diff --git a/libcxx/docs/Status/Cxx20.rst b/libcxx/docs/Status/Cxx20.rst
--- a/libcxx/docs/Status/Cxx20.rst
+++ b/libcxx/docs/Status/Cxx20.rst
@@ -49,7 +49,7 @@
    .. [#note-P0883.1] P0883: shared_ptr and floating-point changes weren't applied as they themselves aren't implemented yet.
    .. [#note-P0883.2] P0883: ``ATOMIC_FLAG_INIT`` was marked deprecated in version 14.0, but was undeprecated with the implementation of LWG3659 in version 15.0.
    .. [#note-P2231] P2231: Optional is complete. The changes to variant haven't been implemented yet.
-   .. [#note-P0408] P0408: All `stringbuf` and `istringstream` members plus `view()` in all classes implemented.
+   .. [#note-P0408] P0408: Implemented all `stringbuf`/`istringstream`/`ostringstream` members and `view()` in `stringstream`.
    .. [#note-P0660] P0660: Section 32.3 Stop Tokens is complete. ``jthread`` hasn't been implemented yet.
 
 .. _issues-status-cxx20:
diff --git a/libcxx/include/sstream b/libcxx/include/sstream
--- a/libcxx/include/sstream
+++ b/libcxx/include/sstream
@@ -159,12 +159,23 @@
     typedef Allocator                      allocator_type;
 
     // [ostringstream.cons] Constructors:
-    explicit basic_ostringstream(ios_base::openmode which = ios_base::out); // before C++20
-    basic_ostringstream() : basic_ostringstream(ios_base::out) {}           // C++20
-    explicit basic_ostringstream(ios_base::openmode which);                 // C++20
-
-    explicit basic_ostringstream(const basic_string<char_type, traits_type, allocator_type>& str,
+    explicit basic_ostringstream(ios_base::openmode which = ios_base::out);            // before C++20
+    basic_ostringstream() : basic_ostringstream(ios_base::out) {}                      // C++20
+    explicit basic_ostringstream(ios_base::openmode which);                            // C++20
+    explicit basic_ostringstream(const basic_string<char_type, traits_type, allocator_type>& s,
                                  ios_base::openmode which = ios_base::out);
+    basic_ostringstream(ios_base::openmode which, const allocator_type& a);            // C++20
+    explicit basic_ostringstream(basic_string<char_type, traits_type, allocator_type>&& s,
+                                 ios_base::openmode which = ios_base::out);            // C++20
+    template <class SAlloc>
+    basic_ostringstream(const basic_string<char_type, traits_type, SAlloc>& s, const allocator_type& a)
+        : basic_ostringstream(s, ios_base::out, a) {}                                  // C++20
+    template <class SAlloc>
+    basic_ostringstream(const basic_string<char_type, traits_type, SAlloc>& s,
+                        ios_base::openmode which, const allocator_type& a);            // C++20
+    template <class SAlloc>
+    explicit basic_ostringstream(const basic_string<char_type, traits_type, SAlloc>& s,
+                                 ios_base::openmode which = ios_base::out);            // C++20
     basic_ostringstream(basic_ostringstream&& rhs);
 
     // [ostringstream.assign] Assign and swap:
@@ -173,9 +184,16 @@
 
     // [ostringstream.members] Member functions:
     basic_stringbuf<char_type, traits_type, allocator_type>* rdbuf() const;
-    basic_string<char_type, traits_type, allocator_type> str() const;
+    basic_string<char_type, traits_type, allocator_type> str() const;                  // before C++20
+    basic_string<char_type, traits_type, allocator_type> str() const &;                // C++20
+    template <class SAlloc>
+    basic_string<char_type, traits_type, SAlloc> str(const SAlloc& sa) const;          // C++20
+    basic_string<char_type, traits_type, allocator_type> str() &&;                     // C++20
+    basic_string_view<char_type, traits_type> view() const noexcept;                   // C++20
     void str(const basic_string<char_type, traits_type, allocator_type>& s);
