From 6944e5f0c964cae15b2f3741c39c64d2f651539b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller@gmail.com>
Date: Tue, 10 Oct 2023 20:24:06 +0200
Subject: [PATCH 01/28] [libc++] Fix UB in <expected> related to "has value"
 flag (#68552)

The calls to `std::construct_at` might overwrite the previously set
`__has_value_` flag, so reverse the order everywhere. Where possible,
avoid calling `std::construct_at` and construct the value/error
directly into the union.
---
 libcxx/include/__expected/expected.h          | 168 ++++++++----------
 .../observers/has_value.pass.cpp              |  39 ++++
 2 files changed, 117 insertions(+), 90 deletions(-)

diff --git a/libcxx/include/__expected/expected.h b/libcxx/include/__expected/expected.h
index 045370a486fae..08f35b1111f6b 100644
--- a/libcxx/include/__expected/expected.h
+++ b/libcxx/include/__expected/expected.h
@@ -119,9 +119,7 @@ class expected {
   _LIBCPP_HIDE_FROM_ABI constexpr expected()
     noexcept(is_nothrow_default_constructible_v<_Tp>) // strengthened
     requires is_default_constructible_v<_Tp>
-      : __has_val_(true) {
-    std::construct_at(std::addressof(__union_.__val_));
-  }
+      : __union_(__construct_in_place_tag{}), __has_val_(true) {}
 
   _LIBCPP_HIDE_FROM_ABI constexpr expected(const expected&) = delete;
 
@@ -136,14 +134,7 @@ class expected {
     noexcept(is_nothrow_copy_constructible_v<_Tp> && is_nothrow_copy_constructible_v<_Err>) // strengthened
     requires(is_copy_constructible_v<_Tp> && is_copy_constructible_v<_Err> &&
              !(is_trivially_copy_constructible_v<_Tp> && is_trivially_copy_constructible_v<_Err>))
-      : __has_val_(__other.__has_val_) {
-    if (__has_val_) {
-      std::construct_at(std::addressof(__union_.__val_), __other.__union_.__val_);
-    } else {
-      std::construct_at(std::addressof(__union_.__unex_), __other.__union_.__unex_);
-    }
-  }
-
+      : __union_(__union_from_expected(__other)), __has_val_(__other.__has_val_) { }
 
   _LIBCPP_HIDE_FROM_ABI constexpr expected(expected&&)
     requires(is_move_constructible_v<_Tp> && is_move_constructible_v<_Err>
@@ -154,13 +145,7 @@ class expected {
     noexcept(is_nothrow_move_constructible_v<_Tp> && is_nothrow_move_constructible_v<_Err>)
     requires(is_move_constructible_v<_Tp> && is_move_constructible_v<_Err> &&
              !(is_trivially_move_constructible_v<_Tp> && is_trivially_move_constructible_v<_Err>))
-      : __has_val_(__other.__has_val_) {
-    if (__has_val_) {
-      std::construct_at(std::addressof(__union_.__val_), std::move(__other.__union_.__val_));
-    } else {
-      std::construct_at(std::addressof(__union_.__unex_), std::move(__other.__union_.__unex_));
-    }
-  }
+      : __union_(__union_from_expected(std::move(__other))), __has_val_(__other.__has_val_) { }
 
 private:
   template <class _Up, class _OtherErr, class _UfQual, class _OtherErrQual>
@@ -200,26 +185,14 @@ class expected {
   expected(const expected<_Up, _OtherErr>& __other)
     noexcept(is_nothrow_constructible_v<_Tp, const _Up&> &&
              is_nothrow_constructible_v<_Err, const _OtherErr&>) // strengthened
-      : __has_val_(__other.__has_val_) {
-    if (__has_val_) {
-      std::construct_at(std::addressof(__union_.__val_), __other.__union_.__val_);
-    } else {
-      std::construct_at(std::addressof(__union_.__unex_), __other.__union_.__unex_);
-    }
-  }
+      : __union_(__union_from_expected(__other)), __has_val_(__other.__has_val_) {}
 
   template <class _Up, class _OtherErr>
     requires __can_convert<_Up, _OtherErr, _Up, _OtherErr>::value
   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_Up, _Tp> || !is_convertible_v<_OtherErr, _Err>)
   expected(expected<_Up, _OtherErr>&& __other)
     noexcept(is_nothrow_constructible_v<_Tp, _Up> && is_nothrow_constructible_v<_Err, _OtherErr>) // strengthened
-      : __has_val_(__other.__has_val_) {
-    if (__has_val_) {
-      std::construct_at(std::addressof(__union_.__val_), std::move(__other.__union_.__val_));
-    } else {
-      std::construct_at(std::addressof(__union_.__unex_), std::move(__other.__union_.__unex_));
-    }
-  }
+      : __union_(__union_from_expected(std::move(__other))), __has_val_(__other.__has_val_) {}
 
   template <class _Up = _Tp>
     requires(!is_same_v<remove_cvref_t<_Up>, in_place_t> && !is_same_v<expected, remove_cvref_t<_Up>> &&
@@ -227,61 +200,47 @@ class expected {
              (!is_same_v<remove_cv_t<_Tp>, bool> || !__is_std_expected<remove_cvref_t<_Up>>::value))
   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_Up, _Tp>)
       expected(_Up&& __u) noexcept(is_nothrow_constructible_v<_Tp, _Up>) // strengthened
-      : __has_val_(true) {
-    std::construct_at(std::addressof(__union_.__val_), std::forward<_Up>(__u));
-  }
+      : __union_(__construct_in_place_tag{}, std::forward<_Up>(__u)), __has_val_(true) {}
 
   template <class _OtherErr>
     requires is_constructible_v<_Err, const _OtherErr&>
   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<const _OtherErr&, _Err>)
   expected(const unexpected<_OtherErr>& __unex)
     noexcept(is_nothrow_constructible_v<_Err, const _OtherErr&>) // strengthened
-      : __has_val_(false) {
-    std::construct_at(std::addressof(__union_.__unex_), __unex.error());
-  }
+      : __union_(__construct_unexpected_tag{}, __unex.error()), __has_val_(false) {}
 
   template <class _OtherErr>
     requires is_constructible_v<_Err, _OtherErr>
   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_OtherErr, _Err>)
   expected(unexpected<_OtherErr>&& __unex)
     noexcept(is_nothrow_constructible_v<_Err, _OtherErr>) // strengthened
-      : __has_val_(false) {
-    std::construct_at(std::addressof(__union_.__unex_), std::move(__unex.error()));
-  }
+      : __union_(__construct_unexpected_tag{}, std::move(__unex.error())), __has_val_(false) {}
 
   template <class... _Args>
     requires is_constructible_v<_Tp, _Args...>
   _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(in_place_t, _Args&&... __args)
     noexcept(is_nothrow_constructible_v<_Tp, _Args...>) // strengthened
-      : __has_val_(true) {
-    std::construct_at(std::addressof(__union_.__val_), std::forward<_Args>(__args)...);
-  }
+      : __union_(__construct_in_place_tag{}, std::forward<_Args>(__args)...), __has_val_(true) {}
 
   template <class _Up, class... _Args>
     requires is_constructible_v< _Tp, initializer_list<_Up>&, _Args... >
   _LIBCPP_HIDE_FROM_ABI constexpr explicit
   expected(in_place_t, initializer_list<_Up> __il, _Args&&... __args)
     noexcept(is_nothrow_constructible_v<_Tp, initializer_list<_Up>&, _Args...>) // strengthened
-      : __has_val_(true) {
-    std::construct_at(std::addressof(__union_.__val_), __il, std::forward<_Args>(__args)...);
-  }
+      : __union_(__construct_in_place_tag{}, __il, std::forward<_Args>(__args)...), __has_val_(true) {}
 
   template <class... _Args>
     requires is_constructible_v<_Err, _Args...>
   _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(unexpect_t, _Args&&... __args)
-    noexcept(is_nothrow_constructible_v<_Err, _Args...>)  // strengthened
-      : __has_val_(false) {
-    std::construct_at(std::addressof(__union_.__unex_), std::forward<_Args>(__args)...);
-  }
+    noexcept(is_nothrow_constructible_v<_Err, _Args...>) // strengthened
+      : __union_(__construct_unexpected_tag{}, std::forward<_Args>(__args)...), __has_val_(false) {}
 
   template <class _Up, class... _Args>
     requires is_constructible_v< _Err, initializer_list<_Up>&, _Args... >
   _LIBCPP_HIDE_FROM_ABI constexpr explicit
   expected(unexpect_t, initializer_list<_Up> __il, _Args&&... __args)
     noexcept(is_nothrow_constructible_v<_Err, initializer_list<_Up>&, _Args...>) // strengthened
-      : __has_val_(false) {
-    std::construct_at(std::addressof(__union_.__unex_), __il, std::forward<_Args>(__args)...);
-  }
+      : __union_(__construct_unexpected_tag{}, __il, std::forward<_Args>(__args)...), __has_val_(false) {}
 
   // [expected.object.dtor], destructor
 
@@ -440,9 +399,10 @@ class expected {
       std::destroy_at(std::addressof(__union_.__val_));
     } else {
       std::destroy_at(std::addressof(__union_.__unex_));
-      __has_val_ = true;
     }
-    return *std::construct_at(std::addressof(__union_.__val_), std::forward<_Args>(__args)...);
+    std::construct_at(std::addressof(__union_.__val_), std::forward<_Args>(__args)...);
+    __has_val_ = true;
+    return *std::addressof(__union_.__val_);
   }
 
   template <class _Up, class... _Args>
@@ -452,9 +412,10 @@ class expected {
       std::destroy_at(std::addressof(__union_.__val_));
     } else {
       std::destroy_at(std::addressof(__union_.__unex_));
-      __has_val_ = true;
     }
-    return *std::construct_at(std::addressof(__union_.__val_), __il, std::forward<_Args>(__args)...);
+    std::construct_at(std::addressof(__union_.__val_), __il, std::forward<_Args>(__args)...);
+    __has_val_ = true;
+    return *std::addressof(__union_.__val_);
   }
 
 
@@ -894,11 +855,21 @@ class expected {
 
 private:
   struct __empty_t {};
+  struct __construct_in_place_tag {};
+  struct __construct_unexpected_tag {};
 
   template <class _ValueType, class _ErrorType>
   union __union_t {
     _LIBCPP_HIDE_FROM_ABI constexpr __union_t() {}
 
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(__construct_in_place_tag, _Args&&... __args)
+        : __val_(std::forward<_Args>(__args)...) {}
+
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(__construct_unexpected_tag, _Args&&... __args)
+        : __unex_(std::forward<_Args>(__args)...) {}
+
     template <class _Func, class... _Args>
     _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(
         std::__expected_construct_in_place_from_invoke_tag, _Func&& __f, _Args&&... __args)
@@ -931,6 +902,14 @@ class expected {
     _LIBCPP_HIDE_FROM_ABI constexpr __union_t(const __union_t&) = default;
     _LIBCPP_HIDE_FROM_ABI constexpr __union_t& operator=(const __union_t&) = default;
 
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(__construct_in_place_tag, _Args&&... __args)
+        : __val_(std::forward<_Args>(__args)...) {}
+
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(__construct_unexpected_tag, _Args&&... __args)
+        : __unex_(std::forward<_Args>(__args)...) {}
+
     template <class _Func, class... _Args>
     _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(
         std::__expected_construct_in_place_from_invoke_tag, _Func&& __f, _Args&&... __args)
@@ -955,6 +934,18 @@ class expected {
     _LIBCPP_NO_UNIQUE_ADDRESS _ErrorType __unex_;
   };
 
+  template <class _Up, class _OtherErr>
+  _LIBCPP_HIDE_FROM_ABI constexpr __union_t<_Tp, _Err> __union_from_expected(const expected<_Up, _OtherErr>& __other) {
+    return __other.__has_val_ ? __union_t<_Tp, _Err>(__construct_in_place_tag{}, __other.__union_.__val_)
+                              : __union_t<_Tp, _Err>(__construct_unexpected_tag{}, __other.__union_.__unex_);
+  }
+
+  template <class _Up, class _OtherErr>
+  _LIBCPP_HIDE_FROM_ABI constexpr __union_t<_Tp, _Err> __union_from_expected(expected<_Up, _OtherErr>&& __other) {
+    return __other.__has_val_ ? __union_t<_Tp, _Err>(__construct_in_place_tag{}, std::move(__other.__union_.__val_))
+                              : __union_t<_Tp, _Err>(__construct_unexpected_tag{}, std::move(__other.__union_.__unex_));
+  }
+
   _LIBCPP_NO_UNIQUE_ADDRESS __union_t<_Tp, _Err> __union_;
   bool __has_val_;
 };
@@ -998,11 +989,7 @@ class expected<_Tp, _Err> {
   _LIBCPP_HIDE_FROM_ABI constexpr expected(const expected& __rhs)
     noexcept(is_nothrow_copy_constructible_v<_Err>) // strengthened
     requires(is_copy_constructible_v<_Err> && !is_trivially_copy_constructible_v<_Err>)
-      : __has_val_(__rhs.__has_val_) {
-    if (!__rhs.__has_val_) {
-      std::construct_at(std::addressof(__union_.__unex_), __rhs.__union_.__unex_);
-    }
-  }
+      : __union_(__union_from_expected(__rhs)), __has_val_(__rhs.__has_val_) {}
 
   _LIBCPP_HIDE_FROM_ABI constexpr expected(expected&&)
     requires(is_move_constructible_v<_Err> && is_trivially_move_constructible_v<_Err>)
@@ -1011,51 +998,35 @@ class expected<_Tp, _Err> {
   _LIBCPP_HIDE_FROM_ABI constexpr expected(expected&& __rhs)
     noexcept(is_nothrow_move_constructible_v<_Err>)
     requires(is_move_constructible_v<_Err> && !is_trivially_move_constructible_v<_Err>)
-      : __has_val_(__rhs.__has_val_) {
-    if (!__rhs.__has_val_) {
-      std::construct_at(std::addressof(__union_.__unex_), std::move(__rhs.__union_.__unex_));
-    }
-  }
+      : __union_(__union_from_expected(std::move(__rhs))), __has_val_(__rhs.__has_val_) {}
 
   template <class _Up, class _OtherErr>
     requires __can_convert<_Up, _OtherErr, const _OtherErr&>::value
   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<const _OtherErr&, _Err>)
   expected(const expected<_Up, _OtherErr>& __rhs)
     noexcept(is_nothrow_constructible_v<_Err, const _OtherErr&>) // strengthened
-      : __has_val_(__rhs.__has_val_) {
-    if (!__rhs.__has_val_) {
-      std::construct_at(std::addressof(__union_.__unex_), __rhs.__union_.__unex_);
-    }
-  }
+      : __union_(__union_from_expected(__rhs)), __has_val_(__rhs.__has_val_) {}
 
   template <class _Up, class _OtherErr>
     requires __can_convert<_Up, _OtherErr, _OtherErr>::value
   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_OtherErr, _Err>)
   expected(expected<_Up, _OtherErr>&& __rhs)
     noexcept(is_nothrow_constructible_v<_Err, _OtherErr>) // strengthened
-      : __has_val_(__rhs.__has_val_) {
-    if (!__rhs.__has_val_) {
-      std::construct_at(std::addressof(__union_.__unex_), std::move(__rhs.__union_.__unex_));
-    }
-  }
+      : __union_(__union_from_expected(std::move(__rhs))), __has_val_(__rhs.__has_val_) {}
 
   template <class _OtherErr>
     requires is_constructible_v<_Err, const _OtherErr&>
   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<const _OtherErr&, _Err>)
   expected(const unexpected<_OtherErr>& __unex)
     noexcept(is_nothrow_constructible_v<_Err, const _OtherErr&>) // strengthened
-      : __has_val_(false) {
-    std::construct_at(std::addressof(__union_.__unex_), __unex.error());
-  }
+      : __union_(__construct_unexpected_tag{}, __unex.error()), __has_val_(false) {}
 
   template <class _OtherErr>
     requires is_constructible_v<_Err, _OtherErr>
   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_OtherErr, _Err>)
   expected(unexpected<_OtherErr>&& __unex)
     noexcept(is_nothrow_constructible_v<_Err, _OtherErr>) // strengthened
-      : __has_val_(false) {
-    std::construct_at(std::addressof(__union_.__unex_), std::move(__unex.error()));
-  }
+      : __union_(__construct_unexpected_tag{}, std::move(__unex.error())), __has_val_(false) {}
 
