[libc++] Set _LIBCPP_DLL_VIS on _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS in MinGW mode

[mstorsjo]

Summary

Contrary to MSVC, MinGW compilers wants the dllexport attribute on the declaration of an explicit template instantiation, not on the definition.

GCC has always handled these attributes this way, and Clang is being made to match in D61118.

This is one of the parts necessary for building libc++ as a DLL on MinGW without forcibly exporting all symbols using --export-all-symbols. (In addition to this, another few tweaks to clang are needed.)

[smeenai]

LGTM, though you'll wanna wait for a libc++ code owner too.

[ldionne]

Contrary to MSVC, MinGW compilers wants the dllexport attribute on the declaration of an explicit template instantiation, not on the definition.

That sounds weird, I would expect an attribute on the explicit instantiation definition to work, since you don't technically need an explicit instantiation declaration to be visible in order to provide an explicit instantiation definition.

[mstorsjo]

In D61123#1479053, @ldionne wrote:

Contrary to MSVC, MinGW compilers wants the dllexport attribute on the declaration of an explicit template instantiation, not on the definition.

That sounds weird, I would expect an attribute on the explicit instantiation definition to work, since you don't technically need an explicit instantiation declaration to be visible in order to provide an explicit instantiation definition.

You're right; if there's no instantiation declaration, then GCC doesn't warn and uses the attribute from the definition. I'ts only ignored (and warned about) by GCC if there's an earlier declaration.

I'll amend the corresponding clang patch to match this aspect, and land this one.

include/__config

 #if defined(_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS)
 #  define _LIBCPP_DLL_VIS
 #  define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS
 #  define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS
 #  define _LIBCPP_OVERRIDABLE_FUNC_VIS
 #  define _LIBCPP_EXPORTED_FROM_ABI
 #elif defined(_LIBCPP_BUILDING_LIBRARY)
 #  define _LIBCPP_DLL_VIS __declspec(dllexport)
-#  define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS
-#  define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS _LIBCPP_DLL_VIS
+#  if defined(__MINGW32__)
+#    define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS _LIBCPP_DLL_VIS
+#    define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS
+#  else
+#    define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS
+#    define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS _LIBCPP_DLL_VIS
+#  endif
 #  define _LIBCPP_OVERRIDABLE_FUNC_VIS _LIBCPP_DLL_VIS
 #  define _LIBCPP_EXPORTED_FROM_ABI __declspec(dllexport)
 #else
 #  define _LIBCPP_DLL_VIS __declspec(dllimport)
 #  define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS _LIBCPP_DLL_VIS
 #  define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS
 #  define _LIBCPP_OVERRIDABLE_FUNC_VIS
 #  define _LIBCPP_EXPORTED_FROM_ABI __declspec(dllimport)