 1;
+  void* result = std::align(align, 1, p2, space);
+  return (result == ptr);
+}
+#endif
+
+class _LIBCPP_TYPE_VIS __new_delete_memory_resource_imp : public memory_resource {
+  void* do_allocate(size_t bytes, size_t align) override {
+#ifndef _LIBCPP_HAS_NO_ALIGNED_ALLOCATION
+    return std::__libcpp_allocate(bytes, align);
+#else
+    if (bytes == 0)
+      bytes = 1;
+    void* result = std::__libcpp_allocate(bytes, align);
+    if (!is_aligned_to(result, align)) {
+      std::__libcpp_deallocate(result, bytes, align);
+      __throw_bad_alloc();
+    }
+    return result;
+#endif
+  }
+
+  void do_deallocate(void* p, size_t bytes, size_t align) override { std::__libcpp_deallocate(p, bytes, align); }
+
+  bool do_is_equal(const memory_resource& other) const noexcept override { return &other == this; }
+};
+
+// null_memory_resource()
+
+class _LIBCPP_TYPE_VIS __null_memory_resource_imp : public memory_resource {
+  void* do_allocate(size_t, size_t) override { __throw_bad_alloc(); }
+  void do_deallocate(void*, size_t, size_t) override {}
+  bool do_is_equal(const memory_resource& other) const noexcept override { return &other == this; }
+};
+
+namespace {
+
+union ResourceInitHelper {
+  struct {
+    __new_delete_memory_resource_imp new_delete_res;
+    __null_memory_resource_imp null_res;
+  } resources;
+  char dummy;
+  _LIBCPP_CONSTEXPR_SINCE_CXX14 ResourceInitHelper() : resources() {}
+  ~ResourceInitHelper() {}
+};
+
+// Pretend we're inside a system header so the compiler doesn't flag the use of the init_priority
+// attribute with a value that's reserved for the implementation (we're the implementation).
+#include "memory_resource_init_helper.h"
+
+} // end namespace
+
+memory_resource* new_delete_resource() noexcept { return &res_init.resources.new_delete_res; }
+
+memory_resource* null_memory_resource() noexcept { return &res_init.resources.null_res; }
+
+// default_memory_resource()
+
+static memory_resource* __default_memory_resource(bool set = false, memory_resource* new_res = nullptr) noexcept {
+#ifndef _LIBCPP_HAS_NO_ATOMIC_HEADER
+  static constinit atomic<memory_resource*> __res{&res_init.resources.new_delete_res};
+  if (set) {
+    new_res = new_res ? new_res : new_delete_resource();
+    // TODO: Can a weaker ordering be used?
+    return std::atomic_exchange_explicit(&__res, new_res, memory_order_acq_rel);
+  } else {
+    return std::atomic_load_explicit(&__res, memory_order_acquire);
+  }
+#elif !defined(_LIBCPP_HAS_NO_THREADS)
+  static constinit memory_resource* res = &res_init.resources.new_delete_res;
+  static mutex res_lock;
+  if (set) {
+    new_res = new_res ? new_res : new_delete_resource();
+    lock_guard<mutex> guard(res_lock);
+    memory_resource* old_res = res;
+    res = new_res;
+    return old_res;
+  } else {
+    lock_guard<mutex> guard(res_lock);
+    return res;
+  }
+#else
+  static constinit memory_resource* res = &res_init.resources.new_delete_res;
+  if (set) {
+    new_res                  = new_res ? new_res : new_delete_resource();
+    memory_resource* old_res = res;
+    res                      = new_res;
+    return old_res;
+  } else {
+    return res;
+  }
+#endif
+}
+
+memory_resource* get_default_resource() noexcept { return __default_memory_resource(); }
+
+memory_resource* set_default_resource(memory_resource* __new_res) noexcept {
+  return __default_memory_resource(true, __new_res);
+}
+
+} // namespace pmr
+} // namespace std
diff --git a/libcxx/src/memory_resource_init_helper.h b/libcxx/src/support/runtime/memory_resource_init_helper.h
rename from libcxx/src/memory_resource_init_helper.h
rename to libcxx/src/support/runtime/memory_resource_init_helper.h
diff --git a/libcxx/src/support/runtime/memory_resource_msvc.ipp b/libcxx/src/support/runtime/memory_resource_msvc.ipp
new file mode 100644
--- /dev/null
+++ b/libcxx/src/support/runtime/memory_resource_msvc.ipp
@@ -0,0 +1,62 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP_ABI_MICROSOFT
+#error this header can only be used when targeting the MSVC ABI
+#endif
+
+// default_memory_resource()
+
+extern "C" {
+
+_LIBCPP_CRT_FUNC std::pmr::memory_resource* __cdecl _Aligned_set_default_resource(std::pmr::memory_resource*) noexcept;
+_LIBCPP_CRT_FUNC std::pmr::memory_resource* __cdecl _Unaligned_set_default_resource(std::pmr::memory_resource*) noexcept;
+_LIBCPP_CRT_FUNC std::pmr::memory_resource* __cdecl _Aligned_get_default_resource() noexcept;
+_LIBCPP_CRT_FUNC std::pmr::memory_resource* __cdecl _Unaligned_get_default_resource() noexcept;
+_LIBCPP_CRT_FUNC std::pmr::memory_resource* __cdecl _Aligned_new_delete_resource() noexcept;
+_LIBCPP_CRT_FUNC std::pmr::memory_resource* __cdecl _Unaligned_new_delete_resource() noexcept;
+_LIBCPP_CRT_FUNC std::pmr::memory_resource* __cdecl null_memory_resource() noexcept;
+
+};
+
+namespace std {
+
+namespace pmr {
+
+memory_resource* get_default_resource() noexcept {
+#ifdef __cpp_aligned_new
+    return ::_Aligned_get_default_resource();
+#else
+    return ::_Unaligned_get_default_resource();
+#endif
+}
+
+memory_resource* set_default_resource(memory_resource* __new_res) noexcept {
+#ifdef __cpp_aligned_new
+    return ::_Aligned_set_default_resource(__new_res);
+#else
+    return ::_Unaligned_set_default_resource(__new_res);
+#endif
+}
+
+memory_resource* new_delete_resource() noexcept {
+#ifdef __cpp_aligned_new
+    return ::_Aligned_new_delete_resource();
+#else
+    return ::_Unaligned_new_delete_resource();
+#endif
+}
+
+memory_resource* null_memory_resource() noexcept {
+    return ::null_memory_resource();
+}
+
+} // namespace pmr
+
+} // namespace std