   _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(in_place_t) noexcept : __has_val_(true) {}
 
@@ -1063,17 +1034,13 @@ class expected<_Tp, _Err> {
     requires is_constructible_v<_Err, _Args...>
   _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(unexpect_t, _Args&&... __args)
     noexcept(is_nothrow_constructible_v<_Err, _Args...>) // strengthened
-      : __has_val_(false) {
-    std::construct_at(std::addressof(__union_.__unex_), std::forward<_Args>(__args)...);
-  }
+      : __union_(__construct_unexpected_tag{}, std::forward<_Args>(__args)...), __has_val_(false) {}
 
   template <class _Up, class... _Args>
     requires is_constructible_v< _Err, initializer_list<_Up>&, _Args... >
   _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(unexpect_t, initializer_list<_Up> __il, _Args&&... __args)
     noexcept(is_nothrow_constructible_v<_Err, initializer_list<_Up>&, _Args...>) // strengthened
-      : __has_val_(false) {
-    std::construct_at(std::addressof(__union_.__unex_), __il, std::forward<_Args>(__args)...);
-  }
+      : __union_(__construct_unexpected_tag{}, __il, std::forward<_Args>(__args)...), __has_val_(false) {}
 
 private:
   template <class _Func>
@@ -1502,11 +1469,16 @@ class expected<_Tp, _Err> {
 
 private:
   struct __empty_t {};
+  struct __construct_unexpected_tag {};
 
   template <class _ErrorType>
   union __union_t {
     _LIBCPP_HIDE_FROM_ABI constexpr __union_t() : __empty_() {}
 
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(__construct_unexpected_tag, _Args&&... __args)
+        : __unex_(std::forward<_Args>(__args)...) {}
+
     template <class _Func, class... _Args>
     _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(
         __expected_construct_unexpected_from_invoke_tag, _Func&& __f, _Args&&... __args)
@@ -1534,6 +1506,10 @@ class expected<_Tp, _Err> {
     _LIBCPP_HIDE_FROM_ABI constexpr __union_t(const __union_t&) = default;
     _LIBCPP_HIDE_FROM_ABI constexpr __union_t& operator=(const __union_t&) = default;
 
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(__construct_unexpected_tag, _Args&&... __args)
+        : __unex_(std::forward<_Args>(__args)...) {}
+
     template <class _Func, class... _Args>
     _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(
         __expected_construct_unexpected_from_invoke_tag, _Func&& __f, _Args&&... __args)
@@ -1552,6 +1528,18 @@ class expected<_Tp, _Err> {
     _LIBCPP_NO_UNIQUE_ADDRESS _ErrorType __unex_;
   };
 
+  template <class _Up, class _OtherErr>
+  _LIBCPP_HIDE_FROM_ABI constexpr __union_t<_Err> __union_from_expected(const expected<_Up, _OtherErr>& __other) {
+    return __other.__has_val_ ? __union_t<_Err>()
+                              : __union_t<_Err>(__construct_unexpected_tag{}, __other.__union_.__unex_);
+  }
+
+  template <class _Up, class _OtherErr>
+  _LIBCPP_HIDE_FROM_ABI constexpr __union_t<_Err> __union_from_expected(expected<_Up, _OtherErr>&& __other) {
+    return __other.__has_val_ ? __union_t<_Err>()
+                              : __union_t<_Err>(__construct_unexpected_tag{}, std::move(__other.__union_.__unex_));
+  }
+
   _LIBCPP_NO_UNIQUE_ADDRESS __union_t<_Err> __union_;
   bool __has_val_;
 };
diff --git a/libcxx/test/std/utilities/expected/expected.expected/observers/has_value.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/observers/has_value.pass.cpp
index 27d657a065699..8979e0f45d44f 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/observers/has_value.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/observers/has_value.pass.cpp
@@ -12,6 +12,7 @@
 #include <cassert>
 #include <concepts>
 #include <expected>
+#include <optional>
 #include <type_traits>
 #include <utility>
 
@@ -30,6 +31,22 @@ static_assert(!HasValueNoexcept<Foo>);
 static_assert(HasValueNoexcept<std::expected<int, int>>);
 static_assert(HasValueNoexcept<const std::expected<int, int>>);
 
+// This type has one byte of tail padding where `std::expected` will put its
+// "has value" flag. The constructor will clobber all bytes including the
+// tail padding. With this type we can check that `std::expected` will set
+// its "has value" flag _after_ the value/error object is constructed.
+template <int c>
+struct tail_clobberer {
+  constexpr tail_clobberer() {
+    if (!std::is_constant_evaluated()) {
+      // This `memset` might actually be UB (?) but suffices to reproduce bugs
+      // related to the "has value" flag.
+      std::memset(this, c, sizeof(*this));
+    }
+  }
+  alignas(2) bool b;
+};
+
 constexpr bool test() {
   // has_value
   {
@@ -43,6 +60,28 @@ constexpr bool test() {
     assert(!e.has_value());
   }
 
+  // See https://github.com/llvm/llvm-project/issues/68552
+  {
+    static constexpr auto f1 = [] -> std::expected<std::optional<int>, long> { return 0; };
+
+    static constexpr auto f2 = [] -> std::expected<std::optional<int>, int> {
+      return f1().transform_error([](auto) { return 0; });
+    };
+
+    auto e = f2();
+    assert(e.has_value());
+  }
+  {
+    const std::expected<tail_clobberer<0>, bool> e = {};
+    static_assert(sizeof(tail_clobberer<0>) == sizeof(e));
+    assert(e.has_value());
+  }
+  {
+    const std::expected<void, tail_clobberer<1>> e(std::unexpect);
+    static_assert(sizeof(tail_clobberer<1>) == sizeof(e));
+    assert(!e.has_value());
+  }
+
   return true;
 }
 

From 2757d11080f91b53044ed2fe4f09c95ff66b27d0 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller@gmail.com>
Date: Sun, 15 Oct 2023 13:32:14 +0200
Subject: [PATCH 02/28] replace home-grown constructor tags with standard ones

---
 libcxx/include/__expected/expected.h | 51 +++++++++++++---------------
 1 file changed, 24 insertions(+), 27 deletions(-)

diff --git a/libcxx/include/__expected/expected.h b/libcxx/include/__expected/expected.h
index 08f35b1111f6b..6440b31e71832 100644
--- a/libcxx/include/__expected/expected.h
+++ b/libcxx/include/__expected/expected.h
@@ -119,7 +119,7 @@ class expected {
   _LIBCPP_HIDE_FROM_ABI constexpr expected()
     noexcept(is_nothrow_default_constructible_v<_Tp>) // strengthened
     requires is_default_constructible_v<_Tp>
-      : __union_(__construct_in_place_tag{}), __has_val_(true) {}
+      : __union_(std::in_place), __has_val_(true) {}
 
   _LIBCPP_HIDE_FROM_ABI constexpr expected(const expected&) = delete;
 
@@ -200,47 +200,47 @@ class expected {
              (!is_same_v<remove_cv_t<_Tp>, bool> || !__is_std_expected<remove_cvref_t<_Up>>::value))
   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_Up, _Tp>)
       expected(_Up&& __u) noexcept(is_nothrow_constructible_v<_Tp, _Up>) // strengthened
-      : __union_(__construct_in_place_tag{}, std::forward<_Up>(__u)), __has_val_(true) {}
+      : __union_(std::in_place, std::forward<_Up>(__u)), __has_val_(true) {}
 
   template <class _OtherErr>
     requires is_constructible_v<_Err, const _OtherErr&>
   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<const _OtherErr&, _Err>)
   expected(const unexpected<_OtherErr>& __unex)
     noexcept(is_nothrow_constructible_v<_Err, const _OtherErr&>) // strengthened
-      : __union_(__construct_unexpected_tag{}, __unex.error()), __has_val_(false) {}
+      : __union_(std::unexpect, __unex.error()), __has_val_(false) {}
 
   template <class _OtherErr>
     requires is_constructible_v<_Err, _OtherErr>
   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_OtherErr, _Err>)
   expected(unexpected<_OtherErr>&& __unex)
     noexcept(is_nothrow_constructible_v<_Err, _OtherErr>) // strengthened
-      : __union_(__construct_unexpected_tag{}, std::move(__unex.error())), __has_val_(false) {}
+      : __union_(std::unexpect, std::move(__unex.error())), __has_val_(false) {}
 
   template <class... _Args>
     requires is_constructible_v<_Tp, _Args...>
   _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(in_place_t, _Args&&... __args)
     noexcept(is_nothrow_constructible_v<_Tp, _Args...>) // strengthened
-      : __union_(__construct_in_place_tag{}, std::forward<_Args>(__args)...), __has_val_(true) {}
+      : __union_(std::in_place, std::forward<_Args>(__args)...), __has_val_(true) {}
 
   template <class _Up, class... _Args>
     requires is_constructible_v< _Tp, initializer_list<_Up>&, _Args... >
   _LIBCPP_HIDE_FROM_ABI constexpr explicit
   expected(in_place_t, initializer_list<_Up> __il, _Args&&... __args)
     noexcept(is_nothrow_constructible_v<_Tp, initializer_list<_Up>&, _Args...>) // strengthened
-      : __union_(__construct_in_place_tag{}, __il, std::forward<_Args>(__args)...), __has_val_(true) {}
+      : __union_(std::in_place, __il, std::forward<_Args>(__args)...), __has_val_(true) {}
 
   template <class... _Args>
     requires is_constructible_v<_Err, _Args...>
   _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(unexpect_t, _Args&&... __args)
     noexcept(is_nothrow_constructible_v<_Err, _Args...>) // strengthened
-      : __union_(__construct_unexpected_tag{}, std::forward<_Args>(__args)...), __has_val_(false) {}
+      : __union_(std::unexpect, std::forward<_Args>(__args)...), __has_val_(false) {}
 
   template <class _Up, class... _Args>
     requires is_constructible_v< _Err, initializer_list<_Up>&, _Args... >
   _LIBCPP_HIDE_FROM_ABI constexpr explicit
   expected(unexpect_t, initializer_list<_Up> __il, _Args&&... __args)
     noexcept(is_nothrow_constructible_v<_Err, initializer_list<_Up>&, _Args...>) // strengthened
-      : __union_(__construct_unexpected_tag{}, __il, std::forward<_Args>(__args)...), __has_val_(false) {}
+      : __union_(std::unexpect, __il, std::forward<_Args>(__args)...), __has_val_(false) {}
 
   // [expected.object.dtor], destructor
 
@@ -855,19 +855,17 @@ class expected {
 
 private:
   struct __empty_t {};
-  struct __construct_in_place_tag {};
-  struct __construct_unexpected_tag {};
 
   template <class _ValueType, class _ErrorType>
   union __union_t {
     _LIBCPP_HIDE_FROM_ABI constexpr __union_t() {}
 
     template <class... _Args>
-    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(__construct_in_place_tag, _Args&&... __args)
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(std::in_place_t, _Args&&... __args)
         : __val_(std::forward<_Args>(__args)...) {}
 
     template <class... _Args>
-    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(__construct_unexpected_tag, _Args&&... __args)
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(std::unexpect_t, _Args&&... __args)
         : __unex_(std::forward<_Args>(__args)...) {}
 
     template <class _Func, class... _Args>
@@ -903,11 +901,11 @@ class expected {
     _LIBCPP_HIDE_FROM_ABI constexpr __union_t& operator=(const __union_t&) = default;
 
     template <class... _Args>
-    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(__construct_in_place_tag, _Args&&... __args)
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(std::in_place_t, _Args&&... __args)
         : __val_(std::forward<_Args>(__args)...) {}
 
     template <class... _Args>
-    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(__construct_unexpected_tag, _Args&&... __args)
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(std::unexpect_t, _Args&&... __args)
         : __unex_(std::forward<_Args>(__args)...) {}
 
     template <class _Func, class... _Args>
@@ -936,14 +934,14 @@ class expected {
 
   template <class _Up, class _OtherErr>
   _LIBCPP_HIDE_FROM_ABI constexpr __union_t<_Tp, _Err> __union_from_expected(const expected<_Up, _OtherErr>& __other) {
-    return __other.__has_val_ ? __union_t<_Tp, _Err>(__construct_in_place_tag{}, __other.__union_.__val_)
-                              : __union_t<_Tp, _Err>(__construct_unexpected_tag{}, __other.__union_.__unex_);
+    return __other.__has_val_ ? __union_t<_Tp, _Err>(std::in_place, __other.__union_.__val_)
+                              : __union_t<_Tp, _Err>(std::unexpect, __other.__union_.__unex_);
   }
 
   template <class _Up, class _OtherErr>
   _LIBCPP_HIDE_FROM_ABI constexpr __union_t<_Tp, _Err> __union_from_expected(expected<_Up, _OtherErr>&& __other) {
-    return __other.__has_val_ ? __union_t<_Tp, _Err>(__construct_in_place_tag{}, std::move(__other.__union_.__val_))
-                              : __union_t<_Tp, _Err>(__construct_unexpected_tag{}, std::move(__other.__union_.__unex_));
+    return __other.__has_val_ ? __union_t<_Tp, _Err>(std::in_place, std::move(__other.__union_.__val_))
+                              : __union_t<_Tp, _Err>(std::unexpect, std::move(__other.__union_.__unex_));
   }
 
   _LIBCPP_NO_UNIQUE_ADDRESS __union_t<_Tp, _Err> __union_;
@@ -1019,14 +1017,14 @@ class expected<_Tp, _Err> {
   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<const _OtherErr&, _Err>)
   expected(const unexpected<_OtherErr>& __unex)
     noexcept(is_nothrow_constructible_v<_Err, const _OtherErr&>) // strengthened
-      : __union_(__construct_unexpected_tag{}, __unex.error()), __has_val_(false) {}
+      : __union_(std::unexpect, __unex.error()), __has_val_(false) {}
 
   template <class _OtherErr>
     requires is_constructible_v<_Err, _OtherErr>
   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_OtherErr, _Err>)
   expected(unexpected<_OtherErr>&& __unex)
     noexcept(is_nothrow_constructible_v<_Err, _OtherErr>) // strengthened
-      : __union_(__construct_unexpected_tag{}, std::move(__unex.error())), __has_val_(false) {}
+      : __union_(std::unexpect, std::move(__unex.error())), __has_val_(false) {}
 
   _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(in_place_t) noexcept : __has_val_(true) {}
 