-    basic_string_view<char_type, traits_type> view() const noexcept; // C++20
+    template <class SAlloc>
+    void str(const basic_string<char_type, traits_type, SAlloc>& s);                   // C++20
+    void str(basic_string<char_type, traits_type, allocator_type>&& s);                // C++20
 };
 
 template <class charT, class traits, class Allocator>
@@ -825,6 +843,7 @@
         : basic_istream<_CharT, _Traits>(std::addressof(__sb_)), __sb_(__s, __wch | ios_base::in, __a) {}
 
     template <class _SAlloc>
+      requires (!is_same_v<_SAlloc, allocator_type>)
     _LIBCPP_HIDE_FROM_ABI explicit basic_istringstream(const basic_string<_CharT, _Traits, _SAlloc>& __s,
                                                        ios_base::openmode __wch = ios_base::in)
         : basic_istream<_CharT, _Traits>(std::addressof(__sb_)), __sb_(__s, __wch | ios_base::in) {}
@@ -929,6 +948,29 @@
         , __sb_(__s, __wch | ios_base::out)
     { }
 
+#if _LIBCPP_STD_VER >= 20
+    _LIBCPP_HIDE_FROM_ABI basic_ostringstream(ios_base::openmode __wch, const _Allocator& __a)
+        : basic_ostream<_CharT, _Traits>(std::addressof(__sb_)), __sb_(__wch | ios_base::out, __a) {}
+
+    _LIBCPP_HIDE_FROM_ABI explicit basic_ostringstream(string_type&& __s, ios_base::openmode __wch = ios_base::out)
+        : basic_ostream<_CharT, _Traits>(std::addressof(__sb_)), __sb_(std::move(__s), __wch | ios_base::out) {}
+
+    template <class _SAlloc>
+    _LIBCPP_HIDE_FROM_ABI basic_ostringstream(const basic_string<_CharT, _Traits, _SAlloc>& __s, const _Allocator& __a)
+        : basic_ostringstream(__s, ios_base::out, __a) {}
+
+    template <class _SAlloc>
+    _LIBCPP_HIDE_FROM_ABI basic_ostringstream(
+        const basic_string<_CharT, _Traits, _SAlloc>& __s, ios_base::openmode __wch, const _Allocator& __a)
+        : basic_ostream<_CharT, _Traits>(std::addressof(__sb_)), __sb_(__s, __wch | ios_base::out, __a) {}
+
+    template <class _SAlloc>
+      requires (!is_same_v<_SAlloc, allocator_type>)
+    _LIBCPP_HIDE_FROM_ABI explicit basic_ostringstream(const basic_string<_CharT, _Traits, _SAlloc>& __s,
+                                                       ios_base::openmode __wch = ios_base::out)
+        : basic_ostream<_CharT, _Traits>(std::addressof(__sb_)), __sb_(__s, __wch | ios_base::out) {}
+#endif // _LIBCPP_STD_VER >= 20
+
     _LIBCPP_INLINE_VISIBILITY
     basic_ostringstream(basic_ostringstream&& __rhs)
         : basic_ostream<_CharT, _Traits>(_VSTD::move(__rhs))
@@ -955,20 +997,33 @@
     basic_stringbuf<char_type, traits_type, allocator_type>* rdbuf() const {
         return const_cast<basic_stringbuf<char_type, traits_type, allocator_type>*>(&__sb_);
     }
-    _LIBCPP_INLINE_VISIBILITY
-    string_type str() const {
-        return __sb_.str();
-    }
-    _LIBCPP_INLINE_VISIBILITY
-    void str(const string_type& __s) {
-        __sb_.str(__s);
+
+#if _LIBCPP_STD_VER >= 20
+    _LIBCPP_HIDE_FROM_ABI string_type str() const & { return __sb_.str(); }
+
+    template <class _SAlloc>
+      requires __is_allocator<_SAlloc>::value
+    _LIBCPP_HIDE_FROM_ABI basic_string<char_type, traits_type, _SAlloc> str(const _SAlloc& __sa) const {
+        return __sb_.str(__sa);
     }
+
+    _LIBCPP_HIDE_FROM_ABI string_type str() && { return std::move(__sb_).str(); }
+
+    _LIBCPP_HIDE_FROM_ABI basic_string_view<char_type, traits_type> view() const noexcept { return __sb_.view(); }
+#else // _LIBCPP_STD_VER >= 20
+    _LIBCPP_HIDE_FROM_ABI string_type str() const { return __sb_.str(); }
+#endif // _LIBCPP_STD_VER >= 20
+
+    _LIBCPP_HIDE_FROM_ABI void str(const string_type& __s) { __sb_.str(__s); }
+
 #if _LIBCPP_STD_VER >= 20
-    _LIBCPP_HIDE_FROM_ABI
-    basic_string_view<char_type, traits_type> view() const noexcept {
-        return __sb_.view();
+    template <class _SAlloc>
+    _LIBCPP_HIDE_FROM_ABI void str(const basic_string<char_type, traits_type, _SAlloc>& __s) {
