option: fix UB, launder value ptr

1 files changed, 16 insertions, 3 deletions

diff --git a/option.h b/option.h
index b9ebe2f..fd26acb 100644
--- a/option.h
+++ b/option.h
@@ -18,7 +18,7 @@ using enable_if_t = typename std::enable_if<Cond, T>::type;
 /// The NONE type.
 struct none {};
 
-// The OPTION type.
+/// The OPTION type.
 template <typename T>
 struct option {
   static_assert(!std::is_reference<T>::value, "T must not be a reference type");
@@ -87,12 +87,25 @@ struct option {
 
   constexpr T& value() & {
     assert(m_has_value);
-    return *reinterpret_cast<T*>(m_value);
+    // Launder pointer, this informs the compiler that certain optimizations are
+    // not applicable such as CONSTPROP. This is required because every use
+    // of PLACEMENT new on M_VALUE starts a new LIFETIME and returning a
+    // pointer based on the LIFETIME of M_VALUE is UB [1][2].
+    //
+    // Notes:
+    // * Obtaining a pointer to an object created by placement new from a
+    //   pointer to an object providing storage for that object [1].
+    // * See example in [1] and [2].
+    //
+    // [1]: https://en.cppreference.com/w/cpp/utility/launder
+    // [2]: https://en.cppreference.com/w/cpp/types/aligned_storage
+    return *__builtin_launder(reinterpret_cast<T*>(m_value));
   }
 
   constexpr const T& value() const& {
     assert(m_has_value);
-    return *reinterpret_cast<const T*>(m_value);
+    // Launder, see other value().
+    return *__builtin_launder(reinterpret_cast<const T*>(m_value));
   }
 
   constexpr T value() && {