[libc++] Introduce a new attribute keyword for Clang improves compatibility with Mingw-GCC

In MinGW environment, Clang handles dllexport attribute of internal
class that defined in class template in different way from GCC.
This incompatibility should be fixed but breaks ABI of libc++, so
introduce a new keyword to keep ABI in MinGW environment with
old and patched Clang and to stay ABI compatible on other platforms.

This attribute is attached only for basic_ostream::sentry::sentry,
basic_ostream::sentry::~sentry and basic_istream::sentry::sentry.
Other entities won't be affected by patching Clang so doesn't need
to be annotate.

At a time to introduce a new class as a non-template inner type of
a class template, all non-inline member functions of that class
also needs to be attached _LIBCPP_HIDE_FROM_ABI_MINGW_OR_AFTER_V1
or _LIBCPP_HIDE_FROM_ABI, not _LIBCPP_HIDE_FROM_ABI_AFTER_V1.
Otherwise, that member functions contained in DLL will be
inaccessible on MinGW environment.

For time to increase ABI version, _LIBCPP_HIDE_FROM_ABI_MINGW_OR_AFTER_V1
can be simply replaced to _LIBCPP_HIDE_FROM_ABI_AFTER_V1.

Author: kikairoya

libcxx/include/__config

@@ -530,6 +530,26 @@ typedef __char32_t char32_t;
 #    define _LIBCPP_HIDE_FROM_ABI_AFTER_V1 _LIBCPP_HIDE_FROM_ABI
 #  endif
 
+//// ** in-progress: this description block should be here or merged block above? **
+// _LIBCPP_HIDE_FROM_ABI(_AFTER_V1) is mandatory for member functions in a inner class in a class template
+// (e.g. std::basic_ostream<...>::sentry::sentry(...) ) due to strange behavior of MinGW-GCC about
+// instantiation combined with dllimport/dllexport described below. Former clang didn't do this buggy behavior
+// but will be aligned to GCC for platform compatibility (in particluar, required by some of libstdc++ package).
+// Thus, such member functions exist from a past (they are, ostream::sentry::sentry, ostream::ostream::~sentry
+// and istream::sentry::sentry only) need to be HIDE_FROM_ABI but simply attaching simply ABI on other platforms.
+// So it's needed a dedicated keyword named _LIBCPP_HIDE_FROM_ABI_MINGW_OR_AFTER_V1 to affects only for MinGW
+// target (and, Cygwin too). For other platforms, that is expanded to _LIBCPP_HIDE_FROM_ABI_AFTER_V1 because at
+// a time to fix V2 ABI comes, _LIBCPP_HIDE_FROM_ABI_MINGW_OR_AFTER_V1 can be simply replaced into
+// _LIBCPP_HIDE_FROM_ABI_AFTER_V1 and be completely removed.
+// If time to a new class in a class template comes, all non-inline member functions of that new inner class must
+// be declared with _LIBCPP_HIDE_FROM_ABI_AFTER_V1 otherwise they build to DLL will be inaccessible by MinGW-GCC.
+//// ** in-progress: describe peculiar behavior of MinGW-GCC **
+#  if defined(__MINGW32__) || defined(__CYGWIN__)
+#    define _LIBCPP_HIDE_FROM_ABI_MINGW_OR_AFTER_V1 inline _LIBCPP_HIDE_FROM_ABI
+#  else
+#    define _LIBCPP_HIDE_FROM_ABI_MINGW_OR_AFTER_V1 _LIBCPP_HIDE_FROM_ABI_AFTER_V1
+#  endif
+
 // TODO: Remove this workaround once we drop support for Clang 16
 #  if __has_warning("-Wc++23-extensions")
 #    define _LIBCPP_CLANG_DIAGNOSTIC_IGNORED_CXX23_EXTENSION _LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wc++23-extensions")

libcxx/include/__ostream/basic_ostream.h

@@ -186,8 +186,8 @@ class basic_ostream<_CharT, _Traits>::sentry {
   basic_ostream<_CharT, _Traits>& __os_;
 
 public:
-  explicit sentry(basic_ostream<_CharT, _Traits>& __os);
-  ~sentry();
+  explicit _LIBCPP_HIDE_FROM_ABI_MINGW_OR_AFTER_V1 sentry(basic_ostream<_CharT, _Traits>& __os);
+  _LIBCPP_HIDE_FROM_ABI_MINGW_OR_AFTER_V1 ~sentry();
   sentry(const sentry&)            = delete;
   sentry& operator=(const sentry&) = delete;
 

libcxx/include/istream

@@ -309,7 +309,7 @@ class basic_istream<_CharT, _Traits>::sentry {
   bool __ok_;
 
 public:
-  explicit sentry(basic_istream<_CharT, _Traits>& __is, bool __noskipws = false);
+  explicit _LIBCPP_HIDE_FROM_ABI_MINGW_OR_AFTER_V1 sentry(basic_istream<_CharT, _Traits>& __is, bool __noskipws = false);
   //    ~sentry() = default;
 
   _LIBCPP_HIDE_FROM_ABI explicit operator bool() const { return __ok_; }