+        __sb_.str(__s);
     }
-#endif
+
+    _LIBCPP_HIDE_FROM_ABI void str(string_type&& __s) { __sb_.str(std::move(__s)); }
+#endif // _LIBCPP_STD_VER >= 20
 };
 
 template <class _CharT, class _Traits, class _Allocator>
diff --git a/libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.cons/mode.alloc.pass.cpp b/libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.cons/mode.alloc.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.cons/mode.alloc.pass.cpp
@@ -0,0 +1,38 @@
+//===----------------------------------------------------------------------===//
+//
+// 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, c++17
+
+// <sstream>
+
+// template <class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>>
+// class basic_ostringstream
+
+// basic_ostringstream(ios_base::openmode which, const Allocator& a);
+
+#include <sstream>
+#include <cassert>
+
+#include "test_allocator.h"
+#include "test_macros.h"
+
+template <class CharT>
+static void test() {
+  const test_allocator<CharT> a(2);
+  const std::basic_ostringstream<CharT, std::char_traits<CharT>, test_allocator<CharT>> ss(std::ios_base::binary, a);
+  assert(ss.rdbuf()->get_allocator() == a);
+  assert(ss.view().empty());
+}
+
+int main(int, char**) {
+  test<char>();
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+  return 0;
+}
diff --git a/libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.cons/string-alloc.mode.pass.cpp b/libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.cons/string-alloc.mode.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.cons/string-alloc.mode.pass.cpp
@@ -0,0 +1,49 @@
+//===----------------------------------------------------------------------===//
+//
+// 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, c++17
+
+// <sstream>
+
+// template <class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>>
+// class basic_ostringstream
+
+// template <class SAlloc>
+// explicit basic_ostringstream(const basic_string<char_type, traits_type, SAlloc>& s, ios_base::openmode which = ios_base::out)
+
+#include <sstream>
+#include <cassert>
+
+#include "make_string.h"
+#include "test_allocator.h"
+#include "test_macros.h"
+
+#define STR(S) MAKE_STRING(CharT, S)
+#define SV(S) MAKE_STRING_VIEW(CharT, S)
+
+template <class CharT>
+static void test() {
+  {
+    const std::basic_string<CharT> s(STR("testing"));
+    const std::basic_ostringstream<CharT, std::char_traits<CharT>, test_allocator<CharT>> ss(s);
+    assert(ss.view() == SV("testing"));
+  }
+  {
+    const std::basic_string<CharT> s(STR("testing"));
+    const std::basic_ostringstream<CharT, std::char_traits<CharT>, test_allocator<CharT>> ss(s, std::ios_base::binary);
+    assert(ss.view() == SV("testing"));
+  }
+}
+
+int main(int, char**) {
+  test<char>();
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+  return 0;
+}
diff --git a/libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.cons/string.alloc.pass.cpp b/libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.cons/string.alloc.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.cons/string.alloc.pass.cpp
@@ -0,0 +1,45 @@
+//===----------------------------------------------------------------------===//
+//
+// 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, c++17
+
+// <sstream>
+
+// template <class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>>
+// class basic_ostringstream
+
+// template <class SAlloc>
+// basic_ostringstream(const basic_string<char_type, traits_type, SAlloc>& s, const Allocator& a)
+//     : basic_ostringstream(s, ios_base::out, a) {}
+
+#include <sstream>
+#include <cassert>
+
+#include "make_string.h"
+#include "test_allocator.h"
+#include "test_macros.h"
+
+#define STR(S) MAKE_STRING(CharT, S)
+#define SV(S) MAKE_STRING_VIEW(CharT, S)
+
+template <class CharT>
+static void test() {
