[libcxx] Poison memory in variant destroy

libcxx/include/variant

@@ -261,6 +261,10 @@ namespace std {
 #include <new>
 #include <version>
 
+#if __has_feature(memory_sanitizer)
+#  include <sanitizer/msan_interface.h>
+#endif
+
 // standard-mandated includes
 
 // [variant.syn]
@@ -781,6 +785,10 @@ _LIBCPP_VARIANT_DESTRUCTOR(
     _Trait::_TriviallyAvailable,
     _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 ~__dtor() = default,
     _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __destroy() noexcept {
+#  if __has_feature(memory_sanitizer)
+      if (!std::is_constant_evaluated())
+        __sanitizer_dtor_callback(&this->__data, sizeof(this->__data));
+#  endif
       this->__index = __variant_npos<__index_t>;
     } _LIBCPP_EAT_SEMICOLON);
 

libcxx/test/std/utilities/variant/variant.variant/variant_msan.pass.cpp

@@ -0,0 +1,34 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// <variant>
+
+// template <class ...Types> class variant;
+
+#include <variant>
+
+#if __has_feature(memory_sanitizer)
+#  include <sanitizer/msan_interface.h>
+#endif
+
+int main(int, char**) {
+#if __has_feature(memory_sanitizer)
+  std::variant<double, int> v;
+  v.emplace<double>();
+  double& d = std::get<double>(v);
+  v.emplace<int>();
+  if (__msan_test_shadow(&d, sizeof(d)) == -1) {
+    // Unexpected: The entire range is accessible.
+    return 1;
+  }
+#endif
+
+  return 0;
+}