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

[Louis Dionne via libcxx-commits]

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Jan =?utf-8?q?Kokem=C3=BCller?= <jan.kokemueller at gmail.com>,
Jan =?utf-8?q?Kokem=C3=BCller?= <jan.kokemueller at gmail.com>,
Jan =?utf-8?q?Kokem=C3=BCller?= <jan.kokemueller at gmail.com>,
Jan =?utf-8?q?Kokem=C3=BCller?= <jan.kokemueller at gmail.com>,
Jan =?utf-8?q?Kokem=C3=BCller?= <jan.kokemueller at gmail.com>,
Jan =?utf-8?q?Kokem=C3=BCller?= <jan.kokemueller at gmail.com>
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In-Reply-To: <llvm/llvm-project/pull/68733/libcxx at github.com>


================
@@ -43,6 +45,38 @@ constexpr bool test() {
     assert(!e.has_value());
   }
 
+  // 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.
+  {
+#if !defined(TEST_COMPILER_CLANG) || TEST_CLANG_VER >= 1600
----------------
ldionne wrote:

Why is this `#if` required?

https://github.com/llvm/llvm-project/pull/68733