+  const std::basic_string<CharT> s(STR("testing"));
+  const test_allocator<CharT> a(2);
+  const std::basic_ostringstream<CharT, std::char_traits<CharT>, test_allocator<CharT>> ss(s, a);
+  assert(ss.rdbuf()->get_allocator() == a);
+  assert(ss.view() == SV("testing"));
+}
+
+int main(int, char**) {
+  test<char>();
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+  return 0;
+}
diff --git a/libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.cons/string.mode.alloc.pass.cpp b/libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.cons/string.mode.alloc.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.cons/string.mode.alloc.pass.cpp
@@ -0,0 +1,44 @@
+//===----------------------------------------------------------------------===//
+//
+// 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, c++17
+
+// <sstream>
+
+// template <class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>>
+// class basic_ostringstream
+
+// template <class SAlloc>
+// basic_ostringstream(const basic_string<char_type, traits_type, SAlloc>& s, ios_base::openmode which, const Allocator& a)
+
+#include <sstream>
+#include <cassert>
+
+#include "make_string.h"
+#include "test_allocator.h"
+#include "test_macros.h"
+
+#define STR(S) MAKE_STRING(CharT, S)
+#define SV(S) MAKE_STRING_VIEW(CharT, S)
+
+template <class CharT>
+static void test() {
+  const std::basic_string<CharT> s(STR("testing"));
+  const test_allocator<CharT> a(2);
+  const std::basic_ostringstream<CharT, std::char_traits<CharT>, test_allocator<CharT>> ss(s, std::ios_base::binary, a);
+  assert(ss.rdbuf()->get_allocator() == a);
+  assert(ss.view() == SV("testing"));
+}
+
+int main(int, char**) {
+  test<char>();
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+  return 0;
+}
diff --git a/libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.cons/string.move.mode.pass.cpp b/libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.cons/string.move.mode.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.cons/string.move.mode.pass.cpp
@@ -0,0 +1,46 @@
+//===----------------------------------------------------------------------===//
+//
+// 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, c++17
+
+// <sstream>
+
+// template <class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>>
+// class basic_ostringstream
+
+// explicit basic_ostringstream(basic_string<char_type, traits_type, allocator_type>&& s, ios_base::openmode which = ios_base::out);
+
+#include <sstream>
+#include <cassert>
+
+#include "make_string.h"
+#include "test_macros.h"
+
+#define STR(S) MAKE_STRING(CharT, S)
+
+template <class CharT>
+static void test() {
+  {
+    std::basic_string<CharT> s(STR("testing"));
+    const std::basic_ostringstream<CharT> ss(std::move(s));
+    assert(ss.str() == STR("testing"));
+  }
+  {
+    std::basic_string<CharT> s(STR("testing"));
+    const std::basic_ostringstream<CharT> ss(std::move(s), std::ios_base::binary);
+    assert(ss.str() == STR("testing"));
+  }
+}
+
+int main(int, char**) {
+  test<char>();
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+  return 0;
+}
diff --git a/libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.members/str.alloc.pass.cpp b/libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.members/str.alloc.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.members/str.alloc.pass.cpp
@@ -0,0 +1,44 @@
+//===----------------------------------------------------------------------===//
+//
+// 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, c++17
+
+// <sstream>
+
+// template <class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>>
+// class basic_ostringstream
+
+// template <class SAlloc>
+// basic_string<charT, traits, SAlloc> str(const SAlloc& sa) const;
+
+#include <sstream>
+#include <cassert>
+
+#include "make_string.h"
+#include "test_allocator.h"
+#include "test_macros.h"
+
+#define STR(S) MAKE_STRING(CharT, S)
