[libc++] Do not forward-declare syncstream outside experimental (#82511)

We only define the classes in `<syncstream>` when experimental library
features are enabled, but we would forward-declare them in `<iosfwd>`
even when they are disabled. This led to confusing error messages about
being unable to instantiate an undefined template.

Author: ldionne

libcxx/include/iosfwd

@@ -142,7 +142,7 @@ typedef fpos<mbstate_t> u8streampos;
 typedef fpos<mbstate_t> u16streampos;
 typedef fpos<mbstate_t> u32streampos;
 
-#if _LIBCPP_STD_VER >= 20
+#if _LIBCPP_STD_VER >= 20 && !defined(_LIBCPP_HAS_NO_EXPERIMENTAL_SYNCSTREAM)
 
 template <class _CharT, class _Traits = char_traits<_CharT>, class _Allocator = allocator<_CharT>>
 class basic_syncbuf;
@@ -160,7 +160,7 @@ using osyncstream = basic_osyncstream<char>;
 using wosyncstream = basic_osyncstream<wchar_t>;
 #  endif
 
-#endif // _LIBCPP_STD_VER >=20
+#endif // _LIBCPP_STD_VER >= 20 && !defined(_LIBCPP_HAS_NO_EXPERIMENTAL_SYNCSTREAM)
 
 // Include other forward declarations here
 template <class _Tp, class _Alloc = allocator<_Tp> >

libcxx/modules/std/iosfwd.inc

@@ -18,19 +18,23 @@ export namespace std {
   using std::u8streampos;
 #endif
 
+#ifndef _LIBCPP_HAS_NO_EXPERIMENTAL_SYNCSTREAM
   using std::basic_osyncstream;
   using std::basic_syncbuf;
+#endif
 
   using std::istreambuf_iterator;
   using std::ostreambuf_iterator;
 
+#ifndef _LIBCPP_HAS_NO_EXPERIMENTAL_SYNCSTREAM
   using std::osyncstream;
   using std::syncbuf;
 
-#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
+#  ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
   using std::wosyncstream;
   using std::wsyncbuf;
-#endif
+#  endif
+#endif // _LIBCPP_HAS_NO_EXPERIMENTAL_SYNCSTREAM
 
   using std::fpos;
 } // namespace std