update

mention static data member
clarify that relates explicit instantiation declaration
wrong use of dllimport in example description

Author: kikairoya

libcxx/include/__config

@@ -532,8 +532,8 @@ typedef __char32_t char32_t;
 
 // _LIBCPP_HIDE_FROM_ABI is required for member functions defined within an inner class of a class template
 // (e.g., std::basic_ostream<...>::sentry::sentry(...)), due to inconsistent behavior in MinGW-GCC (and
-// Cygwin as well, in all relevant cases) regarding template instantiation and symbol visibility when combined
-// with __declspec(dllexport/dllimport).
+// Cygwin as well, in all relevant cases) regarding explicit instantiation declaration and symbol visibility
+// when combined with __declspec(dllexport).
 //
 // Previous versions of Clang did not exhibit this issue, but upcoming versions are expected to align with
 // GCC's behavior for compatibility. This is particularly important because some of libstdc++ packages
@@ -547,16 +547,18 @@ typedef __char32_t char32_t;
 // Therefore, a dedicated macro _LIBCPP_HIDE_FROM_ABI_MINGW_OR_AFTER_V1 is introduced. This macro expands to
 // _LIBCPP_HIDE_FROM_ABI only when targeting MinGW, and to _LIBCPP_HIDE_FROM_ABI_AFTER_V1 on all other platforms.
 //
-// Going forward, whenever a new (static or non-static) member function is added to an inner class within a
-// class template, it must be annotated with _LIBCPP_HIDE_FROM_ABI to ensure proper symbol visibility when
-// targeting MinGW. Otherwise, the resulting DLL will be unusable due to missing symbols.
+// Going forward, whenever a new (static or non-static) member function or static data member is added to an
+// inner class within a class template that has explicit instantiation declaration, it must be annotated with
+// _LIBCPP_HIDE_FROM_ABI to ensure proper symbol visibility when targeting MinGW. Otherwise, the resulting DLL
+// will be unusable due to missing symbols.
 //
-// The underlying issue arises from how MinGW-GCC handles template instantiation:
+// The underlying issue arises from how MinGW-GCC handles explicit instantiation declaration of a class template:
 //
 //   - When exporting: 'extern template __declspec(dllexport) class TheTemplateClass<T>;'
 //     allows exporting the outer template instantiation, but not its nested types (e.g., InnerClass).
+//     note: this is just a declaration, needs a definition as `template class TheTemplateClass<T>;' at somewere.
 //
-//   - When importing: 'extern template __declspec(dllimport) class TheTemplateClass<T>;'
+//   - When importing: 'extern template class TheTemplateClass<T>;'
 //     causes MinGW-GCC to also try importing nested types such as TheTemplateClass<T>::InnerClass,
 //     even if they were never exported. This leads to linker errors like:
 //     'undefined reference to TheTemplateClass<T>::InnerClass::...'