diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -341,8 +341,10 @@
   __fwd/array.h
   __fwd/get.h
   __fwd/hash.h
+  __fwd/memory_resource.h
   __fwd/pair.h
   __fwd/span.h
+  __fwd/string.h
   __fwd/string_view.h
   __fwd/tuple.h
   __hash_table
diff --git a/libcxx/include/__availability b/libcxx/include/__availability
--- a/libcxx/include/__availability
+++ b/libcxx/include/__availability
@@ -172,6 +172,8 @@
     // be avoided.
 // #   define _LIBCPP_HAS_NO_VERBOSE_ABORT_IN_LIBRARY
 
+#  define _LIBCPP_AVAILABILITY_MEMORY_RESOURCE
+
 #elif defined(__APPLE__)
 
 #   define _LIBCPP_AVAILABILITY_SHARED_MUTEX                                    \
@@ -273,6 +275,8 @@
 
 #   define _LIBCPP_HAS_NO_VERBOSE_ABORT_IN_LIBRARY
 
+#  define _LIBCPP_AVAILABILITY_MEMORY_RESOURCE __attribute__((__unavailable__))
+
 #else
 
 // ...New vendors can add availability markup here...
diff --git a/libcxx/include/__fwd/memory_resource.h b/libcxx/include/__fwd/memory_resource.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__fwd/memory_resource.h
@@ -0,0 +1,27 @@
+//===----------------------------------------------------------------------===//
+//
+// 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___FWD_MEMORY_RESOURCE_H
+#define _LIBCPP___FWD_MEMORY_RESOURCE_H
+
+#include <__config>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+namespace pmr {
+template <class _ValueType>
+class _LIBCPP_TEMPLATE_VIS polymorphic_allocator;
+} // namespace pmr
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___FWD_MEMORY_RESOURCE_H
diff --git a/libcxx/include/__fwd/string.h b/libcxx/include/__fwd/string.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__fwd/string.h
@@ -0,0 +1,110 @@
+//===----------------------------------------------------------------------===//
+//
+// 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___FWD_STRING_H
+#define _LIBCPP___FWD_STRING_H
+
+#include <__config>
+#include <__fwd/memory_resource.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class _CharT>
+struct _LIBCPP_TEMPLATE_VIS char_traits;
+template <>
+struct char_traits<char>;
+
+#ifndef _LIBCPP_HAS_NO_CHAR8_T
+template <>
+struct char_traits<char8_t>;
+#endif
+
+template <>
+struct char_traits<char16_t>;
+template <>
+struct char_traits<char32_t>;
+
+#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
+template <>
+struct char_traits<wchar_t>;
+#endif
+
+template <class _Tp>
+class _LIBCPP_TEMPLATE_VIS allocator;
+
+template <class _CharT, class _Traits = char_traits<_CharT>, class _Allocator = allocator<_CharT> >
+class basic_string;
+
+using string = basic_string<char>;
+
+#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
+using wstring = basic_string<wchar_t>;
+#endif
+
+#ifndef _LIBCPP_HAS_NO_CHAR8_T
+using u8string = basic_string<char8_t>;
+#endif
+
+using u16string = basic_string<char16_t>;
+using u32string = basic_string<char32_t>;
+
+#if _LIBCPP_STD_VER >= 17
+
+namespace pmr {
+template <class _CharT, class _Traits = char_traits<_CharT>>
+using basic_string = std::basic_string<_CharT, _Traits, polymorphic_allocator<_CharT>>;
+
+using string = basic_string<char>;
+
+#  ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
+using wstring = basic_string<wchar_t>;
+#  endif
+
+#  ifndef _LIBCPP_HAS_NO_CHAR8_T
+using u8string = basic_string<char8_t>;
+#  endif
+
+using u16string = basic_string<char16_t>;
+using u32string = basic_string<char32_t>;
+
+} // namespace pmr
+
+#endif // _LIBCPP_STD_VER >= 17
+
+// clang-format off
+template <class _CharT, class _Traits, class _Allocator>
+class _LIBCPP_PREFERRED_NAME(string)
+#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
+      _LIBCPP_PREFERRED_NAME(wstring)
+#endif
+#ifndef _LIBCPP_HAS_NO_CHAR8_T
+      _LIBCPP_PREFERRED_NAME(u8string)
+#endif
+      _LIBCPP_PREFERRED_NAME(u16string)
+      _LIBCPP_PREFERRED_NAME(u32string)
+#if _LIBCPP_STD_VER >= 17
+      _LIBCPP_PREFERRED_NAME(pmr::string)
+#  ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
+      _LIBCPP_PREFERRED_NAME(pmr::wstring)
+#  endif
+#  ifndef _LIBCPP_HAS_NO_CHAR8_T
+      _LIBCPP_PREFERRED_NAME(pmr::u8string)
+#  endif
+      _LIBCPP_PREFERRED_NAME(pmr::u16string)
+      _LIBCPP_PREFERRED_NAME(pmr::u32string)
+#endif
+      basic_string;
+// clang-format on
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___FWD_STRING_H
diff --git a/libcxx/include/__fwd/string_view.h b/libcxx/include/__fwd/string_view.h
--- a/libcxx/include/__fwd/string_view.h
+++ b/libcxx/include/__fwd/string_view.h
@@ -32,6 +32,19 @@
 typedef basic_string_view<wchar_t>  wstring_view;
 #endif
 
