Index: libcxx/docs/DesignDocs/ABIVersioning.rst
===================================================================
--- libcxx/docs/DesignDocs/ABIVersioning.rst
+++ libcxx/docs/DesignDocs/ABIVersioning.rst
@@ -22,3 +22,11 @@
 ``_LIBCPP_ABI_VERSION``, which is controlled by the ``LIBCXX_ABI_VERSION`` set
 at build time. Libc++ does not intend users to interact with these C++ macros
 directly.
+
+-----------------
+MSVC environments
+-----------------
+
+The exception to this is MSVC environments. Libc++ does not currently have users
+that require a stable ABI in MSVC environments, so MSVC-only changes may be
+applied unconditionally.
Index: libcxx/docs/index.rst
===================================================================
--- libcxx/docs/index.rst
+++ libcxx/docs/index.rst
@@ -120,7 +120,7 @@
 macOS 10.9+     i386, x86_64, arm64       Building the shared library itself requires targetting macOS 10.13+
 FreeBSD 12+     i386, x86_64, arm
 Linux           i386, x86_64, arm, arm64  Only glibc-2.24 and later and no other libc is officially supported
-Windows         i386, x86_64              Both MSVC and MinGW style environments
+Windows         i386, x86_64              Both MSVC and MinGW style environments, ABI in MSVC environments is :doc:`unstable <DesignDocs/ABIVersioning>`
 AIX 7.2TL5+     powerpc, powerpc64
 =============== ========================= ============================
 
Index: libcxx/include/optional
===================================================================
--- libcxx/include/optional
+++ libcxx/include/optional
@@ -638,7 +638,7 @@
 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>
Index: libcxx/include/variant
===================================================================
--- libcxx/include/variant
+++ libcxx/include/variant
@@ -1294,7 +1294,7 @@
 } // 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/test/libcxx/utilities/optional/optional.object/optional_size.pass.cpp
===================================================================
--- /dev/null
+++ libcxx/test/libcxx/utilities/optional/optional.object/optional_size.pass.cpp
@@ -0,0 +1,31 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14
+
+// <optional>
+
+// template <class T> class optional;
+
+#include <optional>
+
+template <class T>
+struct type_with_bool {
+  T value;
+  bool has_value;
+};
+
+int main(int, char**) {
+  // Test that std::optional achieves the expected size. See https://llvm.org/PR61095.
+  static_assert(sizeof(std::optional<char>) == sizeof(type_with_bool<char>));
+  static_assert(sizeof(std::optional<int>) == sizeof(type_with_bool<int>));
+  static_assert(sizeof(std::optional<long>) == sizeof(type_with_bool<long>));
+  static_assert(sizeof(std::optional<std::size_t>) == sizeof(type_with_bool<std::size_t>));
+
+  return 0;
+}
Index: libcxx/test/libcxx/utilities/variant/variant.variant/variant_size.pass.cpp
===================================================================
--- libcxx/test/libcxx/utilities/variant/variant.variant/variant_size.pass.cpp
+++ libcxx/test/libcxx/utilities/variant/variant.variant/variant_size.pass.cpp
@@ -60,6 +60,16 @@
   static_assert(std::__variant_npos<IndexT> == IndexLim::max(), "");
 }
 
+template <class LargestType>
+struct type_with_index {
+  LargestType largest;
+#ifdef _LIBCPP_ABI_VARIANT_INDEX_TYPE_OPTIMIZATION
+  unsigned char index;
+#else
+  unsigned int index;
+#endif
+};
+
 int main(int, char**) {
   test_index_type<unsigned char>();
   // This won't compile due to template depth issues.
@@ -67,5 +77,12 @@
   test_index_internals<unsigned char>();
   test_index_internals<unsigned short>();
 
+  // Test that std::variant achieves the expected size. See https://llvm.org/PR61095.
+  static_assert(sizeof(std::variant<char, char, char>) == sizeof(type_with_index<char>));
+  static_assert(sizeof(std::variant<int, int, int>) == sizeof(type_with_index<int>));
+  static_assert(sizeof(std::variant<long, long, long>) == sizeof(type_with_index<long>));
+  static_assert(sizeof(std::variant<char, int, long>) == sizeof(type_with_index<long>));
+  static_assert(sizeof(std::variant<std::size_t, std::size_t, std::size_t>) == sizeof(type_with_index<std::size_t>));
+
   return 0;
 }