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

[Louis Dionne via libcxx-commits]

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


================
@@ -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.
----------------
ldionne wrote:

I would adjust the comment like this so it also applies to other implementations (our test suite is used by others stdlibs too).

```suggestion
// 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.
```

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