+// clang-format off
+template <class _CharT, class _Traits>
+class _LIBCPP_PREFERRED_NAME(string_view)
+#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
+      _LIBCPP_PREFERRED_NAME(wstring_view)
+#endif
+#ifndef _LIBCPP_HAS_NO_CHAR8_T
+      _LIBCPP_PREFERRED_NAME(u8string_view)
+#endif
+      _LIBCPP_PREFERRED_NAME(u16string_view)
+      _LIBCPP_PREFERRED_NAME(u32string_view)
+      basic_string_view;
+// clang-format on
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___FWD_STRING_VIEW_H
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
@@ -26,17 +26,19 @@
 
 _LIBCPP_DIAGNOSTIC_PUSH
 _LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wweak-vtables") // TODO: move destructor into the dylib
-class _LIBCPP_TYPE_VIS memory_resource {
+class _LIBCPP_TYPE_VIS _LIBCPP_AVAILABILITY_MEMORY_RESOURCE memory_resource {
   static const size_t __max_align = alignof(max_align_t);
 
 public:
   virtual ~memory_resource() = default;
 
-  _LIBCPP_HIDE_FROM_ABI void* allocate(size_t __bytes, size_t __align = __max_align) {
+  [[using __gnu__: __returns_nonnull__, __alloc_size__(2), __alloc_align__(3)]] _LIBCPP_HIDE_FROM_ABI void*
+  allocate(size_t __bytes, size_t __align = __max_align) {
     return do_allocate(__bytes, __align);
   }
 
-  _LIBCPP_HIDE_FROM_ABI void deallocate(void* __p, size_t __bytes, size_t __align = __max_align) {
+  [[__gnu__::__nonnull__]] _LIBCPP_HIDE_FROM_ABI void
+  deallocate(void* __p, size_t __bytes, size_t __align = __max_align) {
     do_deallocate(__p, __bytes, __align);
   }
 
@@ -61,13 +63,14 @@
 
 // [mem.res.global]
 
