[committed,1/2] libstdc++: Add <syncstream> declarations to <iosfwd> [PR105284]

diff --git a/libstdc++-v3/include/std/iosfwd b/libstdc++-v3/include/std/iosfwd
index 2d79a131631..c8c67c86c3b 100644
--- a/libstdc++-v3/include/std/iosfwd
+++ b/libstdc++-v3/include/std/iosfwd
@@ -207,6 +207,24 @@  _GLIBCXX_END_NAMESPACE_CXX11
   /// Class for @c wchar_t mixed input and output file streams.
   typedef basic_fstream<wchar_t> 	wfstream;
 #endif
+
+#if __cplusplus >= 202002L && _GLIBCXX_USE_CXX11_ABI
+  template<typename _CharT, typename _Traits = char_traits<_CharT>,
+           typename _Allocator = allocator<_CharT>>
+    class basic_syncbuf;
+  template<typename _CharT, typename _Traits = char_traits<_CharT>,
+           typename _Allocator = allocator<_CharT>>
+    class basic_osyncstream;
+
+  using syncbuf = basic_syncbuf<char>;
+  using osyncstream = basic_osyncstream<char>;
+
+#ifdef _GLIBCXX_USE_WCHAR_T
+  using wsyncbuf = basic_syncbuf<wchar_t>;
+  using wosyncstream = basic_osyncstream<wchar_t>;
+#endif
+#endif // C++20 && CXX11_ABI
+
   /** @}  */
 
 _GLIBCXX_END_NAMESPACE_VERSION
diff --git a/libstdc++-v3/include/std/syncstream b/libstdc++-v3/include/std/syncstream
index 3cf2296f723..7a4f731ddd9 100644
--- a/libstdc++-v3/include/std/syncstream
+++ b/libstdc++-v3/include/std/syncstream
@@ -50,8 +50,7 @@  namespace std _GLIBCXX_VISIBILITY(default)
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
-  template<typename _CharT, typename _Traits = char_traits<_CharT>,
-	    typename _Alloc = allocator<_CharT>>
+  template<typename _CharT, typename _Traits, typename _Alloc>
     class basic_syncbuf : public __syncbuf_base<_CharT, _Traits>
     {
     public:
@@ -241,8 +240,7 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       __mutex _M_mtx;
     };
 
-  template <typename _CharT, typename _Traits = char_traits<_CharT>,
-	    typename _Alloc = allocator<_CharT>>
+  template <typename _CharT, typename _Traits, typename _Alloc>
     class basic_osyncstream : public basic_ostream<_CharT, _Traits>
     {
       using __ostream_type = basic_ostream<_CharT, _Traits>;
diff --git a/libstdc++-v3/testsuite/27_io/headers/iosfwd/synopsis.cc b/libstdc++-v3/testsuite/27_io/headers/iosfwd/synopsis.cc
new file mode 100644
index 00000000000..48f733bf28a
--- /dev/null
+++ b/libstdc++-v3/testsuite/27_io/headers/iosfwd/synopsis.cc
@@ -0,0 +1,119 @@ 
+// { dg-do compile }
+
+#include <iosfwd>
+
+namespace std
+{
+  // [iosfwd.syn]
+
+  template<class charT> struct char_traits;
+  template<> struct char_traits<char>;
+#if __cplusplus >= 202002L
+  template<> struct char_traits<char8_t>;
+#endif
+#if __cplusplus >= 201103L
+  template<> struct char_traits<char16_t>;
+  template<> struct char_traits<char32_t>;
+#endif
+  template<> struct char_traits<wchar_t>;
+
+  template<class T> class allocator;
+
+  template<class charT, class traits>
+    class basic_ios;
+  template<class charT, class traits>
+    class basic_streambuf;
+  template<class charT, class traits>
+    class basic_istream;
+  template<class charT, class traits>
+    class basic_ostream;
+  template<class charT, class traits>
+    class basic_iostream;
+
+_GLIBCXX_BEGIN_NAMESPACE_CXX11
+  template<class charT, class traits, class Allocator>
+    class basic_stringbuf;
+  template<class charT, class traits, class Allocator>
+    class basic_istringstream;
+  template<class charT, class traits, class Allocator>
+    class basic_ostringstream;
