<variant>: Workaround for LLVM-59854 in the case of destructor of variant (#4903)

Co-authored-by: Casey Carter <[email protected]>

Author: frederick-vs-ja

stl/inc/variant

@@ -378,7 +378,11 @@ public:
         _Variant_storage<_Rest...> _Tail;
     };
 
-    _CONSTEXPR20 ~_Variant_storage_() noexcept {
+    _CONSTEXPR20 ~_Variant_storage_()
+#ifndef __clang__ // TRANSITION, LLVM-59854
+        noexcept
+#endif // ^^^ no workaround ^^^
+    {
         // explicitly non-trivial destructor (which would otherwise be defined as deleted
         // since the class has a variant member with a non-trivial destructor)
     }

tests/std/tests/P0088R3_variant_msvc/test.cpp

@@ -741,6 +741,89 @@ namespace msvc {
         }
     } // namespace gh2770
 
+    namespace gh4901 {
+#if _HAS_CXX20
+#define CONSTEXPR20 constexpr
+#else // ^^^ _HAS_CXX20 / !_HAS_CXX20 vvv
+#define CONSTEXPR20 inline
+#endif // ^^^ !_HAS_CXX20  ^^^
+        struct X {
+            CONSTEXPR20 ~X() {}
+        };
+
+        struct Y {
+            X _;
+        };
+
+        struct ZA {
+            std::variant<Y, int> z;
+        };
+
+        struct ZB {
+            std::variant<int, Y> z;
+        };
+
+#if _HAS_CXX20
+        static_assert(ZA{0}.z.index() == 1);
+        static_assert(ZA{Y{}}.z.index() == 0);
+        static_assert(ZB{0}.z.index() == 0);
+        static_assert(ZB{Y{}}.z.index() == 1);
+#endif // _HAS_CXX20
+
+        static_assert(std::is_nothrow_destructible_v<X>);
+        static_assert(std::is_nothrow_destructible_v<Y>);
+        static_assert(std::is_nothrow_destructible_v<std::variant<Y, int>>);
+        static_assert(std::is_nothrow_destructible_v<std::variant<int, Y>>);
+        static_assert(std::is_nothrow_destructible_v<ZA>);
+        static_assert(std::is_nothrow_destructible_v<ZB>);
+
+        // Verify that variant::~variant is noexcept even when an alternative has a potentially-throwing destructor,
+        // per N4988 [res.on.exception.handling]/3.
+        struct X2 {
+            CONSTEXPR20 ~X2() noexcept(false) {}
+        };
+
+        struct Y2 {
+            X2 _;
+        };
+
+        struct ZA2 {
+            std::variant<Y2, int> z;
+        };
+
+        struct ZB2 {
+            std::variant<int, Y2> z;
+        };
+
+#if _HAS_CXX20
+        static_assert(ZA2{0}.z.index() == 1);
+        static_assert(ZA2{Y2{}}.z.index() == 0);
+        static_assert(ZB2{0}.z.index() == 0);
+        static_assert(ZB2{Y2{}}.z.index() == 1);
+#endif // _HAS_CXX20
+
+        static_assert(!std::is_nothrow_destructible_v<X2>);
+        static_assert(!std::is_nothrow_destructible_v<Y2>);
+        static_assert(std::is_nothrow_destructible_v<std::variant<Y2, int>>);
+        static_assert(std::is_nothrow_destructible_v<std::variant<int, Y2>>);
+        static_assert(std::is_nothrow_destructible_v<ZA2>);
+        static_assert(std::is_nothrow_destructible_v<ZB2>);
+
+        struct ZC {
+            std::variant<Y, int, Y2> z;
+        };
+
+#if _HAS_CXX20
+        static_assert(ZC{Y{}}.z.index() == 0);
+        static_assert(ZC{0}.z.index() == 1);
+        static_assert(ZC{Y2{}}.z.index() == 2);
+#endif // _HAS_CXX20
+
+        static_assert(std::is_nothrow_destructible_v<std::variant<Y, int, Y2>>);
+        static_assert(std::is_nothrow_destructible_v<ZC>);
+#undef CONSTEXPR20
+    } // namespace gh4901
+
     namespace assign_cv {
         template <class T>
         struct TypeIdentityImpl {