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

[via libcxx-commits]

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@@ -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
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philnik777 wrote:

I think this should be fine. IIUC the type is allowed to overwrite padding bytes during construction.

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