[libcxx-commits] [libcxx] [libc++] Introduce a new attribute keyword for Clang improves compatibility with Mingw-GCC (PR #141040)

[via libcxx-commits]

================
@@ -530,6 +530,48 @@ typedef __char32_t char32_t;
 #    define _LIBCPP_HIDE_FROM_ABI_AFTER_V1 _LIBCPP_HIDE_FROM_ABI
 #  endif
 
+// _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).
+//
+// 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
+// compiled with --with-default-libstdcxx-abi=gcc4-compatible are incompatible with Clang, resulting in linking
+// errors or runtime crushes.
+//
+// A few such member functions already exist (here are ostream::sentry::sentry, ostream::~sentry and
+// istream::sentry::sentry) were not previously marked with _LIBCPP_HIDE_FROM_ABI but should be to avoid symbol
+// visibility issues. However, adding the macro unconditionally would break the ABI on other platforms.
+//
+// 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.
----------------
jeremyd2019 wrote:

only if there are explicit `extern` instantiations of that outer class template.

https://github.com/llvm/llvm-project/pull/141040