@@ -1034,13 +1032,13 @@ class expected<_Tp, _Err> {
     requires is_constructible_v<_Err, _Args...>
   _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(unexpect_t, _Args&&... __args)
     noexcept(is_nothrow_constructible_v<_Err, _Args...>) // strengthened
-      : __union_(__construct_unexpected_tag{}, std::forward<_Args>(__args)...), __has_val_(false) {}
+      : __union_(std::unexpect, std::forward<_Args>(__args)...), __has_val_(false) {}
 
   template <class _Up, class... _Args>
     requires is_constructible_v< _Err, initializer_list<_Up>&, _Args... >
   _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(unexpect_t, initializer_list<_Up> __il, _Args&&... __args)
     noexcept(is_nothrow_constructible_v<_Err, initializer_list<_Up>&, _Args...>) // strengthened
-      : __union_(__construct_unexpected_tag{}, __il, std::forward<_Args>(__args)...), __has_val_(false) {}
+      : __union_(std::unexpect, __il, std::forward<_Args>(__args)...), __has_val_(false) {}
 
 private:
   template <class _Func>
@@ -1469,14 +1467,13 @@ class expected<_Tp, _Err> {
 
 private:
   struct __empty_t {};
-  struct __construct_unexpected_tag {};
 
   template <class _ErrorType>
   union __union_t {
     _LIBCPP_HIDE_FROM_ABI constexpr __union_t() : __empty_() {}
 
     template <class... _Args>
-    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(__construct_unexpected_tag, _Args&&... __args)
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(std::unexpect_t, _Args&&... __args)
         : __unex_(std::forward<_Args>(__args)...) {}
 
     template <class _Func, class... _Args>
@@ -1507,7 +1504,7 @@ class expected<_Tp, _Err> {
     _LIBCPP_HIDE_FROM_ABI constexpr __union_t& operator=(const __union_t&) = default;
 
     template <class... _Args>
-    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(__construct_unexpected_tag, _Args&&... __args)
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(std::unexpect_t, _Args&&... __args)
         : __unex_(std::forward<_Args>(__args)...) {}
 
     template <class _Func, class... _Args>
@@ -1531,13 +1528,13 @@ class expected<_Tp, _Err> {
   template <class _Up, class _OtherErr>
   _LIBCPP_HIDE_FROM_ABI constexpr __union_t<_Err> __union_from_expected(const expected<_Up, _OtherErr>& __other) {
     return __other.__has_val_ ? __union_t<_Err>()
-                              : __union_t<_Err>(__construct_unexpected_tag{}, __other.__union_.__unex_);
+                              : __union_t<_Err>(std::unexpect, __other.__union_.__unex_);
   }
 
   template <class _Up, class _OtherErr>
   _LIBCPP_HIDE_FROM_ABI constexpr __union_t<_Err> __union_from_expected(expected<_Up, _OtherErr>&& __other) {
     return __other.__has_val_ ? __union_t<_Err>()
-                              : __union_t<_Err>(__construct_unexpected_tag{}, std::move(__other.__union_.__unex_));
+                              : __union_t<_Err>(std::unexpect, std::move(__other.__union_.__unex_));
   }
 
   _LIBCPP_NO_UNIQUE_ADDRESS __union_t<_Err> __union_;

From 7f1c6c7222e7539120654b82ad39b2c7051303ec Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller@gmail.com>
Date: Sun, 15 Oct 2023 13:37:48 +0200
Subject: [PATCH 03/28] replace use of ternary operator by explicit if/else

---
 libcxx/include/__expected/expected.h | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/libcxx/include/__expected/expected.h b/libcxx/include/__expected/expected.h
index 6440b31e71832..55036d3f55c24 100644
--- a/libcxx/include/__expected/expected.h
+++ b/libcxx/include/__expected/expected.h
@@ -934,14 +934,18 @@ class expected {
 
   template <class _Up, class _OtherErr>
   _LIBCPP_HIDE_FROM_ABI constexpr __union_t<_Tp, _Err> __union_from_expected(const expected<_Up, _OtherErr>& __other) {
-    return __other.__has_val_ ? __union_t<_Tp, _Err>(std::in_place, __other.__union_.__val_)
-                              : __union_t<_Tp, _Err>(std::unexpect, __other.__union_.__unex_);
+    if (__other.__has_val_)
+      return __union_t<_Tp, _Err>(std::in_place, __other.__union_.__val_);
+    else
+      return __union_t<_Tp, _Err>(std::unexpect, __other.__union_.__unex_);
   }
 
   template <class _Up, class _OtherErr>
   _LIBCPP_HIDE_FROM_ABI constexpr __union_t<_Tp, _Err> __union_from_expected(expected<_Up, _OtherErr>&& __other) {
-    return __other.__has_val_ ? __union_t<_Tp, _Err>(std::in_place, std::move(__other.__union_.__val_))
-                              : __union_t<_Tp, _Err>(std::unexpect, std::move(__other.__union_.__unex_));
+    if (__other.__has_val_)
+      return __union_t<_Tp, _Err>(std::in_place, std::move(__other.__union_.__val_));
+    else
+      return __union_t<_Tp, _Err>(std::unexpect, std::move(__other.__union_.__unex_));
   }
 
   _LIBCPP_NO_UNIQUE_ADDRESS __union_t<_Tp, _Err> __union_;
@@ -1527,14 +1531,18 @@ class expected<_Tp, _Err> {
 
   template <class _Up, class _OtherErr>
   _LIBCPP_HIDE_FROM_ABI constexpr __union_t<_Err> __union_from_expected(const expected<_Up, _OtherErr>& __other) {
-    return __other.__has_val_ ? __union_t<_Err>()
-                              : __union_t<_Err>(std::unexpect, __other.__union_.__unex_);
+    if (__other.__has_val_)
+      return __union_t<_Err>();
+    else
+      return __union_t<_Err>(std::unexpect, __other.__union_.__unex_);
   }
 
   template <class _Up, class _OtherErr>
   _LIBCPP_HIDE_FROM_ABI constexpr __union_t<_Err> __union_from_expected(expected<_Up, _OtherErr>&& __other) {
-    return __other.__has_val_ ? __union_t<_Err>()
-                              : __union_t<_Err>(std::unexpect, std::move(__other.__union_.__unex_));
+    if (__other.__has_val_)
+      return __union_t<_Err>();
+    else
+      return __union_t<_Err>(std::unexpect, std::move(__other.__union_.__unex_));
   }
 
   _LIBCPP_NO_UNIQUE_ADDRESS __union_t<_Err> __union_;

From fec6013102955da78ee85f99426457fd9e129555 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller@gmail.com>
Date: Sun, 15 Oct 2023 13:47:06 +0200
Subject: [PATCH 04/28] add a short description to the tests

---
 .../expected.expected/observers/has_value.pass.cpp   | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/libcxx/test/std/utilities/expected/expected.expected/observers/has_value.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/observers/has_value.pass.cpp
index 8979e0f45d44f..b203253e89779 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/observers/has_value.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/observers/has_value.pass.cpp
@@ -60,7 +60,17 @@ constexpr bool test() {
     assert(!e.has_value());
   }
 
-  // See https://github.com/llvm/llvm-project/issues/68552
+  // The following tests check that the "has_value" flag is not overwritten
+  // by the constructor of the value. This could happen because the flag is
+  // stored in the tail padding of the value.
+  //
+  // The first test is a simplified version of the real code where this was
+  // first observed.
+  //
+  // The other tests use a synthetic struct that clobbers its tail padding
+  // on construction, making the issue easier to reproduce.
+  //
+  // See https://github.com/llvm/llvm-project/issues/68552 and the linked PR.
   {
     static constexpr auto f1 = [] -> std::expected<std::optional<int>, long> { return 0; };
 

From 99771837135b9d657101adc1b59eceaebae26f38 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller@gmail.com>
Date: Sun, 15 Oct 2023 13:50:57 +0200
Subject: [PATCH 05/28] remove comment about memset being UB here

---
 .../expected/expected.expected/observers/has_value.pass.cpp     | 2 --
 1 file changed, 2 deletions(-)

diff --git a/libcxx/test/std/utilities/expected/expected.expected/observers/has_value.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/observers/has_value.pass.cpp
index b203253e89779..d7591718ae784 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/observers/has_value.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/observers/has_value.pass.cpp
@@ -39,8 +39,6 @@ template <int c>
 struct tail_clobberer {
   constexpr tail_clobberer() {
     if (!std::is_constant_evaluated()) {
-      // This `memset` might actually be UB (?) but suffices to reproduce bugs
-      // related to the "has value" flag.
       std::memset(this, c, sizeof(*this));
     }
   }

From 54db802337e3683ff56242cab7e5772b384e6f77 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller@gmail.com>
Date: Tue, 17 Oct 2023 15:59:37 +0200
Subject: [PATCH 06/28] add constructors to __union_t from expected parts and
 refactor

GCC 13 didn't like the previous approach.
---
 libcxx/include/__expected/expected.h | 80 ++++++++++++++--------------
 1 file changed, 40 insertions(+), 40 deletions(-)

diff --git a/libcxx/include/__expected/expected.h b/libcxx/include/__expected/expected.h
index 55036d3f55c24..06c9a26c8e3a8 100644
--- a/libcxx/include/__expected/expected.h
+++ b/libcxx/include/__expected/expected.h
@@ -134,7 +134,7 @@ class expected {
     noexcept(is_nothrow_copy_constructible_v<_Tp> && is_nothrow_copy_constructible_v<_Err>) // strengthened
     requires(is_copy_constructible_v<_Tp> && is_copy_constructible_v<_Err> &&
              !(is_trivially_copy_constructible_v<_Tp> && is_trivially_copy_constructible_v<_Err>))
-      : __union_(__union_from_expected(__other)), __has_val_(__other.__has_val_) { }
+      : __union_(__other.__has_val_, __other.__union_), __has_val_(__other.__has_val_) { }
 
   _LIBCPP_HIDE_FROM_ABI constexpr expected(expected&&)
     requires(is_move_constructible_v<_Tp> && is_move_constructible_v<_Err>
@@ -145,7 +145,7 @@ class expected {
     noexcept(is_nothrow_move_constructible_v<_Tp> && is_nothrow_move_constructible_v<_Err>)
     requires(is_move_constructible_v<_Tp> && is_move_constructible_v<_Err> &&
              !(is_trivially_move_constructible_v<_Tp> && is_trivially_move_constructible_v<_Err>))
-      : __union_(__union_from_expected(std::move(__other))), __has_val_(__other.__has_val_) { }
+      : __union_(__other.__has_val_, std::move(__other.__union_)), __has_val_(__other.__has_val_) { }
 
 private:
   template <class _Up, class _OtherErr, class _UfQual, class _OtherErrQual>
@@ -185,14 +185,14 @@ class expected {
   expected(const expected<_Up, _OtherErr>& __other)
     noexcept(is_nothrow_constructible_v<_Tp, const _Up&> &&
              is_nothrow_constructible_v<_Err, const _OtherErr&>) // strengthened
-      : __union_(__union_from_expected(__other)), __has_val_(__other.__has_val_) {}
+      : __union_(__other.__has_val_, __other.__union_), __has_val_(__other.__has_val_) {}
 
   template <class _Up, class _OtherErr>
     requires __can_convert<_Up, _OtherErr, _Up, _OtherErr>::value
   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_Up, _Tp> || !is_convertible_v<_OtherErr, _Err>)
   expected(expected<_Up, _OtherErr>&& __other)
     noexcept(is_nothrow_constructible_v<_Tp, _Up> && is_nothrow_constructible_v<_Err, _OtherErr>) // strengthened
-      : __union_(__union_from_expected(std::move(__other))), __has_val_(__other.__has_val_) {}
+      : __union_(__other.__has_val_, std::move(__other.__union_)), __has_val_(__other.__has_val_) {}
 
   template <class _Up = _Tp>
     requires(!is_same_v<remove_cvref_t<_Up>, in_place_t> && !is_same_v<expected, remove_cvref_t<_Up>> &&
@@ -878,6 +878,14 @@ class expected {
         std::__expected_construct_unexpected_from_invoke_tag, _Func&& __f, _Args&&... __args)
         : __unex_(std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}
 
+    template <class _Union>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(bool __has_val, _Union&& __other) {
+      if (__has_val)
+        std::construct_at(this, std::in_place, std::forward<_Union>(__other).__val_);
+      else
+        std::construct_at(this, std::unexpect, std::forward<_Union>(__other).__unex_);
+    }
+
     _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t()
       requires(is_trivially_destructible_v<_ValueType> && is_trivially_destructible_v<_ErrorType>)
     = default;
@@ -918,6 +926,14 @@ class expected {
         std::__expected_construct_unexpected_from_invoke_tag, _Func&& __f, _Args&&... __args)
         : __unex_(std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}
 
+    template <class _Union>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(bool __has_val, _Union&& __other) {
+      if (__has_val)
+        std::construct_at(this, std::in_place, std::forward<_Union>(__other).__val_);
+      else
+        std::construct_at(this, std::unexpect, std::forward<_Union>(__other).__unex_);
+    }
+
     _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t()
       requires(is_trivially_destructible_v<_ValueType> && is_trivially_destructible_v<_ErrorType>)
     = default;
@@ -932,22 +948,6 @@ class expected {
     _LIBCPP_NO_UNIQUE_ADDRESS _ErrorType __unex_;
   };
 
-  template <class _Up, class _OtherErr>
-  _LIBCPP_HIDE_FROM_ABI constexpr __union_t<_Tp, _Err> __union_from_expected(const expected<_Up, _OtherErr>& __other) {
-    if (__other.__has_val_)
-      return __union_t<_Tp, _Err>(std::in_place, __other.__union_.__val_);
-    else
-      return __union_t<_Tp, _Err>(std::unexpect, __other.__union_.__unex_);
-  }
-
-  template <class _Up, class _OtherErr>
-  _LIBCPP_HIDE_FROM_ABI constexpr __union_t<_Tp, _Err> __union_from_expected(expected<_Up, _OtherErr>&& __other) {
-    if (__other.__has_val_)
-      return __union_t<_Tp, _Err>(std::in_place, std::move(__other.__union_.__val_));
-    else
-      return __union_t<_Tp, _Err>(std::unexpect, std::move(__other.__union_.__unex_));
-  }
-
   _LIBCPP_NO_UNIQUE_ADDRESS __union_t<_Tp, _Err> __union_;
   bool __has_val_;
 };
@@ -991,7 +991,7 @@ class expected<_Tp, _Err> {
   _LIBCPP_HIDE_FROM_ABI constexpr expected(const expected& __rhs)
     noexcept(is_nothrow_copy_constructible_v<_Err>) // strengthened
     requires(is_copy_constructible_v<_Err> && !is_trivially_copy_constructible_v<_Err>)
-      : __union_(__union_from_expected(__rhs)), __has_val_(__rhs.__has_val_) {}
+      : __union_(__rhs.__has_val_, __rhs.__union_), __has_val_(__rhs.__has_val_) {}
 
   _LIBCPP_HIDE_FROM_ABI constexpr expected(expected&&)
     requires(is_move_constructible_v<_Err> && is_trivially_move_constructible_v<_Err>)
@@ -1000,21 +1000,21 @@ class expected<_Tp, _Err> {
   _LIBCPP_HIDE_FROM_ABI constexpr expected(expected&& __rhs)
     noexcept(is_nothrow_move_constructible_v<_Err>)
     requires(is_move_constructible_v<_Err> && !is_trivially_move_constructible_v<_Err>)
-      : __union_(__union_from_expected(std::move(__rhs))), __has_val_(__rhs.__has_val_) {}
+      : __union_(__rhs.__has_val_, std::move(__rhs.__union_)), __has_val_(__rhs.__has_val_) {}
 
   template <class _Up, class _OtherErr>
     requires __can_convert<_Up, _OtherErr, const _OtherErr&>::value
   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<const _OtherErr&, _Err>)
   expected(const expected<_Up, _OtherErr>& __rhs)
     noexcept(is_nothrow_constructible_v<_Err, const _OtherErr&>) // strengthened
-      : __union_(__union_from_expected(__rhs)), __has_val_(__rhs.__has_val_) {}
+      : __union_(__rhs.__has_val_, __rhs.__union_), __has_val_(__rhs.__has_val_) {}
 
   template <class _Up, class _OtherErr>
     requires __can_convert<_Up, _OtherErr, _OtherErr>::value
   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_OtherErr, _Err>)
   expected(expected<_Up, _OtherErr>&& __rhs)
     noexcept(is_nothrow_constructible_v<_Err, _OtherErr>) // strengthened
-      : __union_(__union_from_expected(std::move(__rhs))), __has_val_(__rhs.__has_val_) {}
+      : __union_(__rhs.__has_val_, std::move(__rhs.__union_)), __has_val_(__rhs.__has_val_) {}
 
   template <class _OtherErr>
     requires is_constructible_v<_Err, const _OtherErr&>
@@ -1485,6 +1485,14 @@ class expected<_Tp, _Err> {
         __expected_construct_unexpected_from_invoke_tag, _Func&& __f, _Args&&... __args)
         : __unex_(std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}
 
