[libc++] Fix UB in <expected> related to "has value" flag (#68552) (#68733)

The calls to std::construct_at might overwrite the previously set
__has_value_ flag in the case where the flag is overlapping with
the actual value or error being stored (since we use [[no_unique_address]]).
To fix this issue, this patch ensures that we initialize the
__has_value_ flag after we call std::construct_at.

Fixes #68552

Author: jiixyj

libcxx/include/__expected/expected.h

@@ -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;
 }
 

libcxx/test/std/utilities/expected/expected.expected/assign/emplace.intializer_list.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;
 }
 

libcxx/test/std/utilities/expected/expected.expected/assign/emplace.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>
@@ -161,13 +162,19 @@ 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;
 }
 
 void testException() {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-  struct Except {};
-
   struct ThrowingInt {
     ThrowingInt(int) { throw Except{}; }
   };

libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.convert.copy.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>
@@ -160,13 +161,19 @@ 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;
 }
 
 void testException() {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-  struct Except {};
-
   struct ThrowingInt {
     ThrowingInt(int) { throw Except{}; }
   };

libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.convert.move.pass.cpp

@@ -30,6 +30,7 @@
 #include <utility>
 
 #include "test_macros.h"
+#include "../../types.h"
 
 struct NonCopyable {
   NonCopyable(const NonCopyable&) = delete;
@@ -93,13 +94,26 @@ 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;
 }
 
 void testException() {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-  struct Except {};
-
   struct Throwing {
     Throwing() = default;
     Throwing(const Throwing&) { throw Except{}; }

libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.copy.pass.cpp

@@ -22,6 +22,7 @@
 #include <type_traits>
 
 #include "test_macros.h"
+#include "../../types.h"
 
 struct NoDedefaultCtor {
   NoDedefaultCtor() = delete;
@@ -45,20 +46,20 @@ constexpr void testDefaultCtor() {
 
 template <class T>
 constexpr void testTypes() {
+  testDefaultCtor<T, bool>();
   testDefaultCtor<T, int>();
   testDefaultCtor<T, NoDedefaultCtor>();
 }
 
 constexpr bool test() {
   testTypes<int>();
   testTypes<MyInt>();
+  testTypes<TailClobberer<0>>();
   return true;
 }
 
 void testException() {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-  struct Except {};
-
   struct Throwing {
     Throwing() { throw Except{}; };
   };

libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.default.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
   {
@@ -111,8 +115,6 @@ constexpr bool test() {
 
 void testException() {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-  struct Except {};
-
   struct Throwing {
     Throwing(int) { throw Except{}; };
   };

libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.inplace.pass.cpp

@@ -28,6 +28,7 @@
 
 #include "MoveOnly.h"
 #include "test_macros.h"
+#include "../../types.h"
 
 // Test Constraints:
 static_assert(
@@ -90,13 +91,17 @@ 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;
 }
 
 void testException() {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-  struct Except {};
-
   struct Throwing {
     Throwing(std::initializer_list<int>, int) { throw Except{}; };
   };

libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.inplace_init_list.pass.cpp

@@ -32,6 +32,7 @@
 #include <utility>
 
 #include "test_macros.h"
+#include "../../types.h"
 
 struct NonMovable {
   NonMovable(NonMovable&&) = delete;
@@ -112,13 +113,28 @@ 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());
+    assert(e1.has_value());
+  }
+
+  // move TailClobbererNonTrivialMove as error
+  {
+    std::expected<bool, TailClobbererNonTrivialMove<1>> e1(std::unexpect);
+    auto e2 = std::move(e1);
+    assert(!e2.has_value());
+    assert(!e1.has_value());
+  }
+
   return true;
 }
 
 void testException() {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-  struct Except {};
-
   struct Throwing {
     Throwing() = default;
     Throwing(Throwing&&) { throw Except{}; }

libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.move.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
@@ -153,8 +157,6 @@ constexpr bool test() {
 
 void testException() {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-  struct Except {};
-
   struct Throwing {
     Throwing(int) { throw Except{}; };
   };