[libcxx-commits] [libcxx] [libc++] Fix strict aliasing violation for `deque::const_iterator` (PR #136067)

[A. Jiang via libcxx-commits]

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@@ -110,6 +110,13 @@ ABI Affecting Changes
 - The ABI flag ``_LIBCPP_ABI_NO_REVERSE_ITERATOR_SECOND_MEMBER`` has been split off from
   ``_LIBCPP_ABI_NO_ITERATOR_BASES``. If you are using this flag and care about ABI stability, you should set
   ``_LIBCPP_ABI_NO_REVERSE_ITERATOR_SECOND_MEMBER`` as well.
+- The ``const_iterator`` member type of ``std::deque`` is now corrected to hold a (possibly fancy) pointer to the
+  (possibly fancy) pointer allocated in the internal map. E.g. when the allocators use fancy pointers, the internal map
+  stores ``fancy_ptr<T>`` objects, and the previous strategy accessed these objects via ``const fancy_ptr<const T>``
+  lvalues, caused undefined behavior. Now ``const_iterator`` stores
+  ``fancy_ptr<const fancy_ptr<T>>`` instead of ``fancy_ptr<const fancy_ptr<const T>>``, and ABI break can happen when
+  such two types have incompatible layouts. This is necessary for reducing undefined behavior and ``constexpr`` support
+  for ``deque`` in C++26, so we do not provide any way to opt-out of that behavior.
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frederick-vs-ja wrote:

I think there can be ABI break if the fancy pointers are designed weirdly and `deque::const_iterator` is permanently stored. The intent should be consistent with #100585.

https://github.com/llvm/llvm-project/pull/136067