+    template <class _Union>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(bool __has_val, _Union&& __other) {
+      if (__has_val)
+        std::construct_at(this);
+      else
+        std::construct_at(this, std::unexpect, std::forward<_Union>(__other).__unex_);
+    }
+
     _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t()
       requires(is_trivially_destructible_v<_ErrorType>)
     = default;
@@ -1516,6 +1524,14 @@ class expected<_Tp, _Err> {
         __expected_construct_unexpected_from_invoke_tag, _Func&& __f, _Args&&... __args)
         : __unex_(std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}
 
+    template <class _Union>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(bool __has_val, _Union&& __other) {
+      if (__has_val)
+        std::construct_at(this);
+      else
+        std::construct_at(this, std::unexpect, std::forward<_Union>(__other).__unex_);
+    }
+
     _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t()
       requires(is_trivially_destructible_v<_ErrorType>)
     = default;
@@ -1529,22 +1545,6 @@ class expected<_Tp, _Err> {
     _LIBCPP_NO_UNIQUE_ADDRESS _ErrorType __unex_;
   };
 
-  template <class _Up, class _OtherErr>
-  _LIBCPP_HIDE_FROM_ABI constexpr __union_t<_Err> __union_from_expected(const expected<_Up, _OtherErr>& __other) {
-    if (__other.__has_val_)
-      return __union_t<_Err>();
-    else
-      return __union_t<_Err>(std::unexpect, __other.__union_.__unex_);
-  }
-
-  template <class _Up, class _OtherErr>
-  _LIBCPP_HIDE_FROM_ABI constexpr __union_t<_Err> __union_from_expected(expected<_Up, _OtherErr>&& __other) {
-    if (__other.__has_val_)
-      return __union_t<_Err>();
-    else
-      return __union_t<_Err>(std::unexpect, std::move(__other.__union_.__unex_));
-  }
-
   _LIBCPP_NO_UNIQUE_ADDRESS __union_t<_Err> __union_;
   bool __has_val_;
 };

From 6411556590d66095ddb0976cce909ec435a0b65e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller@gmail.com>
Date: Tue, 17 Oct 2023 16:06:19 +0200
Subject: [PATCH 07/28] put new expected<void> test into appropriate folder

---
 .../observers/has_value.pass.cpp              | 24 +++----------------
 .../observers/has_value.pass.cpp              |  8 +++++++
 libcxx/test/std/utilities/expected/types.h    | 16 +++++++++++++
 3 files changed, 27 insertions(+), 21 deletions(-)

diff --git a/libcxx/test/std/utilities/expected/expected.expected/observers/has_value.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/observers/has_value.pass.cpp
index d7591718ae784..74569cfa2083e 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/observers/has_value.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/observers/has_value.pass.cpp
@@ -17,6 +17,7 @@
 #include <utility>
 
 #include "test_macros.h"
+#include "../../types.h"
 
 // Test noexcept
 template <class T>
@@ -31,20 +32,6 @@ static_assert(!HasValueNoexcept<Foo>);
 static_assert(HasValueNoexcept<std::expected<int, int>>);
 static_assert(HasValueNoexcept<const std::expected<int, int>>);
 
-// This type has one byte of tail padding where `std::expected` will put its
-// "has value" flag. The constructor will clobber all bytes including the
-// tail padding. With this type we can check that `std::expected` will set
-// its "has value" flag _after_ the value/error object is constructed.
-template <int c>
-struct tail_clobberer {
-  constexpr tail_clobberer() {
-    if (!std::is_constant_evaluated()) {
-      std::memset(this, c, sizeof(*this));
-    }
-  }
-  alignas(2) bool b;
-};
-
 constexpr bool test() {
   // has_value
   {
@@ -80,15 +67,10 @@ constexpr bool test() {
     assert(e.has_value());
   }
   {
-    const std::expected<tail_clobberer<0>, bool> e = {};
-    static_assert(sizeof(tail_clobberer<0>) == sizeof(e));
+    const std::expected<TailClobberer<0>, bool> e = {};
+    static_assert(sizeof(TailClobberer<0>) == sizeof(e));
     assert(e.has_value());
   }
-  {
-    const std::expected<void, tail_clobberer<1>> e(std::unexpect);
-    static_assert(sizeof(tail_clobberer<1>) == sizeof(e));
-    assert(!e.has_value());
-  }
 
   return true;
 }
diff --git a/libcxx/test/std/utilities/expected/expected.void/observers/has_value.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/observers/has_value.pass.cpp
index 42a173d60c898..89c79541f9955 100644
--- a/libcxx/test/std/utilities/expected/expected.void/observers/has_value.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.void/observers/has_value.pass.cpp
@@ -16,6 +16,7 @@
 #include <utility>
 
 #include "test_macros.h"
+#include "../../types.h"
 
 // Test noexcept
 template <class T>
@@ -43,6 +44,13 @@ constexpr bool test() {
     assert(!e.has_value());
   }
 
+  // See comments of the corresponding test in "expected.expected".
+  {
+    const std::expected<void, TailClobberer<1>> e(std::unexpect);
+    static_assert(sizeof(TailClobberer<1>) == sizeof(e));
+    assert(!e.has_value());
+  }
+
   return true;
 }
 
diff --git a/libcxx/test/std/utilities/expected/types.h b/libcxx/test/std/utilities/expected/types.h
index 7c7e517785b4f..874b72b64e8ae 100644
--- a/libcxx/test/std/utilities/expected/types.h
+++ b/libcxx/test/std/utilities/expected/types.h
@@ -9,7 +9,9 @@
 #ifndef TEST_STD_UTILITIES_EXPECTED_TYPES_H
 #define TEST_STD_UTILITIES_EXPECTED_TYPES_H
 
+#include <cstring>
 #include <utility>
+#include <type_traits>
 #include "test_macros.h"
 
 template <bool copyMoveNoexcept, bool convertNoexcept = true>
@@ -102,6 +104,20 @@ struct TrackedMove {
   }
 };
 
+// This type has one byte of tail padding where `std::expected` will put its
+// "has value" flag. The constructor will clobber all bytes including the
+// tail padding. With this type we can check that `std::expected` will set
+// its "has value" flag _after_ the value/error object is constructed.
+template <int c>
+struct TailClobberer {
+  constexpr TailClobberer() {
+    if (!std::is_constant_evaluated()) {
+      std::memset(this, c, sizeof(*this));
+    }
+  }
+  alignas(2) bool b;
+};
+
 #ifndef TEST_HAS_NO_EXCEPTIONS
 struct Except {};
 

From cd1a2b8530d2391b7a60b7fb5b94cbd2a8b724ee Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller@gmail.com>
Date: Tue, 17 Oct 2023 18:21:47 +0200
Subject: [PATCH 08/28] disable new tests on compilers that cannot build them

---
 .../expected/expected.expected/observers/has_value.pass.cpp  | 5 +++++
 .../expected/expected.void/observers/has_value.pass.cpp      | 3 +++
 2 files changed, 8 insertions(+)

diff --git a/libcxx/test/std/utilities/expected/expected.expected/observers/has_value.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/observers/has_value.pass.cpp
index 74569cfa2083e..3cb46bcf79abe 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/observers/has_value.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/observers/has_value.pass.cpp
@@ -57,6 +57,7 @@ constexpr bool test() {
   //
   // See https://github.com/llvm/llvm-project/issues/68552 and the linked PR.
   {
+#if !defined(TEST_COMPILER_CLANG) || TEST_CLANG_VER >= 1600
     static constexpr auto f1 = [] -> std::expected<std::optional<int>, long> { return 0; };
 
     static constexpr auto f2 = [] -> std::expected<std::optional<int>, int> {
@@ -65,10 +66,14 @@ constexpr bool test() {
 
     auto e = f2();
     assert(e.has_value());
+#endif
   }
   {
     const std::expected<TailClobberer<0>, bool> e = {};
+    // clang-cl does not support [[no_unique_address]] yet.
+#if !(defined(TEST_COMPILER_CLANG) && defined(_MSC_VER))
     static_assert(sizeof(TailClobberer<0>) == sizeof(e));
+#endif
     assert(e.has_value());
   }
 
diff --git a/libcxx/test/std/utilities/expected/expected.void/observers/has_value.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/observers/has_value.pass.cpp
index 89c79541f9955..50098d10977b7 100644
--- a/libcxx/test/std/utilities/expected/expected.void/observers/has_value.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.void/observers/has_value.pass.cpp
@@ -47,7 +47,10 @@ constexpr bool test() {
   // See comments of the corresponding test in "expected.expected".
   {
     const std::expected<void, TailClobberer<1>> e(std::unexpect);
+    // clang-cl does not support [[no_unique_address]] yet.
+#if !(defined(TEST_COMPILER_CLANG) && defined(_MSC_VER))
     static_assert(sizeof(TailClobberer<1>) == sizeof(e));
+#endif
     assert(!e.has_value());
   }
 

From b31c8efacc509235779e031626dd7077e2f77d6e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller@gmail.com>
Date: Tue, 17 Oct 2023 19:57:07 +0200
Subject: [PATCH 09/28] make comment in
 libcxx/test/std/utilities/expected/types.h more general

Co-authored-by: Louis Dionne <ldionne.2@gmail.com>
---
 libcxx/test/std/utilities/expected/types.h | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/libcxx/test/std/utilities/expected/types.h b/libcxx/test/std/utilities/expected/types.h
index 874b72b64e8ae..e6fd5ac479266 100644
--- a/libcxx/test/std/utilities/expected/types.h
+++ b/libcxx/test/std/utilities/expected/types.h
@@ -104,10 +104,12 @@ struct TrackedMove {
   }
 };
 
-// This type has one byte of tail padding where `std::expected` will put its
+// This type has one byte of tail padding where `std::expected` may put its
 // "has value" flag. The constructor will clobber all bytes including the
-// tail padding. With this type we can check that `std::expected` will set
-// its "has value" flag _after_ the value/error object is constructed.
+// tail padding. With this type we can check that `std::expected` handles
+// the case where the "has value" flag is an overlapping subobject correctly.
+//
+// See https://github.com/llvm/llvm-project/issues/68552 for details.
 template <int c>
 struct TailClobberer {
   constexpr TailClobberer() {

From 49f59a5972005e401d932572a9c3a6555ae0247a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller@gmail.com>
Date: Tue, 17 Oct 2023 20:12:00 +0200
Subject: [PATCH 10/28] spell out full path to corresponding test file

---
 .../expected/expected.void/observers/has_value.pass.cpp        | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/libcxx/test/std/utilities/expected/expected.void/observers/has_value.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/observers/has_value.pass.cpp
index 50098d10977b7..06a45ed430180 100644
--- a/libcxx/test/std/utilities/expected/expected.void/observers/has_value.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.void/observers/has_value.pass.cpp
@@ -44,7 +44,8 @@ constexpr bool test() {
     assert(!e.has_value());
   }
 
-  // See comments of the corresponding test in "expected.expected".
+  // See comments of the corresponding test in
+  // "expected.expected/observers/has_value.pass.cpp".
   {
     const std::expected<void, TailClobberer<1>> e(std::unexpect);
     // clang-cl does not support [[no_unique_address]] yet.

From 84a61daf9003ff2420e51c524586b04d26687bdb Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller@gmail.com>
Date: Tue, 17 Oct 2023 20:14:28 +0200
Subject: [PATCH 11/28] directly construct into union member instead of trying
 to delegate to another constructor

---
 libcxx/include/__expected/expected.h | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/libcxx/include/__expected/expected.h b/libcxx/include/__expected/expected.h
index 06c9a26c8e3a8..9d09485d22ae4 100644
--- a/libcxx/include/__expected/expected.h
+++ b/libcxx/include/__expected/expected.h
@@ -881,9 +881,9 @@ class expected {
     template <class _Union>
     _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(bool __has_val, _Union&& __other) {
       if (__has_val)
-        std::construct_at(this, std::in_place, std::forward<_Union>(__other).__val_);
+        std::construct_at(std::addressof(__val_), std::forward<_Union>(__other).__val_);
       else
-        std::construct_at(this, std::unexpect, std::forward<_Union>(__other).__unex_);
+        std::construct_at(std::addressof(__unex_), std::forward<_Union>(__other).__unex_);
     }
 
     _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t()
@@ -929,9 +929,9 @@ class expected {
     template <class _Union>
     _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(bool __has_val, _Union&& __other) {
       if (__has_val)
-        std::construct_at(this, std::in_place, std::forward<_Union>(__other).__val_);
+        std::construct_at(std::addressof(__val_), std::forward<_Union>(__other).__val_);
       else
-        std::construct_at(this, std::unexpect, std::forward<_Union>(__other).__unex_);
+        std::construct_at(std::addressof(__unex_), std::forward<_Union>(__other).__unex_);
     }
 
     _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t()
@@ -1488,9 +1488,9 @@ class expected<_Tp, _Err> {
     template <class _Union>
     _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(bool __has_val, _Union&& __other) {
       if (__has_val)
-        std::construct_at(this);
+        std::construct_at(std::addressof(__empty_));
       else
-        std::construct_at(this, std::unexpect, std::forward<_Union>(__other).__unex_);
+        std::construct_at(std::addressof(__unex_), std::forward<_Union>(__other).__unex_);
     }
 
     _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t()
@@ -1527,9 +1527,9 @@ class expected<_Tp, _Err> {
     template <class _Union>
     _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(bool __has_val, _Union&& __other) {
       if (__has_val)
-        std::construct_at(this);
+        std::construct_at(std::addressof(__empty_));
       else
-        std::construct_at(this, std::unexpect, std::forward<_Union>(__other).__unex_);
+        std::construct_at(std::addressof(__unex_), std::forward<_Union>(__other).__unex_);
     }
 
     _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t()

From 5f70208208967b9365311641af151be532ee18f3 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller@gmail.com>
Date: Tue, 17 Oct 2023 20:24:03 +0200
Subject: [PATCH 12/28] remove unneeded '__empty_t' in union of non-void
 expected

---
 libcxx/include/__expected/expected.h | 6 ------
 1 file changed, 6 deletions(-)

diff --git a/libcxx/include/__expected/expected.h b/libcxx/include/__expected/expected.h
index 9d09485d22ae4..f874a3a126a58 100644
--- a/libcxx/include/__expected/expected.h
+++ b/libcxx/include/__expected/expected.h
@@ -854,12 +854,8 @@ class expected {
   }
 
 private:
-  struct __empty_t {};
-
   template <class _ValueType, class _ErrorType>
   union __union_t {
-    _LIBCPP_HIDE_FROM_ABI constexpr __union_t() {}
-
     template <class... _Args>
     _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(std::in_place_t, _Args&&... __args)
         : __val_(std::forward<_Args>(__args)...) {}
@@ -904,7 +900,6 @@ class expected {
   template <class _ValueType, class _ErrorType>
     requires(is_trivially_move_constructible_v<_ValueType> && is_trivially_move_constructible_v<_ErrorType>)
   union __union_t<_ValueType, _ErrorType> {
-    _LIBCPP_HIDE_FROM_ABI constexpr __union_t() : __empty_() {}
     _LIBCPP_HIDE_FROM_ABI constexpr __union_t(const __union_t&) = default;
     _LIBCPP_HIDE_FROM_ABI constexpr __union_t& operator=(const __union_t&) = default;
 
@@ -943,7 +938,6 @@ class expected {
       requires(!is_trivially_destructible_v<_ValueType> || !is_trivially_destructible_v<_ErrorType>)
     {}
 
-    _LIBCPP_NO_UNIQUE_ADDRESS __empty_t __empty_;
     _LIBCPP_NO_UNIQUE_ADDRESS _ValueType __val_;
     _LIBCPP_NO_UNIQUE_ADDRESS _ErrorType __unex_;
   };

From 1ab8ff96ef4d3838139ed0ec59ea4a6d2e2be1d7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller@gmail.com>
Date: Wed, 18 Oct 2023 03:22:39 +0200
Subject: [PATCH 13/28] make lambdas automatic variables

