`<expected>`: Workaround LLVM-59854 (#3326)

stl/inc/expected

@@ -308,7 +308,11 @@ public:
         : _Unexpected(_Ilist, _STD forward<_Args>(_Vals)...), _Has_value(false) {}
 
     // [expected.object.dtor]
-    constexpr ~expected() noexcept {
+    constexpr ~expected()
+#ifndef __clang__ // TRANSITION, LLVM-59854
+        noexcept
+#endif // __clang__
+    {
         if (_Has_value) {
             if constexpr (!is_trivially_destructible_v<_Ty>) {
                 _Value.~_Ty();
@@ -834,7 +838,11 @@ public:
         : _Unexpected(_Ilist, _STD forward<_Args>(_Vals)...), _Has_value(false) {}
 
     // [expected.void.dtor]
-    constexpr ~expected() noexcept {
+    constexpr ~expected()
+#ifndef __clang__ // TRANSITION, LLVM-59854
+        noexcept
+#endif // __clang__
+    {
         if (!_Has_value) {
             _Unexpected.~_Err();
         }

tests/std/tests/P0323R12_expected/test.cpp

@@ -7,9 +7,9 @@
 #include <concepts>
 #include <exception>
 #include <expected>
-#include <initializer_list>
 #include <type_traits>
 #include <utility>
+#include <vector>
 
 using namespace std;
 
@@ -2044,6 +2044,14 @@ void test_reinit_regression() {
     }
 }
 
+// Defend against regression of llvm-project#59854, in which clang is confused
+// by the explicit `noexcept` on `expected`'s destructors.
+struct Data {
+    vector<int> vec_;
+    constexpr Data(initializer_list<int> il) : vec_(il) {}
+};
+static_assert(((void) expected<void, Data>{unexpect, {1, 2, 3}}, true));
+
 int main() {
     test_unexpected::test_all();
     static_assert(test_unexpected::test_all());