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

This is affected to only Clang for MinGW target.

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 ensure nothing affects to other
environment/platforms.

The new attribute keyword _LIBCPP_INNER_CLASS_IN_TEMPLATE_VIS does
nothing in almost all situations except if included from client
(not in building libc++ itself) by clang (not by GCC or others).

If clang does include libc++, the keyword will be expanded to
__attribute__((__exclude_from_explicit_instantiation__)), results
attached class will be away from explicit instantiation declaration
so will be instanciated implicitly as formar Clang does. Thus,
it will no-op for old Clang that has incompatibility with MinGW-GCC
or emulate old behavior for patched Clang.

This attribute is attached only for std::basic_ostream::sentry and
std::basic_istream::sentry. Other entities won't be affected by
patching Clang so doesn't need to be annotate. Notably, at a time
to introduce a new class as a non-template inner type of a class
template, that class also needs to be attached
_LIBCPP_INNER_CLASS_ININ_TEMPLATE_VIS.

Author: kikairoya

libcxx/docs/DesignDocs/VisibilityMacros.rst

@@ -93,6 +93,13 @@ Visibility Macros
   the extern template declaration) as exported on Windows, as discussed above.
   On all other platforms, this macro has an empty definition.
 
+**_LIBCPP_INNER_CLASS_IN_TEMPLATE_VIS**
+  To let dll-importing and dll-exporting consitent on windows-gnu environment,
+  this annotation must be attached to a non-template class that isn't marked
+  as `dllexport` defined in a class template. Currentry here are ostream::sentry
+  and istream::sentry only but if a time to such a new class be introduced,
+  the class has to be attached this annotation.
+
 Links
 =====
 

libcxx/include/__config

@@ -365,6 +365,11 @@ typedef __char32_t char32_t;
 #      define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS
 #      define _LIBCPP_OVERRIDABLE_FUNC_VIS
 #      define _LIBCPP_EXPORTED_FROM_ABI
+#      if defined(_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS) || !__has_attribute(exclude_from_explicit_instantiation)
+#        define _LIBCPP_INNER_CLASS_IN_TEMPLATE_VIS
+#      else
+#        define _LIBCPP_INNER_CLASS_IN_TEMPLATE_VIS __attribute__((__exclude_from_explicit_instantiation__))
+#      endif
 #    elif defined(_LIBCPP_BUILDING_LIBRARY)
 #      if defined(__MINGW32__)
 #        define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS __declspec(dllexport)
@@ -375,11 +380,13 @@ typedef __char32_t char32_t;
 #      endif
 #      define _LIBCPP_OVERRIDABLE_FUNC_VIS __declspec(dllexport)
 #      define _LIBCPP_EXPORTED_FROM_ABI __declspec(dllexport)
+#      define _LIBCPP_INNER_CLASS_IN_TEMPLATE_VIS
 #    else
 #      define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS __declspec(dllimport)
 #      define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS
 #      define _LIBCPP_OVERRIDABLE_FUNC_VIS
 #      define _LIBCPP_EXPORTED_FROM_ABI __declspec(dllimport)
+#      define _LIBCPP_INNER_CLASS_IN_TEMPLATE_VIS
 #    endif
 
 #    define _LIBCPP_HIDDEN
@@ -399,6 +406,7 @@ typedef __char32_t char32_t;
 #    define _LIBCPP_EXPORTED_FROM_ABI _LIBCPP_VISIBILITY("default")
 #    define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS _LIBCPP_VISIBILITY("default")
 #    define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS
+#    define _LIBCPP_INNER_CLASS_IN_TEMPLATE_VIS
 
 // TODO: Make this a proper customization point or remove the option to override it.
 #    ifndef _LIBCPP_OVERRIDABLE_FUNC_VIS

libcxx/include/__ostream/basic_ostream.h

@@ -71,7 +71,7 @@ class basic_ostream : virtual public basic_ios<_CharT, _Traits> {
 
 public:
   // 27.7.2.4 Prefix/suffix:
-  class sentry;
+  class _LIBCPP_INNER_CLASS_IN_TEMPLATE_VIS sentry;
 
   // 27.7.2.6 Formatted output:
   inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1 basic_ostream& operator<<(basic_ostream& (*__pf)(basic_ostream&)) {

libcxx/include/istream

@@ -228,7 +228,7 @@ public:
   basic_istream& operator=(const basic_istream& __rhs) = delete;
 
   // 27.7.1.1.3 Prefix/suffix:
-  class sentry;
+  class _LIBCPP_INNER_CLASS_IN_TEMPLATE_VIS sentry;
 
   // 27.7.1.2 Formatted input:
   inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1 basic_istream& operator>>(basic_istream& (*__pf)(basic_istream&)) {