[libc++] Use _LIBCPP_TYPE_VIS_ONLY with enum class

[smeenai]

Summary

An enum class has associated type info. In the Microsoft ABI, type info
is emitted in the COMDAT section and isn't exported, so clang rightfully
complains about __declspec(dllexport) being unused for an enum class.
On other platforms, we still want to export the type info.

[compnerd]

include/__config

719 ↗

(On Diff #69786)

I don't think that this is right. On non-Windows, this would potentially expand out to __attribute__ (( __type_visibility__ ("default") )). I don't believe that enum classes have anything like RTTI associated with them, so we don't really want type visibility for that.

[smeenai]

include/__config

719 ↗

(On Diff #69786)

Good point. Should I just drop the visibility macro entirely then?

[compnerd]

include/__config

719 ↗

(On Diff #69786)

I think that might be the correct approach here. The definition will be inlined in the header, so the values should always be available, so the visibility attributes shouldn't change anything anyways.

[smeenai]

Removing export entirely, per compnerd's suggestion

[compnerd]

Ugh, I was wrong. There is type information associated with a scoped enum. This means that _LIBCPP_TYPE_VIS is the right annotation here.

[smeenai]

Switching back to _LIBCPP_TYPE_VIS_ONLY after discussion with @compnerd

[smeenai]

Friendly ping :)

libcxx/trunk/include/__config

 #define _LIBCPP_DECLARE_STRONG_ENUM(x) struct _LIBCPP_TYPE_VIS x { enum __lx
 #define _LIBCPP_DECLARE_STRONG_ENUM_EPILOG(x) \
     __lx __v_; \
     _LIBCPP_ALWAYS_INLINE x(__lx __v) : __v_(__v) {} \
     _LIBCPP_ALWAYS_INLINE explicit x(int __v) : __v_(static_cast<__lx>(__v)) {} \
     _LIBCPP_ALWAYS_INLINE operator int() const {return __v_;} \
     };
 #else  // _LIBCPP_HAS_NO_STRONG_ENUMS
-#define _LIBCPP_DECLARE_STRONG_ENUM(x) enum class _LIBCPP_TYPE_VIS x
+#define _LIBCPP_DECLARE_STRONG_ENUM(x) enum class _LIBCPP_TYPE_VIS_ONLY x
 #define _LIBCPP_DECLARE_STRONG_ENUM_EPILOG(x)
 #endif  // _LIBCPP_HAS_NO_STRONG_ENUMS
 
 #ifdef _LIBCPP_DEBUG
 #   if _LIBCPP_DEBUG == 0
 #       define _LIBCPP_DEBUG_LEVEL 1
 #   elif _LIBCPP_DEBUG == 1
 #       define _LIBCPP_DEBUG_LEVEL 2