-_LIBCPP_FUNC_VIS memory_resource* get_default_resource() noexcept;
-
-_LIBCPP_FUNC_VIS memory_resource* set_default_resource(memory_resource*) noexcept;
-
-_LIBCPP_FUNC_VIS memory_resource* new_delete_resource() noexcept;
-
-_LIBCPP_FUNC_VIS memory_resource* null_memory_resource() noexcept;
+[[__gnu__::__returns_nonnull__]] _LIBCPP_AVAILABILITY_MEMORY_RESOURCE _LIBCPP_FUNC_VIS memory_resource*
+get_default_resource() noexcept;
+[[__gnu__::__returns_nonnull__]] _LIBCPP_AVAILABILITY_MEMORY_RESOURCE _LIBCPP_FUNC_VIS memory_resource*
+set_default_resource(memory_resource*) noexcept;
+[[using __gnu__: __returns_nonnull__, __const__]] _LIBCPP_AVAILABILITY_MEMORY_RESOURCE _LIBCPP_FUNC_VIS memory_resource*
+new_delete_resource() noexcept;
+[[using __gnu__: __returns_nonnull__, __const__]] _LIBCPP_AVAILABILITY_MEMORY_RESOURCE _LIBCPP_FUNC_VIS memory_resource*
+null_memory_resource() noexcept;
 
 } // namespace pmr
 
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
@@ -26,7 +26,7 @@
 
 // [mem.res.monotonic.buffer]
 
-class _LIBCPP_TYPE_VIS monotonic_buffer_resource : public memory_resource {
+class _LIBCPP_TYPE_VIS _LIBCPP_AVAILABILITY_MEMORY_RESOURCE monotonic_buffer_resource : public memory_resource {
   static const size_t __default_buffer_capacity  = 1024;
   static const size_t __default_buffer_alignment = 16;
 
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
@@ -34,7 +34,7 @@
 // [mem.poly.allocator.class]
 
 template <class _ValueType>
-class _LIBCPP_TEMPLATE_VIS polymorphic_allocator {
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_MEMORY_RESOURCE polymorphic_allocator {
 public:
   using value_type = _ValueType;
 
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
@@ -30,7 +30,7 @@
 
 // [mem.res.pool.overview]
 
-class _LIBCPP_TYPE_VIS synchronized_pool_resource : public memory_resource {
+class _LIBCPP_TYPE_VIS _LIBCPP_AVAILABILITY_MEMORY_RESOURCE synchronized_pool_resource : public memory_resource {
 public:
   _LIBCPP_HIDE_FROM_ABI synchronized_pool_resource(const pool_options& __opts, memory_resource* __upstream)
       : __unsync_(__opts, __upstream) {}
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
@@ -27,7 +27,7 @@
 
 // [mem.res.pool.overview]
 
-class _LIBCPP_TYPE_VIS unsynchronized_pool_resource : public memory_resource {
+class _LIBCPP_TYPE_VIS _LIBCPP_AVAILABILITY_MEMORY_RESOURCE unsynchronized_pool_resource : public memory_resource {
   class __fixed_pool;
 
   class __adhoc_pool {
@@ -78,7 +78,7 @@
 
   _LIBCPP_HIDE_FROM_ABI memory_resource* upstream_resource() const { return __res_; }
 
-  pool_options options() const;
+  [[__gnu__::__pure__]] pool_options options() const;
 
 protected:
   void* do_allocate(size_t __bytes, size_t __align) override; // key function
diff --git a/libcxx/include/iosfwd b/libcxx/include/iosfwd
--- a/libcxx/include/iosfwd
+++ b/libcxx/include/iosfwd
@@ -96,6 +96,7 @@
 
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
+#include <__fwd/string.h>
 #include <__mbstate_t.h>
 #include <version>
 
@@ -107,19 +108,6 @@
 
 class _LIBCPP_TYPE_VIS ios_base;
 
-template<class _CharT>  struct _LIBCPP_TEMPLATE_VIS char_traits;
-template<> struct char_traits<char>;
-#ifndef _LIBCPP_HAS_NO_CHAR8_T
-template<> struct char_traits<char8_t>;
-#endif
-template<> struct char_traits<char16_t>;
-template<> struct char_traits<char32_t>;
-#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
-template<> struct char_traits<wchar_t>;
-#endif
-
-template<class _Tp>     class _LIBCPP_TEMPLATE_VIS allocator;
-
 template <class _CharT, class _Traits = char_traits<_CharT> >
     class _LIBCPP_TEMPLATE_VIS basic_ios;
 