+  template<class charT, class traits, class Allocator>
+    class basic_stringstream;
+_GLIBCXX_END_NAMESPACE_CXX11
+
+  template<class charT, class traits>
+    class basic_filebuf;
+  template<class charT, class traits>
+    class basic_ifstream;
+  template<class charT, class traits>
+    class basic_ofstream;
+  template<class charT, class traits>
+    class basic_fstream;
+
+#if __cplusplus >= 202002L && _GLIBCXX_USE_CXX11_ABI
+  template<class charT, class traits, class Allocator>
+    class basic_syncbuf;
+  template<class charT, class traits, class Allocator>
+    class basic_osyncstream;
+#endif
+
+  template<class charT, class traits>
+    class istreambuf_iterator;
+  template<class charT, class traits>
+    class ostreambuf_iterator;
+
+  typedef basic_ios<char>    ios;
+  typedef basic_ios<wchar_t> wios;
+
+  typedef basic_streambuf<char> streambuf;
+  typedef basic_istream<char>   istream;
+  typedef basic_ostream<char>   ostream;
+  typedef basic_iostream<char>  iostream;
+
+  typedef basic_stringbuf<char>     stringbuf;
+  typedef basic_istringstream<char> istringstream;
+  typedef basic_ostringstream<char> ostringstream;
+  typedef basic_stringstream<char>  stringstream;
+
+
+  typedef basic_filebuf<char>  filebuf;
+  typedef basic_ifstream<char> ifstream;
+  typedef basic_ofstream<char> ofstream;
+  typedef basic_fstream<char>  fstream;
+
+#if __cplusplus >= 202002L && _GLIBCXX_USE_CXX11_ABI
+  typedef basic_syncbuf<char>     syncbuf;
+  typedef basic_osyncstream<char> osyncstream;
+#endif
+
+  typedef basic_streambuf<wchar_t> wstreambuf;
+  typedef basic_istream<wchar_t>   wistream;
+  typedef basic_ostream<wchar_t>   wostream;
+  typedef basic_iostream<wchar_t>  wiostream;
+
+  typedef basic_stringbuf<wchar_t>     wstringbuf;
+  typedef basic_istringstream<wchar_t> wistringstream;
+  typedef basic_ostringstream<wchar_t> wostringstream;
+  typedef basic_stringstream<wchar_t>  wstringstream;
+
+  typedef basic_filebuf<wchar_t>  wfilebuf;
+  typedef basic_ifstream<wchar_t> wifstream;
+  typedef basic_ofstream<wchar_t> wofstream;
+  typedef basic_fstream<wchar_t>  wfstream;
+
+#if __cplusplus >= 202002L
+  typedef basic_syncbuf<wchar_t>     wsyncbuf;
+  typedef basic_osyncstream<wchar_t> wosyncstream;
+#endif
+
+  template<class state> class fpos;
+  typedef fpos<std::mbstate_t> streampos;
+  typedef fpos<std::mbstate_t> wstreampos;
+#if __cplusplus >= 202002L
+  typedef fpos<std::mbstate_t> u8streampos;
+#endif
+#if __cplusplus >= 201103L
+  typedef fpos<std::mbstate_t> u16streampos;
+  typedef fpos<std::mbstate_t> u32streampos;
+#endif
+}
diff --git a/libstdc++-v3/testsuite/27_io/headers/iosfwd/types.cc b/libstdc++-v3/testsuite/27_io/headers/iosfwd/types.cc
new file mode 100644
index 00000000000..e2c9c9b8dfa
--- /dev/null
+++ b/libstdc++-v3/testsuite/27_io/headers/iosfwd/types.cc
@@ -0,0 +1,214 @@ 
+// { dg-do compile { target c++11 } }
+
+#include <iosfwd>
+
+template<class, class> struct Same { static constexpr bool value = false; };
+template<class T> struct Same<T, T> { static constexpr bool value = true; };
+
+template<class T> using Traits = std::char_traits<T>;
+template<class T> using Alloc = std::allocator<T>;
+
+using std::basic_ios;
+
+static_assert(Same<basic_ios<char>,
+		   basic_ios<char, Traits<char>>
+		   >::value,
+    "std::basic_ios<char> has the correct default template argument");
+
+static_assert(Same<basic_ios<wchar_t>,