---
 .../expected/expected.expected/observers/has_value.pass.cpp | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/libcxx/test/std/utilities/expected/expected.expected/observers/has_value.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/observers/has_value.pass.cpp
index 3cb46bcf79abe..0e19da0655946 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/observers/has_value.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/observers/has_value.pass.cpp
@@ -57,16 +57,14 @@ constexpr bool test() {
   //
   // See https://github.com/llvm/llvm-project/issues/68552 and the linked PR.
   {
-#if !defined(TEST_COMPILER_CLANG) || TEST_CLANG_VER >= 1600
-    static constexpr auto f1 = [] -> std::expected<std::optional<int>, long> { return 0; };
+    auto f1 = [] -> std::expected<std::optional<int>, long> { return 0; };
 
-    static constexpr auto f2 = [] -> std::expected<std::optional<int>, int> {
+    auto f2 = [&f1] -> std::expected<std::optional<int>, int> {
       return f1().transform_error([](auto) { return 0; });
     };
 
     auto e = f2();
     assert(e.has_value());
-#endif
   }
   {
     const std::expected<TailClobberer<0>, bool> e = {};

From 58c523cf52ea98cc119f332277763b55a08a58c5 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller@gmail.com>
Date: Fri, 20 Oct 2023 18:28:34 +0200
Subject: [PATCH 14/28] rename template parameter from "c" to "constant"

Co-authored-by: Louis Dionne <ldionne.2@gmail.com>
---
 libcxx/test/std/utilities/expected/types.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libcxx/test/std/utilities/expected/types.h b/libcxx/test/std/utilities/expected/types.h
index e6fd5ac479266..697a410cea5fe 100644
--- a/libcxx/test/std/utilities/expected/types.h
+++ b/libcxx/test/std/utilities/expected/types.h
@@ -110,7 +110,7 @@ struct TrackedMove {
 // the case where the "has value" flag is an overlapping subobject correctly.
 //
 // See https://github.com/llvm/llvm-project/issues/68552 for details.
-template <int c>
+template <int constant>
 struct TailClobberer {
   constexpr TailClobberer() {
     if (!std::is_constant_evaluated()) {

From de8a4b4dfc741a3a430f582d747a863f19a982b9 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller@gmail.com>
Date: Fri, 20 Oct 2023 20:55:20 +0200
Subject: [PATCH 15/28] fix build

---
 libcxx/test/std/utilities/expected/types.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libcxx/test/std/utilities/expected/types.h b/libcxx/test/std/utilities/expected/types.h
index 697a410cea5fe..6a7990383ada8 100644
--- a/libcxx/test/std/utilities/expected/types.h
+++ b/libcxx/test/std/utilities/expected/types.h
@@ -114,7 +114,7 @@ template <int constant>
 struct TailClobberer {
   constexpr TailClobberer() {
     if (!std::is_constant_evaluated()) {
-      std::memset(this, c, sizeof(*this));
+      std::memset(this, constant, sizeof(*this));
     }
   }
   alignas(2) bool b;

From 07e02585674c667516e3ed9763d534dd6b9977f6 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller@gmail.com>
Date: Sat, 21 Oct 2023 18:23:27 +0200
Subject: [PATCH 16/28] add TailClobberer tests for expected.expected

---
 .../assign/emplace.intializer_list.pass.cpp   |  8 +++++++
 .../expected.expected/assign/emplace.pass.cpp |  7 ++++++
 .../ctor/ctor.convert.copy.pass.cpp           |  9 +++++++
 .../ctor/ctor.convert.move.pass.cpp           |  9 +++++++
 .../expected.expected/ctor/ctor.copy.pass.cpp | 16 +++++++++++++
 .../ctor/ctor.default.pass.cpp                |  3 +++
 .../ctor/ctor.inplace.pass.cpp                | 16 ++++++++-----
 .../ctor/ctor.inplace_init_list.pass.cpp      |  7 ++++++
 .../expected.expected/ctor/ctor.move.pass.cpp | 16 +++++++++++++
 .../expected.expected/ctor/ctor.u.pass.cpp    | 16 ++++++++-----
 .../ctor/ctor.unexpect.pass.cpp               | 16 ++++++++-----
 .../ctor/ctor.unexpect_init_list.pass.cpp     |  7 ++++++
 .../ctor/ctor.unexpected.copy.pass.cpp        |  6 +++--
 .../ctor/ctor.unexpected.move.pass.cpp        |  6 +++--
 libcxx/test/std/utilities/expected/types.h    | 24 ++++++++++++++++++-
 15 files changed, 143 insertions(+), 23 deletions(-)

diff --git a/libcxx/test/std/utilities/expected/expected.expected/assign/emplace.intializer_list.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/assign/emplace.intializer_list.pass.cpp
index 3cdfcde3f4d69..922200a8c0263 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/assign/emplace.intializer_list.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/assign/emplace.intializer_list.pass.cpp
@@ -81,6 +81,14 @@ constexpr bool test() {
     assert(e.value().i == 10);
   }
 
+  // TailClobberer
+  {
+    std::expected<TailClobberer<0>, bool> e(std::unexpect);
+    auto list = {4, 5, 6};
+    e.emplace(list);
+    assert(e.has_value());
+  }
+
   return true;
 }
 
diff --git a/libcxx/test/std/utilities/expected/expected.expected/assign/emplace.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/assign/emplace.pass.cpp
index c62e628935020..491de2dff0331 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/assign/emplace.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/assign/emplace.pass.cpp
@@ -73,6 +73,13 @@ constexpr bool test() {
     assert(e.value() == 10);
   }
 
+  // TailClobberer
+  {
+    std::expected<TailClobberer<0>, bool> e(std::unexpect);
+    e.emplace();
+    assert(e.has_value());
+  }
+
   return true;
 }
 
diff --git a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.convert.copy.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.convert.copy.pass.cpp
index 9274b9a2c030e..2cc952358e821 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.convert.copy.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.convert.copy.pass.cpp
@@ -45,6 +45,7 @@
 #include <utility>
 
 #include "test_macros.h"
+#include "../../types.h"
 
 // Test Constraints:
 template <class T1, class Err1, class T2, class Err2>
@@ -161,6 +162,14 @@ constexpr bool test() {
     assert(e1.error() == 5);
   }
 
+  // convert TailClobberer
+  {
+    const std::expected<TailClobbererNonTrivialMove<0>, char> e1;
+    std::expected<TailClobberer<0>, char> e2 = e1;
+    assert(e2.has_value());
+    assert(e1.has_value());
+  }
+
   return true;
 }
 
diff --git a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.convert.move.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.convert.move.pass.cpp
index 71979311bfd10..31819cd33ef6c 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.convert.move.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.convert.move.pass.cpp
@@ -46,6 +46,7 @@
 
 #include "MoveOnly.h"
 #include "test_macros.h"
+#include "../../types.h"
 
 // Test Constraints:
 template <class T1, class Err1, class T2, class Err2>
@@ -160,6 +161,14 @@ constexpr bool test() {
     assert(e1.error().get() == 0);
   }
 
+  // convert TailClobberer
+  {
+    std::expected<TailClobbererNonTrivialMove<0>, char> e1;
+    std::expected<TailClobberer<0>, char> e2 = std::move(e1);
+    assert(e2.has_value());
+    assert(e1.has_value());
+  }
+
   return true;
 }
 
diff --git a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.copy.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.copy.pass.cpp
index 77d73485025ab..1b1c4a5960692 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.copy.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.copy.pass.cpp
@@ -30,6 +30,7 @@
 #include <utility>
 
 #include "test_macros.h"
+#include "../../types.h"
 
 struct NonCopyable {
   NonCopyable(const NonCopyable&) = delete;
@@ -93,6 +94,21 @@ constexpr bool test() {
     assert(!e2.has_value());
     assert(e2.error() == 5);
   }
+
+  // copy TailClobberer as value
+  {
+    const std::expected<TailClobberer<0>, bool> e1;
+    auto e2 = e1;
+    assert(e2.has_value());
+  }
+
+  // copy TailClobberer as error
+  {
+    const std::expected<bool, TailClobberer<1>> e1(std::unexpect);
+    auto e2 = e1;
+    assert(!e2.has_value());
+  }
+
   return true;
 }
 
diff --git a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.default.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.default.pass.cpp
index 431e604e8b692..ea23d8aa735c8 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.default.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.default.pass.cpp
@@ -22,6 +22,7 @@
 #include <type_traits>
 
 #include "test_macros.h"
+#include "../../types.h"
 
 struct NoDedefaultCtor {
   NoDedefaultCtor() = delete;
@@ -45,6 +46,7 @@ constexpr void testDefaultCtor() {
 
 template <class T>
 constexpr void testTypes() {
+  testDefaultCtor<T, bool>();
   testDefaultCtor<T, int>();
   testDefaultCtor<T, NoDedefaultCtor>();
 }
@@ -52,6 +54,7 @@ constexpr void testTypes() {
 constexpr bool test() {
   testTypes<int>();
   testTypes<MyInt>();
+  testTypes<TailClobberer<0>>();
   return true;
 }
 
diff --git a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.inplace.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.inplace.pass.cpp
index 92952551711e0..0b41eb44883e4 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.inplace.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.inplace.pass.cpp
@@ -26,6 +26,7 @@
 
 #include "MoveOnly.h"
 #include "test_macros.h"
+#include "../../types.h"
 
 // Test Constraints:
 static_assert(std::is_constructible_v<std::expected<int, int>, std::in_place_t>);
@@ -54,24 +55,24 @@ struct CopyOnly {
   friend constexpr bool operator==(const CopyOnly& mi, int ii) { return mi.i == ii; }
 };
 
-template <class T>
+template <class T, class E = int>
 constexpr void testInt() {
-  std::expected<T, int> e(std::in_place, 5);
+  std::expected<T, E> e(std::in_place, 5);
   assert(e.has_value());
   assert(e.value() == 5);
 }
 
-template <class T>
+template <class T, class E = int>
 constexpr void testLValue() {
   T t(5);
-  std::expected<T, int> e(std::in_place, t);
+  std::expected<T, E> e(std::in_place, t);
   assert(e.has_value());
   assert(e.value() == 5);
 }
 
-template <class T>
+template <class T, class E = int>
 constexpr void testRValue() {
-  std::expected<T, int> e(std::in_place, T(5));
+  std::expected<T, E> e(std::in_place, T(5));
   assert(e.has_value());
   assert(e.value() == 5);
 }
@@ -80,10 +81,13 @@ constexpr bool test() {
   testInt<int>();
   testInt<CopyOnly>();
   testInt<MoveOnly>();
+  testInt<TailClobberer<0>, bool>();
   testLValue<int>();
   testLValue<CopyOnly>();
+  testLValue<TailClobberer<0>, bool>();
   testRValue<int>();
   testRValue<MoveOnly>();
+  testRValue<TailClobberer<0>, bool>();
 
   // no arg
   {
diff --git a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.inplace_init_list.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.inplace_init_list.pass.cpp
index b4cad54b860e9..143f9d7dab113 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.inplace_init_list.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.inplace_init_list.pass.cpp
@@ -28,6 +28,7 @@
 
 #include "MoveOnly.h"
 #include "test_macros.h"
+#include "../../types.h"
 
 // Test Constraints:
 static_assert(
@@ -90,6 +91,12 @@ constexpr bool test() {
     assert(m.get() == 0);
   }
 
+  // TailClobberer
+  {
+    std::expected<TailClobberer<0>, bool> e(std::in_place, {1, 2, 3});
+    assert(e.has_value());
+  }
+
   return true;
 }
 
diff --git a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.move.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.move.pass.cpp
index 5e6749e50c16c..f542778b3d5ff 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.move.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.move.pass.cpp
@@ -32,6 +32,7 @@
 #include <utility>
 
 #include "test_macros.h"
+#include "../../types.h"
 
 struct NonMovable {
   NonMovable(NonMovable&&) = delete;
@@ -112,6 +113,21 @@ constexpr bool test() {
     assert(e2.error() == 5);
     assert(!e1.has_value());
   }
+
+  // move TailClobbererNonTrivialMove as value
+  {
+    std::expected<TailClobbererNonTrivialMove<0>, bool> e1;
+    auto e2 = std::move(e1);
+    assert(e2.has_value());
+  }
+
+  // move TailClobbererNonTrivialMove as error
+  {
+    std::expected<bool, TailClobbererNonTrivialMove<1>> e1(std::unexpect);
+    auto e2 = std::move(e1);
+    assert(!e2.has_value());
+  }
+
   return true;
 }
 
diff --git a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.u.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.u.pass.cpp
index 9e82943f9f314..16dd531a410c2 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.u.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.u.pass.cpp
@@ -29,6 +29,7 @@
 
 #include "MoveOnly.h"
 #include "test_macros.h"
+#include "../../types.h"
 
 // Test Constraints:
 static_assert(std::is_constructible_v<std::expected<int, int>, int>);
@@ -70,24 +71,24 @@ struct CopyOnly {
 struct BaseError {};
 struct DerivedError : BaseError {};
 
-template <class T>
+template <class T, class E = int>
 constexpr void testInt() {
-  std::expected<T, int> e(5);
+  std::expected<T, E> e(5);
   assert(e.has_value());
   assert(e.value() == 5);
 }
 
-template <class T>
+template <class T, class E = int>
 constexpr void testLValue() {
   T t(5);
-  std::expected<T, int> e(t);
+  std::expected<T, E> e(t);
   assert(e.has_value());
   assert(e.value() == 5);
 }
 
-template <class T>
+template <class T, class E = int>
 constexpr void testRValue() {
-  std::expected<T, int> e(T(5));
+  std::expected<T, E> e(T(5));
   assert(e.has_value());
   assert(e.value() == 5);
 }
@@ -96,10 +97,13 @@ constexpr bool test() {
   testInt<int>();
   testInt<CopyOnly>();
   testInt<MoveOnly>();
+  testInt<TailClobberer<0>, bool>();
   testLValue<int>();
   testLValue<CopyOnly>();
+  testLValue<TailClobberer<0>, bool>();
   testRValue<int>();
   testRValue<MoveOnly>();
+  testRValue<TailClobberer<0>, bool>();
 
   // Test default template argument.
   // Without it, the template parameter cannot be deduced from an initializer list
diff --git a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpect.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpect.pass.cpp
index 5a78e41dfcae2..a0b334a25b0cb 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpect.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpect.pass.cpp
@@ -26,6 +26,7 @@
 
 #include "MoveOnly.h"
 #include "test_macros.h"
+#include "../../types.h"
 
 // Test Constraints:
 static_assert(std::is_constructible_v<std::expected<int, int>, std::unexpect_t>);
@@ -54,24 +55,24 @@ struct CopyOnly {
   friend constexpr bool operator==(const CopyOnly& mi, int ii) { return mi.i == ii; }
 };
 
-template <class T>
+template <class T, class V = int>
 constexpr void testInt() {
-  std::expected<int, T> e(std::unexpect, 5);
+  std::expected<V, T> e(std::unexpect, 5);
   assert(!e.has_value());
   assert(e.error() == 5);
 }
 
-template <class T>
+template <class T, class V = int>
 constexpr void testLValue() {
   T t(5);
-  std::expected<int, T> e(std::unexpect, t);
+  std::expected<V, T> e(std::unexpect, t);
   assert(!e.has_value());
   assert(e.error() == 5);
 }
 
-template <class T>
+template <class T, class V = int>
 constexpr void testRValue() {
-  std::expected<int, T> e(std::unexpect, T(5));
+  std::expected<V, T> e(std::unexpect, T(5));
   assert(!e.has_value());
   assert(e.error() == 5);
 }
@@ -80,10 +81,13 @@ constexpr bool test() {
   testInt<int>();
   testInt<CopyOnly>();
   testInt<MoveOnly>();
+  testInt<TailClobberer<1>, bool>();
   testLValue<int>();
   testLValue<CopyOnly>();
+  testLValue<TailClobberer<1>, bool>();
   testRValue<int>();
   testRValue<MoveOnly>();
+  testRValue<TailClobberer<1>, bool>();
 
   // no arg
   {
diff --git a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpect_init_list.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpect_init_list.pass.cpp
index 7cc36b51e4153..4c280fc32a4e9 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpect_init_list.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpect_init_list.pass.cpp
@@ -28,6 +28,7 @@
 
 #include "MoveOnly.h"
 #include "test_macros.h"
+#include "../../types.h"
 