@@ -242,18 +230,6 @@
 typedef long long streamoff;        // for char_traits in <string>
 #endif
 
-template <class _CharT,             // for <stdexcept>
-          class _Traits = char_traits<_CharT>,
-          class _Allocator = allocator<_CharT> >
-    class _LIBCPP_TEMPLATE_VIS basic_string;
-typedef basic_string<char, char_traits<char>, allocator<char> > string;
-#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
-typedef basic_string<wchar_t, char_traits<wchar_t>, allocator<wchar_t> > wstring;
-#endif
-
-template <class _CharT, class _Traits, class _Allocator>
-    class _LIBCPP_PREFERRED_NAME(string) _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wstring)) basic_string;
-
 // Include other forward declarations here
 template <class _Tp, class _Alloc = allocator<_Tp> >
 class _LIBCPP_TEMPLATE_VIS vector;
diff --git a/libcxx/include/module.modulemap.in b/libcxx/include/module.modulemap.in
--- a/libcxx/include/module.modulemap.in
+++ b/libcxx/include/module.modulemap.in
@@ -887,6 +887,7 @@
     export *
 
     module __memory_resource {
+      module memory_resource_fwd          { private header "__fwd/memory_resource.h" }
       module memory_resource              { private header "__memory_resource/memory_resource.h" }
       module monotonic_buffer_resource    { private header "__memory_resource/monotonic_buffer_resource.h" }
       module polymorphic_allocator        { private header "__memory_resource/polymorphic_allocator.h" }
@@ -1117,6 +1118,7 @@
     module __string {
       module char_traits           { private header "__string/char_traits.h" }
       module extern_template_lists { private header "__string/extern_template_lists.h" }
+      module string_fwd            { private header "__fwd/string.h" }
     }
     export *
   }
diff --git a/libcxx/include/string b/libcxx/include/string
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -539,6 +539,7 @@
 #include <__format/enable_insertable.h>
 #include <__functional/hash.h>
 #include <__functional/unary_function.h>
+#include <__fwd/string.h>
 #include <__ios/fpos.h>
 #include <__iterator/distance.h>
 #include <__iterator/iterator_traits.h>
@@ -565,7 +566,6 @@
 #include <cstdio>  // EOF
 #include <cstdlib>
 #include <cstring>
-#include <iosfwd>
 #include <limits>
 #include <stdexcept>
 #include <string_view>
@@ -655,14 +655,7 @@
 struct __uninitialized_size_tag {};
 
 template<class _CharT, class _Traits, class _Allocator>
-class
-    _LIBCPP_TEMPLATE_VIS
-#ifndef _LIBCPP_HAS_NO_CHAR8_T
-    _LIBCPP_PREFERRED_NAME(u8string)
-#endif
-    _LIBCPP_PREFERRED_NAME(u16string)
-    _LIBCPP_PREFERRED_NAME(u32string)
-    basic_string
+class basic_string
 {
 public:
     typedef basic_string                                 __self;
@@ -4674,20 +4667,6 @@
 
 _LIBCPP_END_NAMESPACE_STD
 
-#if _LIBCPP_STD_VER > 14
-_LIBCPP_BEGIN_NAMESPACE_STD
-namespace pmr {
-template <class _CharT, class _TraitsT = std::char_traits<_CharT>>
-using basic_string = std::basic_string<_CharT, _TraitsT, polymorphic_allocator<_CharT>>;
-
-using string    = basic_string<char>;
-using u16string = basic_string<char16_t>;
-using u32string = basic_string<char32_t>;
-using wstring   = basic_string<wchar_t>;
-} // namespace pmr
-_LIBCPP_END_NAMESPACE_STD
-#endif
-
 _LIBCPP_POP_MACROS
 
