[libcxx-commits] [PATCH] D120064: Fix EBO on std::optional and std::variant when targeting the MSVC ABI

[Jan Wilken Dörrie via Phabricator via libcxx-commits]

jdoerrie created this revision.
jdoerrie added reviewers: Quuxplusone, ldionne.
jdoerrie requested review of this revision.
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Herald added a subscriber: libcxx-commits.
Herald added a reviewer: libc++.

In the current implementation both `std::optional` and `std::variant` don't perform the EBO on MSVC's ABI. This is because both classes inherit from multiple empty base classes, which breaks the EBO for MSVC. This patch fixes this issue by applying the `empty_bases` declspec attribute, which is already used to fix a similar issue for std::tuple.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D120064

Files:
  libcxx/include/optional
  libcxx/include/variant


Index: libcxx/include/variant
===================================================================
--- libcxx/include/variant
+++ libcxx/include/variant
@@ -1266,7 +1266,7 @@
 } // namespace __variant_detail
 
 template <class... _Types>
-class _LIBCPP_TEMPLATE_VIS variant
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_DECLSPEC_EMPTY_BASES variant
     : private __sfinae_ctor_base<
           __all<is_copy_constructible_v<_Types>...>::value,
           __all<is_move_constructible_v<_Types>...>::value>,
Index: libcxx/include/optional
===================================================================
--- libcxx/include/optional
+++ libcxx/include/optional
@@ -622,7 +622,7 @@
 template <class _Tp> struct __is_std_optional<optional<_Tp>> : true_type {};
 
 template <class _Tp>
-class optional
+class _LIBCPP_DECLSPEC_EMPTY_BASES optional
     : private __optional_move_assign_base<_Tp>
     , private __optional_sfinae_ctor_base_t<_Tp>
     , private __optional_sfinae_assign_base_t<_Tp>


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