+#define SV(S) MAKE_STRING_VIEW(CharT, S)
+
+template <class CharT>
+static void test() {
+  const std::basic_ostringstream<CharT> ss(STR("testing"));
+  const test_allocator<CharT> a(1);
+  const std::basic_string<CharT, std::char_traits<CharT>, test_allocator<CharT>> s = ss.str(a);
+  assert(s == SV("testing"));
+  assert(s.get_allocator() == a);
+}
+
+int main(int, char**) {
+  test<char>();
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+  return 0;
+}
diff --git a/libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.members/str.move.pass.cpp b/libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.members/str.move.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.members/str.move.pass.cpp
@@ -0,0 +1,42 @@
+//===----------------------------------------------------------------------===//
+//
+// 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, c++17
+
+// <sstream>
+
+// template <class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>>
+// class basic_ostringstream
+
+// basic_string<charT, traits, Allocator> str() &&;
+
+#include <sstream>
+#include <cassert>
+
+#include "make_string.h"
+#include "test_macros.h"
+
+#define STR(S) MAKE_STRING(CharT, S)
+
+template <class CharT>
+static void test() {
+  {
+    std::basic_ostringstream<CharT> ss(STR("testing"));
+    std::basic_string<CharT> s = std::move(ss).str();
+    assert(s == STR("testing"));
+    assert(ss.view().empty());
+  }
+}
+
+int main(int, char**) {
+  test<char>();
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+  return 0;
+}
diff --git a/libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.members/str.string-alloc.pass.cpp b/libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.members/str.string-alloc.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.members/str.string-alloc.pass.cpp
@@ -0,0 +1,45 @@
+//===----------------------------------------------------------------------===//
+//
+// 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, c++17
+
+// <sstream>
+
+// template <class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>>
+// class basic_ostringstream
+
+// template <class SAlloc>
+// void str(const basic_string<charT, traits, SAlloc>& s);
+
+#include <sstream>
+#include <cassert>
+
+#include "make_string.h"
+#include "test_allocator.h"
+#include "test_macros.h"
+
+#define STR(S) MAKE_STRING(CharT, S)
+
+template <class CharT>
+static void test() {
+  {
+    const test_allocator<CharT> a(6);
+    const std::basic_string<CharT, std::char_traits<CharT>, test_allocator<CharT>> s(STR("testing"), a);
+    std::basic_ostringstream<CharT> ss;
+    ss.str(s);
+    assert(ss.str() == STR("testing"));
+  }
+}
+
+int main(int, char**) {
+  test<char>();
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+  return 0;
+}
diff --git a/libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.members/str.string.move.pass.cpp b/libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.members/str.string.move.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.members/str.string.move.pass.cpp
@@ -0,0 +1,42 @@
+//===----------------------------------------------------------------------===//
+//
+// 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, c++17
+
+// <sstream>
+
+// template <class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>>
+// class basic_ostringstream
+
+// void str(basic_string<charT, traits, Allocator>&& s);
+
+#include <sstream>
+#include <cassert>
+
+#include "make_string.h"
+#include "test_macros.h"
+
+#define STR(S) MAKE_STRING(CharT, S)
+
+template <class CharT>
+static void test() {
+  {
+    std::basic_ostringstream<CharT> ss;
+    std::basic_string<CharT> s(STR("testing"));
+    ss.str(std::move(s));
+    assert(ss.str() == STR("testing"));
+  }
+}
+
+int main(int, char**) {
+  test<char>();
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+  return 0;
+}