 // Test Constraints:
 static_assert(
@@ -90,6 +91,12 @@ constexpr bool test() {
     assert(m.get() == 0);
   }
 
+  // TailClobberer
+  {
+    std::expected<bool, TailClobberer<1>> e(std::unexpect, {1, 2, 3});
+    assert(!e.has_value());
+  }
+
   return true;
 }
 
diff --git a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpected.copy.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpected.copy.pass.cpp
index 09ac91182b3b8..b3bb71cf36837 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpected.copy.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpected.copy.pass.cpp
@@ -27,6 +27,7 @@
 
 #include "MoveOnly.h"
 #include "test_macros.h"
+#include "../../types.h"
 
 // Test Constraints
 static_assert(std::is_constructible_v<std::expected<int, int>, const std::unexpected<int>&>);
@@ -49,10 +50,10 @@ struct MyInt {
   friend constexpr bool operator==(const MyInt&, const MyInt&) = default;
 };
 
-template <class T>
+template <class T, class V = int>
 constexpr void testUnexpected() {
   const std::unexpected<int> u(5);
-  std::expected<int, T> e(u);
+  std::expected<V, T> e(u);
   assert(!e.has_value());
   assert(e.error() == 5);
 }
@@ -60,6 +61,7 @@ constexpr void testUnexpected() {
 constexpr bool test() {
   testUnexpected<int>();
   testUnexpected<MyInt>();
+  testUnexpected<TailClobberer<1>, bool>();
   return true;
 }
 
diff --git a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpected.move.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpected.move.pass.cpp
index 9aaaa3fe1a448..419c789c86d51 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpected.move.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpected.move.pass.cpp
@@ -27,6 +27,7 @@
 
 #include "MoveOnly.h"
 #include "test_macros.h"
+#include "../../types.h"
 
 // Test Constraints
 static_assert(std::is_constructible_v<std::expected<int, int>, std::unexpected<int>>);
@@ -49,10 +50,10 @@ struct MyInt {
   friend constexpr bool operator==(const MyInt&, const MyInt&) = default;
 };
 
-template <class Err>
+template <class Err, class V = int>
 constexpr void testInt() {
   std::unexpected<int> u(5);
-  std::expected<int, Err> e(std::move(u));
+  std::expected<V, Err> e(std::move(u));
   assert(!e.has_value());
   assert(e.error() == 5);
 }
@@ -69,6 +70,7 @@ constexpr bool test() {
   testInt<int>();
   testInt<MyInt>();
   testInt<MoveOnly>();
+  testInt<TailClobberer<1>, bool>();
   testMoveOnly();
   return true;
 }
diff --git a/libcxx/test/std/utilities/expected/types.h b/libcxx/test/std/utilities/expected/types.h
index 6a7990383ada8..58c2df504bfb2 100644
--- a/libcxx/test/std/utilities/expected/types.h
+++ b/libcxx/test/std/utilities/expected/types.h
@@ -112,13 +112,35 @@ struct TrackedMove {
 // See https://github.com/llvm/llvm-project/issues/68552 for details.
 template <int constant>
 struct TailClobberer {
-  constexpr TailClobberer() {
+  constexpr TailClobberer() noexcept {
     if (!std::is_constant_evaluated()) {
       std::memset(this, constant, sizeof(*this));
     }
+    // Always set `b` itself to `false` so that the comparison works.
+    b = false;
   }
+  constexpr TailClobberer(const TailClobberer&) : TailClobberer() {}
+  constexpr TailClobberer(TailClobberer&&) = default;
+  // Converts from `int`/`std::initializer_list<int>, used in some tests.
+  constexpr TailClobberer(int) : TailClobberer() {}
+  constexpr TailClobberer(std::initializer_list<int>) noexcept : TailClobberer() {}
+
+  friend constexpr bool operator==(const TailClobberer&, const TailClobberer&) = default;
+
+private:
   alignas(2) bool b;
 };
+static_assert(!std::is_trivially_copy_constructible_v<TailClobberer<0>>);
+static_assert(std::is_trivially_move_constructible_v<TailClobberer<0>>);
+
+template <int constant>
+struct TailClobbererNonTrivialMove : TailClobberer<constant> {
+  using TailClobberer<constant>::TailClobberer;
+  constexpr TailClobbererNonTrivialMove(TailClobbererNonTrivialMove&&) noexcept : TailClobberer<constant>() {}
+};
+static_assert(!std::is_trivially_copy_constructible_v<TailClobbererNonTrivialMove<0>>);
+static_assert(std::is_move_constructible_v<TailClobbererNonTrivialMove<0>>);
+static_assert(!std::is_trivially_move_constructible_v<TailClobbererNonTrivialMove<0>>);
 
 #ifndef TEST_HAS_NO_EXCEPTIONS
 struct Except {};

From 4471615ef8ab85421bddc577718d9c49a821184e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller@gmail.com>
Date: Sat, 21 Oct 2023 18:37:48 +0200
Subject: [PATCH 17/28] add some missing checks

---
 .../expected/expected.expected/ctor/ctor.move.pass.cpp          | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.move.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.move.pass.cpp
index f542778b3d5ff..aaf357fbc518a 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.move.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.move.pass.cpp
@@ -119,6 +119,7 @@ constexpr bool test() {
     std::expected<TailClobbererNonTrivialMove<0>, bool> e1;
     auto e2 = std::move(e1);
     assert(e2.has_value());
+    assert(e1.has_value());
   }
 
   // move TailClobbererNonTrivialMove as error
@@ -126,6 +127,7 @@ constexpr bool test() {
     std::expected<bool, TailClobbererNonTrivialMove<1>> e1(std::unexpect);
     auto e2 = std::move(e1);
     assert(!e2.has_value());
+    assert(!e1.has_value());
   }
 
   return true;

From dd7b119819b6555f1a1299f05acca9a83e37e8fa Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller@gmail.com>
Date: Sat, 21 Oct 2023 18:38:04 +0200
Subject: [PATCH 18/28] add tests for expected<void>

---
 .../expected.void/ctor/ctor.convert.copy.pass.cpp        | 9 +++++++++
 .../expected.void/ctor/ctor.convert.move.pass.cpp        | 9 +++++++++
 .../expected/expected.void/ctor/ctor.copy.pass.cpp       | 8 ++++++++
 .../expected/expected.void/ctor/ctor.move.pass.cpp       | 9 +++++++++
 .../expected/expected.void/ctor/ctor.unexpect.pass.cpp   | 4 ++++
 .../expected.void/ctor/ctor.unexpect_init_list.pass.cpp  | 7 +++++++
 .../expected.void/ctor/ctor.unexpected.copy.pass.cpp     | 2 ++
 .../expected.void/ctor/ctor.unexpected.move.pass.cpp     | 2 ++
 8 files changed, 50 insertions(+)

diff --git a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.convert.copy.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.convert.copy.pass.cpp
index 40f8efa5f94bf..118e4274e9ef8 100644
--- a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.convert.copy.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.convert.copy.pass.cpp
@@ -33,6 +33,7 @@
 #include <utility>
 
 #include "test_macros.h"
+#include "../../types.h"
 
 // Test Constraints:
 template <class T1, class Err1, class T2, class Err2>
@@ -97,6 +98,14 @@ constexpr bool test() {
     assert(e1.error() == 5);
   }
 
+  // convert TailClobberer
+  {
+    const std::expected<void, TailClobbererNonTrivialMove<1>> e1;
+    std::expected<void, TailClobberer<1>> e2 = e1;
+    assert(!e2.has_value());
+    assert(!e1.has_value());
+  }
+
   return true;
 }
 
diff --git a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.convert.move.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.convert.move.pass.cpp
index b28fc7a03bb34..32f6055bc5c61 100644
--- a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.convert.move.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.convert.move.pass.cpp
@@ -34,6 +34,7 @@
 
 #include "MoveOnly.h"
 #include "test_macros.h"
+#include "../../types.h"
 
 // Test Constraints:
 template <class T1, class Err1, class T2, class Err2>
@@ -98,6 +99,14 @@ constexpr bool test() {
     assert(e1.error().get() == 0);
   }
 
+  // convert TailClobberer
+  {
+    std::expected<void, TailClobbererNonTrivialMove<1>> e1;
+    std::expected<void, TailClobberer<1>> e2 = std::move(e1);
+    assert(!e2.has_value());
+    assert(!e1.has_value());
+  }
+
   return true;
 }
 
diff --git a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.copy.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.copy.pass.cpp
index 689f152a3ac55..f46826a4e3289 100644
--- a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.copy.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.copy.pass.cpp
@@ -62,6 +62,14 @@ constexpr bool test() {
     assert(!e2.has_value());
     assert(e2.error() == 5);
   }
+
+  // copy TailClobberer as error
+  {
+    const std::expected<void, TailClobberer<1>> e1(std::unexpect);
+    auto e2 = e1;
+    assert(!e2.has_value());
+  }
+
   return true;
 }
 
diff --git a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.move.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.move.pass.cpp
index 61bce2be4897f..38affdcadde3f 100644
--- a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.move.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.move.pass.cpp
@@ -76,6 +76,15 @@ constexpr bool test() {
     assert(e2.error() == 5);
     assert(!e1.has_value());
   }
+
+  // move TailClobbererNonTrivialMove as error
+  {
+    std::expected<void, TailClobbererNonTrivialMove<1>> e1(std::unexpect);
+    auto e2 = std::move(e1);
+    assert(!e2.has_value());
+    assert(!e1.has_value());
+  }
+
   return true;
 }
 
diff --git a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpect.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpect.pass.cpp
index 0a857c77d9c7a..62c3817a8a66f 100644
--- a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpect.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpect.pass.cpp
@@ -26,6 +26,7 @@
 
 #include "MoveOnly.h"
 #include "test_macros.h"
+#include "../../types.h"
 
 // Test Constraints:
 static_assert(std::is_constructible_v<std::expected<void, int>, std::unexpect_t>);
@@ -80,10 +81,13 @@ constexpr bool test() {
   testInt<int>();
   testInt<CopyOnly>();
   testInt<MoveOnly>();
+  testInt<TailClobberer<1>>();
   testLValue<int>();
   testLValue<CopyOnly>();
+  testLValue<TailClobberer<1>>();
   testRValue<int>();
   testRValue<MoveOnly>();
+  testRValue<TailClobberer<1>>();
 
   // no arg
   {
diff --git a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpect_init_list.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpect_init_list.pass.cpp
index a73921225f1fa..28b4165bc27dc 100644
--- a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpect_init_list.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpect_init_list.pass.cpp
@@ -28,6 +28,7 @@
 
 #include "MoveOnly.h"
 #include "test_macros.h"
+#include "../../types.h"
 
 // Test Constraints:
 static_assert(
@@ -89,6 +90,12 @@ constexpr bool test() {
     assert(m.get() == 0);
   }
 
+  // TailClobberer
+  {
+    std::expected<void, TailClobberer<1>> e(std::unexpect, {1, 2, 3});
+    assert(!e.has_value());
+  }
+
   return true;
 }
 
diff --git a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpected.copy.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpected.copy.pass.cpp
index 89e1c9275e3e0..17063a97cb133 100644
--- a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpected.copy.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpected.copy.pass.cpp
@@ -27,6 +27,7 @@
 
 #include "MoveOnly.h"
 #include "test_macros.h"
+#include "../../types.h"
 
 // Test Constraints
 static_assert(std::is_constructible_v<std::expected<void, int>, const std::unexpected<int>&>);
@@ -60,6 +61,7 @@ constexpr void testUnexpected() {
 constexpr bool test() {
   testUnexpected<int>();
   testUnexpected<MyInt>();
+  testUnexpected<TailClobberer<1>>();
   return true;
 }
 
diff --git a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpected.move.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpected.move.pass.cpp
index 2ddcb63c085f0..792ef341879ea 100644
--- a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpected.move.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpected.move.pass.cpp
@@ -27,6 +27,7 @@
 
 #include "MoveOnly.h"
 #include "test_macros.h"
+#include "../../types.h"
 
 // Test Constraints
 static_assert(std::is_constructible_v<std::expected<void, int>, std::unexpected<int>>);
@@ -69,6 +70,7 @@ constexpr bool test() {
   testInt<int>();
   testInt<MyInt>();
   testInt<MoveOnly>();
+  testInt<TailClobberer<1>>();
   testMoveOnly();
   return true;
 }

From 5f5fab011fd0c7d05574c73435df56f2584a0b62 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller@gmail.com>
Date: Sun, 22 Oct 2023 08:24:40 +0200
Subject: [PATCH 19/28] fix compile errors

---
 .../expected/expected.void/ctor/ctor.convert.copy.pass.cpp      | 2 +-
 .../expected/expected.void/ctor/ctor.convert.move.pass.cpp      | 2 +-
 .../utilities/expected/expected.void/ctor/ctor.copy.pass.cpp    | 1 +
 .../utilities/expected/expected.void/ctor/ctor.move.pass.cpp    | 1 +
 4 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.convert.copy.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.convert.copy.pass.cpp
index 118e4274e9ef8..30533206e65a3 100644
--- a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.convert.copy.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.convert.copy.pass.cpp
@@ -100,7 +100,7 @@ constexpr bool test() {
 
   // convert TailClobberer
   {
-    const std::expected<void, TailClobbererNonTrivialMove<1>> e1;
+    const std::expected<void, TailClobbererNonTrivialMove<1>> e1(std::unexpect);
     std::expected<void, TailClobberer<1>> e2 = e1;
     assert(!e2.has_value());
     assert(!e1.has_value());
diff --git a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.convert.move.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.convert.move.pass.cpp
index 32f6055bc5c61..d9db49c273831 100644
--- a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.convert.move.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.convert.move.pass.cpp
@@ -101,7 +101,7 @@ constexpr bool test() {
 
   // convert TailClobberer
   {
-    std::expected<void, TailClobbererNonTrivialMove<1>> e1;
+    std::expected<void, TailClobbererNonTrivialMove<1>> e1(std::unexpect);
     std::expected<void, TailClobberer<1>> e2 = std::move(e1);
     assert(!e2.has_value());
     assert(!e1.has_value());
diff --git a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.copy.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.copy.pass.cpp
index f46826a4e3289..83e049857a93d 100644
--- a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.copy.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.copy.pass.cpp
@@ -25,6 +25,7 @@
 #include <utility>
 
 #include "test_macros.h"
+#include "../../types.h"
 
 struct NonCopyable {
   NonCopyable(const NonCopyable&) = delete;
diff --git a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.move.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.move.pass.cpp
index 38affdcadde3f..777b412f65f64 100644
--- a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.move.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.move.pass.cpp
@@ -25,6 +25,7 @@
 #include <utility>
 
 #include "test_macros.h"
+#include "../../types.h"
 
 struct NonMovable {
   NonMovable(NonMovable&&) = delete;

From ebaccf9ca66a209238f41b04ce75a6fcf153f460 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller@gmail.com>
Date: Sun, 22 Oct 2023 08:28:14 +0200
Subject: [PATCH 20/28] remove declaration of local Except struct