+		   basic_ios<wchar_t, Traits<wchar_t>>
+		   >::value,
+    "std::basic_ios<wchar_t> has the correct default template argument");
+
+using std::basic_streambuf;
+
+static_assert(Same<basic_streambuf<char>,
+		   basic_streambuf<char, Traits<char>>
+		   >::value,
+    "std::basic_streambuf<char> has the correct default template argument");
+
+static_assert(Same<basic_streambuf<wchar_t>,
+		   basic_streambuf<wchar_t, Traits<wchar_t>>
+		   >::value,
+    "std::basic_streambuf<wchar_t> has the correct default template argument");
+
+using std::basic_istream;
+
+static_assert(Same<basic_istream<char>,
+		   basic_istream<char, Traits<char>>
+		   >::value,
+    "std::basic_istream<char> has the correct default template argument");
+
+static_assert(Same<basic_istream<wchar_t>,
+		   basic_istream<wchar_t, Traits<wchar_t>>
+		   >::value,
+    "std::basic_istream<wchar_t> has the correct default template argument");
+
+using std::basic_ostream;
+
+static_assert(Same<basic_ostream<char>,
+		   basic_ostream<char, Traits<char>>
+		   >::value,
+    "std::basic_ostream<char> has the correct default template argument");
+
+static_assert(Same<basic_ostream<wchar_t>,
+		   basic_ostream<wchar_t, Traits<wchar_t>>
+		   >::value,
+    "std::basic_ostream<wchar_t> has the correct default template argument");
+
+using std::basic_iostream;
+
+static_assert(Same<basic_iostream<char>,
+		   basic_iostream<char, Traits<char>>>::value,
+    "std::basic_iostream<char> has the correct default template argument");
+
+static_assert(Same<basic_iostream<wchar_t>,
+		   basic_iostream<wchar_t, Traits<wchar_t>>
+		   >::value,
+    "std::basic_iostream<wchar_t> has the correct default template argument");
+
+using std::basic_stringbuf;
+
+static_assert(Same<basic_stringbuf<char>,
+		   basic_stringbuf<char, Traits<char>, Alloc<char>>
+		   >::value,
+    "std::basic_stringbuf<char> has the correct default template argument");
+
+static_assert(Same<basic_stringbuf<wchar_t>,
+		   basic_stringbuf<wchar_t, Traits<wchar_t>, Alloc<wchar_t>>
+		   >::value,
+    "std::basic_stringbuf<wchar_t> has the correct default template argument");
+
+using std::basic_istringstream;
+
+static_assert(Same<basic_istringstream<char>,
+		   basic_istringstream<char, Traits<char>, Alloc<char>>
+		   >::value,
+    "std::basic_istringstream<char> has the correct default template argument");
+
+static_assert(Same<basic_istringstream<wchar_t>,
+		   basic_istringstream<wchar_t, Traits<wchar_t>, Alloc<wchar_t>>
+		   >::value,
+    "std::basic_istringstream<wchar_t> has the correct default template argument");
+
+using std::basic_ostringstream;
+
+static_assert(Same<basic_ostringstream<char>,
+		   basic_ostringstream<char, Traits<char>, Alloc<char>>
+		   >::value,
+    "std::basic_ostringstream<char> has the correct default template argument");
+
+static_assert(Same<basic_ostringstream<wchar_t>,
+		   basic_ostringstream<wchar_t, Traits<wchar_t>, Alloc<wchar_t>>
+		   >::value,
+    "std::basic_ostringstream<wchar_t> has the correct default template argument");
+
+using std::basic_stringstream;
+
+static_assert(Same<basic_stringstream<char>,
+		   basic_stringstream<char, Traits<char>, Alloc<char>>
+		   >::value,
+    "std::basic_stringstream<char> has the correct default template argument");
+
+static_assert(Same<basic_stringstream<wchar_t>,
+		   basic_stringstream<wchar_t, Traits<wchar_t>, Alloc<wchar_t>>
+		   >::value,