 #if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
diff --git a/libcxx/include/string_view b/libcxx/include/string_view
--- a/libcxx/include/string_view
+++ b/libcxx/include/string_view
@@ -256,15 +256,7 @@
 }
 
 template<class _CharT, class _Traits>
-class
-    _LIBCPP_PREFERRED_NAME(string_view)
-#ifndef _LIBCPP_HAS_NO_CHAR8_T
-    _LIBCPP_PREFERRED_NAME(u8string_view)
-#endif
-    _LIBCPP_PREFERRED_NAME(u16string_view)
-    _LIBCPP_PREFERRED_NAME(u32string_view)
-    _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wstring_view))
-    basic_string_view {
+class basic_string_view {
 public:
     // types
     typedef _Traits                                    traits_type;
diff --git a/libcxx/test/libcxx/mem/mem.res/availability.verify.cpp b/libcxx/test/libcxx/mem/mem.res/availability.verify.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/libcxx/mem/mem.res/availability.verify.cpp
@@ -0,0 +1,24 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14
+// REQUIRES: use_system_cxx_lib && (target={{.+}}-apple-macosx10.{{9|10|11|12|13|14|15}} || target={{.+}}-apple-macosx{{11.0|12.0}})
+
+#include <memory_resource>
+
+void f() {
+  [[maybe_unused]] std::pmr::polymorphic_allocator<int> poly; // expected-error {{is unavailable}}
+  [[maybe_unused]] std::pmr::memory_resource* res;            // expected-error {{is unavailable}}
+  [[maybe_unused]] std::pmr::synchronized_pool_resource r1;   // expected-error {{is unavailable}}
+  [[maybe_unused]] std::pmr::monotonic_buffer_resource r2;    // expected-error {{is unavailable}}
+  [[maybe_unused]] std::pmr::unsynchronized_pool_resource r3; // expected-error {{is unavailable}}
+  (void)std::pmr::get_default_resource();                     // expected-error {{is unavailable}}
+  (void)std::pmr::set_default_resource(nullptr);              // expected-error {{is unavailable}}
+  (void)std::pmr::new_delete_resource();                      // expected-error {{is unavailable}}
+  (void)std::pmr::null_memory_resource();                     // expected-error {{is unavailable}}
+}
diff --git a/libcxx/test/libcxx/private_headers.verify.cpp b/libcxx/test/libcxx/private_headers.verify.cpp
--- a/libcxx/test/libcxx/private_headers.verify.cpp
+++ b/libcxx/test/libcxx/private_headers.verify.cpp
@@ -373,8 +373,10 @@
 #include <__fwd/array.h> // expected-error@*:* {{use of private header from outside its module: '__fwd/array.h'}}
 #include <__fwd/get.h> // expected-error@*:* {{use of private header from outside its module: '__fwd/get.h'}}
 #include <__fwd/hash.h> // expected-error@*:* {{use of private header from outside its module: '__fwd/hash.h'}}
+#include <__fwd/memory_resource.h> // expected-error@*:* {{use of private header from outside its module: '__fwd/memory_resource.h'}}
 #include <__fwd/pair.h> // expected-error@*:* {{use of private header from outside its module: '__fwd/pair.h'}}
 #include <__fwd/span.h> // expected-error@*:* {{use of private header from outside its module: '__fwd/span.h'}}
+#include <__fwd/string.h> // expected-error@*:* {{use of private header from outside its module: '__fwd/string.h'}}
 #include <__fwd/string_view.h> // expected-error@*:* {{use of private header from outside its module: '__fwd/string_view.h'}}
 #include <__fwd/tuple.h> // expected-error@*:* {{use of private header from outside its module: '__fwd/tuple.h'}}
 #include <__ios/fpos.h> // expected-error@*:* {{use of private header from outside its module: '__ios/fpos.h'}}