---
 .../expected/expected.expected/ctor/ctor.convert.copy.pass.cpp  | 2 --
 .../expected/expected.expected/ctor/ctor.convert.move.pass.cpp  | 2 --
 .../expected/expected.expected/ctor/ctor.copy.pass.cpp          | 2 --
 .../expected/expected.expected/ctor/ctor.default.pass.cpp       | 2 --
 .../expected/expected.expected/ctor/ctor.inplace.pass.cpp       | 2 --
 .../expected/expected.expected/ctor/ctor.move.pass.cpp          | 2 --
 .../expected/expected.void/ctor/ctor.convert.copy.pass.cpp      | 2 --
 .../expected/expected.void/ctor/ctor.convert.move.pass.cpp      | 2 --
 .../utilities/expected/expected.void/ctor/ctor.copy.pass.cpp    | 2 --
 .../utilities/expected/expected.void/ctor/ctor.move.pass.cpp    | 2 --
 .../expected/expected.void/ctor/ctor.unexpect.pass.cpp          | 2 --
 .../expected.void/ctor/ctor.unexpect_init_list.pass.cpp         | 2 --
 .../expected/expected.void/ctor/ctor.unexpected.copy.pass.cpp   | 2 --
 .../expected/expected.void/ctor/ctor.unexpected.move.pass.cpp   | 2 --
 14 files changed, 28 deletions(-)

diff --git a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.convert.copy.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.convert.copy.pass.cpp
index 2cc952358e821..16de28d970396 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.convert.copy.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.convert.copy.pass.cpp
@@ -175,8 +175,6 @@ constexpr bool test() {
 
 void testException() {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-  struct Except {};
-
   struct ThrowingInt {
     ThrowingInt(int) { throw Except{}; }
   };
diff --git a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.convert.move.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.convert.move.pass.cpp
index 31819cd33ef6c..0e30ea2c7fe0b 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.convert.move.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.convert.move.pass.cpp
@@ -174,8 +174,6 @@ constexpr bool test() {
 
 void testException() {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-  struct Except {};
-
   struct ThrowingInt {
     ThrowingInt(int) { throw Except{}; }
   };
diff --git a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.copy.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.copy.pass.cpp
index 1b1c4a5960692..581df51207da2 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.copy.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.copy.pass.cpp
@@ -114,8 +114,6 @@ constexpr bool test() {
 
 void testException() {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-  struct Except {};
-
   struct Throwing {
     Throwing() = default;
     Throwing(const Throwing&) { throw Except{}; }
diff --git a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.default.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.default.pass.cpp
index ea23d8aa735c8..dcd046bdd9d89 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.default.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.default.pass.cpp
@@ -60,8 +60,6 @@ constexpr bool test() {
 
 void testException() {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-  struct Except {};
-
   struct Throwing {
     Throwing() { throw Except{}; };
   };
diff --git a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.inplace.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.inplace.pass.cpp
index 0b41eb44883e4..88ec41939439a 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.inplace.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.inplace.pass.cpp
@@ -115,8 +115,6 @@ constexpr bool test() {
 
 void testException() {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-  struct Except {};
-
   struct Throwing {
     Throwing(int) { throw Except{}; };
   };
diff --git a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.move.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.move.pass.cpp
index aaf357fbc518a..cd89e2445860a 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.move.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.move.pass.cpp
@@ -135,8 +135,6 @@ constexpr bool test() {
 
 void testException() {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-  struct Except {};
-
   struct Throwing {
     Throwing() = default;
     Throwing(Throwing&&) { throw Except{}; }
diff --git a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.convert.copy.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.convert.copy.pass.cpp
index 30533206e65a3..05f556e25eac1 100644
--- a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.convert.copy.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.convert.copy.pass.cpp
@@ -111,8 +111,6 @@ constexpr bool test() {
 
 void testException() {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-  struct Except {};
-
   struct ThrowingInt {
     ThrowingInt(int) { throw Except{}; }
   };
diff --git a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.convert.move.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.convert.move.pass.cpp
index d9db49c273831..a48888be53ee0 100644
--- a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.convert.move.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.convert.move.pass.cpp
@@ -112,8 +112,6 @@ constexpr bool test() {
 
 void testException() {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-  struct Except {};
-
   struct ThrowingInt {
     ThrowingInt(int) { throw Except{}; }
   };
diff --git a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.copy.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.copy.pass.cpp
index 83e049857a93d..7c04a5fa9d044 100644
--- a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.copy.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.copy.pass.cpp
@@ -76,8 +76,6 @@ constexpr bool test() {
 
 void testException() {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-  struct Except {};
-
   struct Throwing {
     Throwing() = default;
     Throwing(const Throwing&) { throw Except{}; }
diff --git a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.move.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.move.pass.cpp
index 777b412f65f64..bfb5028c9264d 100644
--- a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.move.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.move.pass.cpp
@@ -91,8 +91,6 @@ constexpr bool test() {
 
 void testException() {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-  struct Except {};
-
   struct Throwing {
     Throwing() = default;
     Throwing(Throwing&&) { throw Except{}; }
diff --git a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpect.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpect.pass.cpp
index 62c3817a8a66f..85bc98d7f462d 100644
--- a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpect.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpect.pass.cpp
@@ -115,8 +115,6 @@ constexpr bool test() {
 
 void testException() {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-  struct Except {};
-
   struct Throwing {
     Throwing(int) { throw Except{}; };
   };
diff --git a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpect_init_list.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpect_init_list.pass.cpp
index 28b4165bc27dc..4128668a6b07b 100644
--- a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpect_init_list.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpect_init_list.pass.cpp
@@ -101,8 +101,6 @@ constexpr bool test() {
 
 void testException() {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-  struct Except {};
-
   struct Throwing {
     Throwing(std::initializer_list<int>, int) { throw Except{}; };
   };
diff --git a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpected.copy.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpected.copy.pass.cpp
index 17063a97cb133..ba738a3e339de 100644
--- a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpected.copy.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpected.copy.pass.cpp
@@ -67,8 +67,6 @@ constexpr bool test() {
 
 void testException() {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-  struct Except {};
-
   struct Throwing {
     Throwing(int) { throw Except{}; }
   };
diff --git a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpected.move.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpected.move.pass.cpp
index 792ef341879ea..33a5e7293df21 100644
--- a/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpected.move.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.void/ctor/ctor.unexpected.move.pass.cpp
@@ -77,8 +77,6 @@ constexpr bool test() {
 
 void testException() {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-  struct Except {};
-
   struct Throwing {
     Throwing(int) { throw Except{}; }
   };

From c54292e74fe026bcdc8882ef0604677dd2c760e2 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller@gmail.com>
Date: Sun, 22 Oct 2023 09:23:43 +0200
Subject: [PATCH 21/28] remove more declarations of local Except structs

---
 .../expected.expected/ctor/ctor.inplace_init_list.pass.cpp      | 2 --
 .../utilities/expected/expected.expected/ctor/ctor.u.pass.cpp   | 2 --
 .../expected/expected.expected/ctor/ctor.unexpect.pass.cpp      | 2 --
 .../expected.expected/ctor/ctor.unexpect_init_list.pass.cpp     | 2 --
 .../expected.expected/ctor/ctor.unexpected.copy.pass.cpp        | 2 --
 .../expected.expected/ctor/ctor.unexpected.move.pass.cpp        | 2 --
 6 files changed, 12 deletions(-)

diff --git a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.inplace_init_list.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.inplace_init_list.pass.cpp
index 143f9d7dab113..a97086fcc9195 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.inplace_init_list.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.inplace_init_list.pass.cpp
@@ -102,8 +102,6 @@ constexpr bool test() {
 
 void testException() {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-  struct Except {};
-
   struct Throwing {
     Throwing(std::initializer_list<int>, int) { throw Except{}; };
   };
diff --git a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.u.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.u.pass.cpp
index 16dd531a410c2..1cf3d9cc2ef49 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.u.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.u.pass.cpp
@@ -157,8 +157,6 @@ constexpr bool test() {
 
 void testException() {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-  struct Except {};
-
   struct Throwing {
     Throwing(int) { throw Except{}; };
   };
diff --git a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpect.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpect.pass.cpp
index a0b334a25b0cb..27ce97737d288 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpect.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpect.pass.cpp
@@ -115,8 +115,6 @@ constexpr bool test() {
 
 void testException() {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-  struct Except {};
-
   struct Throwing {
     Throwing(int) { throw Except{}; };
   };
diff --git a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpect_init_list.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpect_init_list.pass.cpp
index 4c280fc32a4e9..4f5d3d1492d37 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpect_init_list.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpect_init_list.pass.cpp
@@ -102,8 +102,6 @@ constexpr bool test() {
 
 void testException() {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-  struct Except {};
-
   struct Throwing {
     Throwing(std::initializer_list<int>, int) { throw Except{}; };
   };
diff --git a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpected.copy.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpected.copy.pass.cpp
index b3bb71cf36837..bbfd3048533c7 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpected.copy.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpected.copy.pass.cpp
@@ -67,8 +67,6 @@ constexpr bool test() {
 
 void testException() {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-  struct Except {};
-
   struct Throwing {
     Throwing(int) { throw Except{}; }
   };
diff --git a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpected.move.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpected.move.pass.cpp
index 419c789c86d51..800d47bda6958 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpected.move.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.unexpected.move.pass.cpp
@@ -77,8 +77,6 @@ constexpr bool test() {
 
 void testException() {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-  struct Except {};
-
   struct Throwing {
     Throwing(int) { throw Except{}; }
   };

From 737f6dd0af3b62fdd8af48451f272f781c858cbc Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller@gmail.com>
Date: Fri, 27 Oct 2023 21:41:06 +0200
Subject: [PATCH 22/28] just return a reference to the union value in emplace()

---
 libcxx/include/__expected/expected.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/libcxx/include/__expected/expected.h b/libcxx/include/__expected/expected.h
index f874a3a126a58..bf16c8f720d26 100644
--- a/libcxx/include/__expected/expected.h
+++ b/libcxx/include/__expected/expected.h
@@ -402,7 +402,7 @@ class expected {
     }
     std::construct_at(std::addressof(__union_.__val_), std::forward<_Args>(__args)...);
     __has_val_ = true;
-    return *std::addressof(__union_.__val_);
+    return __union_.__val_;
   }
 
   template <class _Up, class... _Args>
@@ -415,7 +415,7 @@ class expected {
     }
     std::construct_at(std::addressof(__union_.__val_), __il, std::forward<_Args>(__args)...);
     __has_val_ = true;
-    return *std::addressof(__union_.__val_);
+    return __union_.__val_;
   }
 
 

From 63e4c457064caefd8051e7de88f762c45b969a18 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller@gmail.com>
Date: Fri, 27 Oct 2023 21:58:16 +0200
Subject: [PATCH 23/28] use LIBCPP_STATIC_ASSERT to check size of std::expected
 containing a TailClobberer

---
 .../expected/expected.expected/observers/has_value.pass.cpp     | 2 +-
 .../expected/expected.void/observers/has_value.pass.cpp         | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/libcxx/test/std/utilities/expected/expected.expected/observers/has_value.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/observers/has_value.pass.cpp
index 0e19da0655946..2b24b0ac24ddb 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/observers/has_value.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/observers/has_value.pass.cpp
@@ -70,7 +70,7 @@ constexpr bool test() {
     const std::expected<TailClobberer<0>, bool> e = {};
     // clang-cl does not support [[no_unique_address]] yet.
 #if !(defined(TEST_COMPILER_CLANG) && defined(_MSC_VER))
-    static_assert(sizeof(TailClobberer<0>) == sizeof(e));
+    LIBCPP_STATIC_ASSERT(sizeof(TailClobberer<0>) == sizeof(e));
 #endif
     assert(e.has_value());
   }
diff --git a/libcxx/test/std/utilities/expected/expected.void/observers/has_value.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/observers/has_value.pass.cpp
index 06a45ed430180..fe92bb401643d 100644
--- a/libcxx/test/std/utilities/expected/expected.void/observers/has_value.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.void/observers/has_value.pass.cpp
@@ -50,7 +50,7 @@ constexpr bool test() {
     const std::expected<void, TailClobberer<1>> e(std::unexpect);
     // clang-cl does not support [[no_unique_address]] yet.
 #if !(defined(TEST_COMPILER_CLANG) && defined(_MSC_VER))
-    static_assert(sizeof(TailClobberer<1>) == sizeof(e));
+    LIBCPP_STATIC_ASSERT(sizeof(TailClobberer<1>) == sizeof(e));
 #endif
     assert(!e.has_value());
   }

From d5c5626d52eee3c4709ea5d14b8952208dd7615c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller@gmail.com>
Date: Mon, 30 Oct 2023 11:11:15 +0100
Subject: [PATCH 24/28] add swap tests involving TailClobberer

---
 .../expected.expected/swap/free.swap.pass.cpp | 62 ++++++++++++++
 .../swap/member.swap.pass.cpp                 | 62 ++++++++++++++
 .../expected.void/swap/free.swap.pass.cpp     | 28 ++++++
 .../expected.void/swap/member.swap.pass.cpp   | 28 ++++++
 libcxx/test/std/utilities/expected/types.h    | 85 ++++++++++---------
 5 files changed, 227 insertions(+), 38 deletions(-)

diff --git a/libcxx/test/std/utilities/expected/expected.expected/swap/free.swap.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/swap/free.swap.pass.cpp
index 15c66d2b75076..ca612a84fee30 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/swap/free.swap.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/swap/free.swap.pass.cpp
@@ -180,6 +180,35 @@ constexpr bool test() {
     assert(!e2.error().swapCalled);
   }
 
+  // TailClobberer
+  {
+    // is_nothrow_move_constructible_v<E>
+    {
+      std::expected<TailClobbererNonTrivialMove<0, true>, TailClobbererNonTrivialMove<1, true>> x(std::in_place);
+      std::expected<TailClobbererNonTrivialMove<0, true>, TailClobbererNonTrivialMove<1, true>> y(std::unexpect);
+
+      swap(x, y);
+
+      // Both of these would fail if adjusting the "has value" flags happened
+      // _before_ constructing the member objects inside the `swap`.
+      assert(!x.has_value());
+      assert(y.has_value());
+    }
+
+    // !is_nothrow_move_constructible_v<E>
+    {
+      std::expected<TailClobbererNonTrivialMove<0, true>, TailClobbererNonTrivialMove<1, false>> x(std::in_place);
+      std::expected<TailClobbererNonTrivialMove<0, true>, TailClobbererNonTrivialMove<1, false>> y(std::unexpect);
+
+      swap(x, y);
+
+      // Both of these would fail if adjusting the "has value" flags happened
+      // _before_ constructing the member objects inside the `swap`.
+      assert(!x.has_value());
+      assert(y.has_value());
+    }
+  }
+
   return true;
 }
 
@@ -210,6 +239,39 @@ void testException() {
       assert(*e1 == 5);
     }
   }
+
+  // TailClobberer
+  {
+    // is_nothrow_move_constructible_v<E>
+    {
+      std::expected<TailClobbererNonTrivialMove<0, false, true>, TailClobbererNonTrivialMove<1>> x(std::in_place);
+      std::expected<TailClobbererNonTrivialMove<0, false, true>, TailClobbererNonTrivialMove<1>> y(std::unexpect);
+      try {
+        swap(x, y);
+        assert(false);
+      } catch (Except) {
+        assert(x.has_value());
+        // This would fail if `TailClobbererNonTrivialMove<1>` clobbered the
+        // flag when rolling back the swap.
+        assert(!y.has_value());
+      }
+    }
+
+    // !is_nothrow_move_constructible_v<E>
+    {
+      std::expected<TailClobbererNonTrivialMove<0>, TailClobbererNonTrivialMove<1, false, true>> x(std::in_place);
+      std::expected<TailClobbererNonTrivialMove<0>, TailClobbererNonTrivialMove<1, false, true>> y(std::unexpect);
+      try {
+        swap(x, y);
+        assert(false);
+      } catch (Except) {
+        // This would fail if `TailClobbererNonTrivialMove<0>` clobbered the
+        // flag when rolling back the swap.
+        assert(x.has_value());
+        assert(!y.has_value());
+      }
+    }
+  }
 #endif // TEST_HAS_NO_EXCEPTIONS
 }
 
diff --git a/libcxx/test/std/utilities/expected/expected.expected/swap/member.swap.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/swap/member.swap.pass.cpp
index d2d4a09922092..97b871ed9cffe 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/swap/member.swap.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/swap/member.swap.pass.cpp
@@ -198,6 +198,35 @@ constexpr bool test() {
     assert(!e2.error().swapCalled);
   }
 