+    "std::basic_stringstream<wchar_t> has the correct default template argument");
+
+using std::basic_filebuf;
+
+static_assert(Same<basic_filebuf<char>,
+		   basic_filebuf<char, Traits<char>>
+		   >::value,
+    "std::basic_filebuf<char> has the correct default template argument");
+
+static_assert(Same<basic_filebuf<wchar_t>,
+		   basic_filebuf<wchar_t, Traits<wchar_t>>
+		   >::value,
+    "std::basic_filebuf<wchar_t> has the correct default template argument");
+
+using std::basic_ifstream;
+
+static_assert(Same<basic_ifstream<char>,
+		   basic_ifstream<char, Traits<char>>
+		   >::value,
+    "std::basic_ifstream<char> has the correct default template argument");
+
+static_assert(Same<basic_ifstream<wchar_t>,
+		   basic_ifstream<wchar_t, Traits<wchar_t>>
+		   >::value,
+    "std::basic_ifstream<wchar_t> has the correct default template argument");
+
+using std::basic_ofstream;
+
+static_assert(Same<basic_ofstream<char>,
+		   basic_ofstream<char, Traits<char>>
+		   >::value,
+    "std::basic_ofstream<char> has the correct default template argument");
+
+static_assert(Same<basic_ofstream<wchar_t>,
+		   basic_ofstream<wchar_t, Traits<wchar_t>>
+		   >::value,
+    "std::basic_ofstream<wchar_t> has the correct default template argument");
+
+using std::basic_fstream;
+
+static_assert(Same<basic_fstream<char>,
+		   basic_fstream<char, Traits<char>>
+		   >::value,
+    "std::basic_fstream<char> has the correct default template argument");
+
+static_assert(Same<basic_fstream<wchar_t>,
+		   basic_fstream<wchar_t, Traits<wchar_t>>
+		   >::value,
+    "std::basic_fstream<wchar_t> has the correct default template argument");
+
+#if __cplusplus >= 202002L && _GLIBCXX_USE_CXX11_ABI
+using std::basic_syncbuf;
+
+static_assert(Same<basic_syncbuf<char>,
+		   basic_syncbuf<char, Traits<char>, Alloc<char>>
+		   >::value,
+    "std::basic_syncbuf<char> has the correct default template argument");
+
+static_assert(Same<basic_syncbuf<wchar_t>,
+		   basic_syncbuf<wchar_t, Traits<wchar_t>, Alloc<wchar_t>>
+		   >::value,
+    "std::basic_syncbuf<wchar_t> has the correct default template argument");
+
+using std::basic_osyncstream;
+
+static_assert(Same<basic_osyncstream<char>,
+		   basic_osyncstream<char, Traits<char>, Alloc<char>>
+		   >::value,
+    "std::basic_osyncstream<char> has the correct default template argument");
+
+static_assert(Same<basic_osyncstream<wchar_t>,
+		   basic_osyncstream<wchar_t, Traits<wchar_t>, Alloc<wchar_t>>
+		   >::value,
+    "std::basic_osyncstream<wchar_t> has the correct default template argument");
+#endif
+
+using std::istreambuf_iterator;
+
+static_assert(Same<istreambuf_iterator<char>,
+		   istreambuf_iterator<char, Traits<char>>
+		   >::value,
+    "std::istreambuf_iterator<char> has the correct default template argument");
+
+static_assert(Same<istreambuf_iterator<wchar_t>,
+		   istreambuf_iterator<wchar_t, Traits<wchar_t>>
+		   >::value,
+    "std::istreambuf_iterator<wchar_t> has the correct default template argument");
+
+using std::ostreambuf_iterator;
+
+static_assert(Same<ostreambuf_iterator<char>,
+		   ostreambuf_iterator<char, Traits<char>>
+		   >::value,
+    "std::ostreambuf_iterator<char> has the correct default template argument");
+
+static_assert(Same<ostreambuf_iterator<wchar_t>,
+		   ostreambuf_iterator<wchar_t, Traits<wchar_t>>
+		   >::value,
+    "std::ostreambuf_iterator<wchar_t> has the correct default template argument");