+  // TailClobberer
+  {
+    // is_nothrow_move_constructible_v<E>
+    {
+      std::expected<TailClobbererNonTrivialMove<0, true>, TailClobbererNonTrivialMove<1, true>> x(std::in_place);
+      std::expected<TailClobbererNonTrivialMove<0, true>, TailClobbererNonTrivialMove<1, true>> y(std::unexpect);
+
+      x.swap(y);
+
+      // Both of these would fail if adjusting the "has value" flags happened
+      // _before_ constructing the member objects inside the `swap`.
+      assert(!x.has_value());
+      assert(y.has_value());
+    }
+
+    // !is_nothrow_move_constructible_v<E>
+    {
+      std::expected<TailClobbererNonTrivialMove<0, true>, TailClobbererNonTrivialMove<1, false>> x(std::in_place);
+      std::expected<TailClobbererNonTrivialMove<0, true>, TailClobbererNonTrivialMove<1, false>> y(std::unexpect);
+
+      x.swap(y);
+
+      // Both of these would fail if adjusting the "has value" flags happened
+      // _before_ constructing the member objects inside the `swap`.
+      assert(!x.has_value());
+      assert(y.has_value());
+    }
+  }
+
   return true;
 }
 
@@ -228,6 +257,39 @@ void testException() {
       assert(*e1 == 5);
     }
   }
+
+  // TailClobberer
+  {
+    // is_nothrow_move_constructible_v<E>
+    {
+      std::expected<TailClobbererNonTrivialMove<0, false, true>, TailClobbererNonTrivialMove<1>> x(std::in_place);
+      std::expected<TailClobbererNonTrivialMove<0, false, true>, TailClobbererNonTrivialMove<1>> y(std::unexpect);
+      try {
+        x.swap(y);
+        assert(false);
+      } catch (Except) {
+        assert(x.has_value());
+        // This would fail if `TailClobbererNonTrivialMove<1>` clobbered the
+        // flag when rolling back the swap.
+        assert(!y.has_value());
+      }
+    }
+
+    // !is_nothrow_move_constructible_v<E>
+    {
+      std::expected<TailClobbererNonTrivialMove<0>, TailClobbererNonTrivialMove<1, false, true>> x(std::in_place);
+      std::expected<TailClobbererNonTrivialMove<0>, TailClobbererNonTrivialMove<1, false, true>> y(std::unexpect);
+      try {
+        x.swap(y);
+        assert(false);
+      } catch (Except) {
+        // This would fail if `TailClobbererNonTrivialMove<0>` clobbered the
+        // flag when rolling back the swap.
+        assert(x.has_value());
+        assert(!y.has_value());
+      }
+    }
+  }
 #endif // TEST_HAS_NO_EXCEPTIONS
 }
 
diff --git a/libcxx/test/std/utilities/expected/expected.void/swap/free.swap.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/swap/free.swap.pass.cpp
index ad1eb3f0bd20d..d49485f0d402b 100644
--- a/libcxx/test/std/utilities/expected/expected.void/swap/free.swap.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.void/swap/free.swap.pass.cpp
@@ -117,6 +117,19 @@ constexpr bool test() {
     assert(s.dtorCalled);
   }
 
+  // TailClobberer
+  {
+    std::expected<void, TailClobbererNonTrivialMove<1>> x(std::in_place);
+    std::expected<void, TailClobbererNonTrivialMove<1>> y(std::unexpect);
+
+    swap(x, y);
+
+    // The next line would fail if adjusting the "has value" flag happened
+    // _before_ constructing the member object inside the `swap`.
+    assert(!x.has_value());
+    assert(y.has_value());
+  }
+
   return true;
 }
 
@@ -151,6 +164,21 @@ void testException() {
       assert(!e2Destroyed);
     }
   }
+
+  // TailClobberer
+  {
+    std::expected<void, TailClobbererNonTrivialMove<0, false, true>> x(std::in_place);
+    std::expected<void, TailClobbererNonTrivialMove<0, false, true>> y(std::unexpect);
+    try {
+      swap(x, y);
+      assert(false);
+    } catch (Except) {
+      // This would fail if `TailClobbererNonTrivialMove<0, false, true>`
+      // clobbered the flag before throwing the exception.
+      assert(x.has_value());
+      assert(!y.has_value());
+    }
+  }
 #endif // TEST_HAS_NO_EXCEPTIONS
 }
 
diff --git a/libcxx/test/std/utilities/expected/expected.void/swap/member.swap.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/swap/member.swap.pass.cpp
index a71c701a46930..166dca558f99b 100644
--- a/libcxx/test/std/utilities/expected/expected.void/swap/member.swap.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.void/swap/member.swap.pass.cpp
@@ -126,6 +126,19 @@ constexpr bool test() {
     assert(s.dtorCalled);
   }
 
+  // TailClobberer
+  {
+    std::expected<void, TailClobbererNonTrivialMove<1>> x(std::in_place);
+    std::expected<void, TailClobbererNonTrivialMove<1>> y(std::unexpect);
+
+    x.swap(y);
+
+    // The next line would fail if adjusting the "has value" flag happened
+    // _before_ constructing the member object inside the `swap`.
+    assert(!x.has_value());
+    assert(y.has_value());
+  }
+
   return true;
 }
 
@@ -160,6 +173,21 @@ void testException() {
       assert(!e2Destroyed);
     }
   }
+
+  // TailClobberer
+  {
+    std::expected<void, TailClobbererNonTrivialMove<0, false, true>> x(std::in_place);
+    std::expected<void, TailClobbererNonTrivialMove<0, false, true>> y(std::unexpect);
+    try {
+      x.swap(y);
+      assert(false);
+    } catch (Except) {
+      // This would fail if `TailClobbererNonTrivialMove<0, false, true>`
+      // clobbered the flag before throwing the exception.
+      assert(x.has_value());
+      assert(!y.has_value());
+    }
+  }
 #endif // TEST_HAS_NO_EXCEPTIONS
 }
 
diff --git a/libcxx/test/std/utilities/expected/types.h b/libcxx/test/std/utilities/expected/types.h
index 58c2df504bfb2..d2224cf869dfa 100644
--- a/libcxx/test/std/utilities/expected/types.h
+++ b/libcxx/test/std/utilities/expected/types.h
@@ -104,44 +104,6 @@ struct TrackedMove {
   }
 };
 
-// This type has one byte of tail padding where `std::expected` may put its
-// "has value" flag. The constructor will clobber all bytes including the
-// tail padding. With this type we can check that `std::expected` handles
-// the case where the "has value" flag is an overlapping subobject correctly.
-//
-// See https://github.com/llvm/llvm-project/issues/68552 for details.
-template <int constant>
-struct TailClobberer {
-  constexpr TailClobberer() noexcept {
-    if (!std::is_constant_evaluated()) {
-      std::memset(this, constant, sizeof(*this));
-    }
-    // Always set `b` itself to `false` so that the comparison works.
-    b = false;
-  }
-  constexpr TailClobberer(const TailClobberer&) : TailClobberer() {}
-  constexpr TailClobberer(TailClobberer&&) = default;
-  // Converts from `int`/`std::initializer_list<int>, used in some tests.
-  constexpr TailClobberer(int) : TailClobberer() {}
-  constexpr TailClobberer(std::initializer_list<int>) noexcept : TailClobberer() {}
-
-  friend constexpr bool operator==(const TailClobberer&, const TailClobberer&) = default;
-
-private:
-  alignas(2) bool b;
-};
-static_assert(!std::is_trivially_copy_constructible_v<TailClobberer<0>>);
-static_assert(std::is_trivially_move_constructible_v<TailClobberer<0>>);
-
-template <int constant>
-struct TailClobbererNonTrivialMove : TailClobberer<constant> {
-  using TailClobberer<constant>::TailClobberer;
-  constexpr TailClobbererNonTrivialMove(TailClobbererNonTrivialMove&&) noexcept : TailClobberer<constant>() {}
-};
-static_assert(!std::is_trivially_copy_constructible_v<TailClobbererNonTrivialMove<0>>);
-static_assert(std::is_move_constructible_v<TailClobbererNonTrivialMove<0>>);
-static_assert(!std::is_trivially_move_constructible_v<TailClobbererNonTrivialMove<0>>);
-
 #ifndef TEST_HAS_NO_EXCEPTIONS
 struct Except {};
 
@@ -190,4 +152,51 @@ struct MoveOnlyErrorType {
   MoveOnlyErrorType& operator=(const MoveOnlyErrorType&) = delete;
 };
 
+// This type has one byte of tail padding where `std::expected` may put its
+// "has value" flag. The constructor will clobber all bytes including the
+// tail padding. With this type we can check that `std::expected` handles
+// the case where the "has value" flag is an overlapping subobject correctly.
+//
+// See https://github.com/llvm/llvm-project/issues/68552 for details.
+template <int Constant>
+struct TailClobberer {
+  constexpr TailClobberer() noexcept {
+    if (!std::is_constant_evaluated()) {
+      std::memset(this, Constant, sizeof(*this));
+    }
+    // Always set `b` itself to `false` so that the comparison works.
+    b = false;
+  }
+  constexpr TailClobberer(const TailClobberer&) : TailClobberer() {}
+  constexpr TailClobberer(TailClobberer&&) = default;
+  // Converts from `int`/`std::initializer_list<int>, used in some tests.
+  constexpr TailClobberer(int) : TailClobberer() {}
+  constexpr TailClobberer(std::initializer_list<int>) noexcept : TailClobberer() {}
+
+  friend constexpr bool operator==(const TailClobberer&, const TailClobberer&) = default;
+
+  friend constexpr void swap(TailClobberer&, TailClobberer&){};
+
+private:
+  alignas(2) bool b;
+};
+static_assert(!std::is_trivially_copy_constructible_v<TailClobberer<0>>);
+static_assert(std::is_trivially_move_constructible_v<TailClobberer<0>>);
+
+template <int Constant, bool Noexcept = true, bool ThrowOnMove = false>
+struct TailClobbererNonTrivialMove : TailClobberer<Constant> {
+  using TailClobberer<Constant>::TailClobberer;
+  constexpr TailClobbererNonTrivialMove(TailClobbererNonTrivialMove&&) noexcept(Noexcept) : TailClobberer<Constant>() {
+#ifndef TEST_HAS_NO_EXCEPTIONS
+    if (ThrowOnMove)
+      throw Except{};
+#endif
+  }
+};
+static_assert(!std::is_trivially_copy_constructible_v<TailClobbererNonTrivialMove<0>>);
+static_assert(std::is_move_constructible_v<TailClobbererNonTrivialMove<0>>);
+static_assert(!std::is_trivially_move_constructible_v<TailClobbererNonTrivialMove<0>>);
+static_assert(std::is_nothrow_move_constructible_v<TailClobbererNonTrivialMove<0, true>>);
+static_assert(!std::is_nothrow_move_constructible_v<TailClobbererNonTrivialMove<0, false>>);
+
 #endif // TEST_STD_UTILITIES_EXPECTED_TYPES_H

From 8e866c64045f12fcd8393d255227843179645dee Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller@gmail.com>
Date: Mon, 30 Oct 2023 11:12:18 +0100
Subject: [PATCH 25/28] drive by fix: these should be free swaps

---
 .../expected/expected.expected/swap/free.swap.pass.cpp        | 2 +-
 .../utilities/expected/expected.void/swap/free.swap.pass.cpp  | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/libcxx/test/std/utilities/expected/expected.expected/swap/free.swap.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/swap/free.swap.pass.cpp
index ca612a84fee30..3c03efd832919 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/swap/free.swap.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/swap/free.swap.pass.cpp
@@ -129,7 +129,7 @@ constexpr bool test() {
     std::expected<TrackedMove<true>, TrackedMove<false>> e1(std::in_place, 5);
     std::expected<TrackedMove<true>, TrackedMove<false>> e2(std::unexpect, 10);
 
-    e1.swap(e2);
+    swap(e1, e2);
 
     assert(!e1.has_value());
     assert(e1.error().i == 10);
diff --git a/libcxx/test/std/utilities/expected/expected.void/swap/free.swap.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/swap/free.swap.pass.cpp
index d49485f0d402b..f7314c79fa6db 100644
--- a/libcxx/test/std/utilities/expected/expected.void/swap/free.swap.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.void/swap/free.swap.pass.cpp
@@ -91,7 +91,7 @@ constexpr bool test() {
     std::expected<void, Traced> e1(std::in_place);
     std::expected<void, Traced> e2(std::unexpect, s, 10);
 
-    e1.swap(e2);
+    swap(e1, e2);
 
     assert(!e1.has_value());
     assert(e1.error().data_ == 10);
@@ -107,7 +107,7 @@ constexpr bool test() {
     std::expected<void, Traced> e1(std::unexpect, s, 10);
     std::expected<void, Traced> e2(std::in_place);
 
-    e1.swap(e2);
+    swap(e1, e2);
 
     assert(e1.has_value());
     assert(!e2.has_value());

From c637421afaf016fbf06f8b9963368d6788fedb69 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller@gmail.com>
Date: Mon, 30 Oct 2023 11:12:44 +0100
Subject: [PATCH 26/28] make both member.swap.pass.cpp files clang-format
 friendly

---
 .../expected/expected.expected/swap/member.swap.pass.cpp        | 2 +-
 .../utilities/expected/expected.void/swap/member.swap.pass.cpp  | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/libcxx/test/std/utilities/expected/expected.expected/swap/member.swap.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/swap/member.swap.pass.cpp
index 97b871ed9cffe..b6b112cfbeb8b 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/swap/member.swap.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/swap/member.swap.pass.cpp
@@ -70,7 +70,7 @@ static_assert(!HasMemberSwap<MoveMayThrow, MoveMayThrow>);
 
 // Test noexcept
 template <class T, class E>
-concept MemberSwapNoexcept =
+concept MemberSwapNoexcept = //
     requires(std::expected<T, E> x, std::expected<T, E> y) {
       { x.swap(y) } noexcept;
     };
diff --git a/libcxx/test/std/utilities/expected/expected.void/swap/member.swap.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/swap/member.swap.pass.cpp
index 166dca558f99b..70e004abef684 100644
--- a/libcxx/test/std/utilities/expected/expected.void/swap/member.swap.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.void/swap/member.swap.pass.cpp
@@ -52,7 +52,7 @@ struct MoveMayThrow {
 };
 
 template <class E>
-concept MemberSwapNoexcept =
+concept MemberSwapNoexcept = //
     requires(std::expected<void, E> x, std::expected<void, E> y) {
       { x.swap(y) } noexcept;
     };

From 19cfd8a5a8eff7d34184bce66886a449a58de1bf Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller@gmail.com>
Date: Mon, 30 Oct 2023 11:42:35 +0100
Subject: [PATCH 27/28] try to avoid -Werror=terminate error on GCC

---
 libcxx/test/std/utilities/expected/types.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libcxx/test/std/utilities/expected/types.h b/libcxx/test/std/utilities/expected/types.h
index d2224cf869dfa..70b0e4ec6f9fa 100644
--- a/libcxx/test/std/utilities/expected/types.h
+++ b/libcxx/test/std/utilities/expected/types.h
@@ -188,7 +188,7 @@ struct TailClobbererNonTrivialMove : TailClobberer<Constant> {
   using TailClobberer<Constant>::TailClobberer;
   constexpr TailClobbererNonTrivialMove(TailClobbererNonTrivialMove&&) noexcept(Noexcept) : TailClobberer<Constant>() {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-    if (ThrowOnMove)
+    if (!Noexcept && ThrowOnMove)
       throw Except{};
 #endif
   }

From 05f7c85be795504de40a84657e17bba9b6679b55 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller@gmail.com>
Date: Mon, 30 Oct 2023 12:09:42 +0100
Subject: [PATCH 28/28] use if constexpr

---
 libcxx/test/std/utilities/expected/types.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libcxx/test/std/utilities/expected/types.h b/libcxx/test/std/utilities/expected/types.h
index 70b0e4ec6f9fa..ac4a82f2baf78 100644
--- a/libcxx/test/std/utilities/expected/types.h
+++ b/libcxx/test/std/utilities/expected/types.h
@@ -188,7 +188,7 @@ struct TailClobbererNonTrivialMove : TailClobberer<Constant> {
   using TailClobberer<Constant>::TailClobberer;
   constexpr TailClobbererNonTrivialMove(TailClobbererNonTrivialMove&&) noexcept(Noexcept) : TailClobberer<Constant>() {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-    if (!Noexcept && ThrowOnMove)
+    if constexpr (!Noexcept && ThrowOnMove)
       throw Except{};
 #endif
   }