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[libc++] Ensure that std::expected has no tail padding (#69673)
Currently std::expected can have some padding bytes in its tail due to [[no_unique_address]]. Those padding bytes can be used by other objects. For example, in the current implementation: sizeof(std::expected<std::optional<int>, bool>) == sizeof(std::expected<std::expected<std::optional<int>, bool>, bool>) As a result, the data layout of an std::expected<std::expected<std::optional<int>, bool>, bool> can look like this: +-- optional "has value" flag | +--padding /---int---\ | | 00 00 00 00 01 00 00 00 | | | +- "outer" expected "has value" flag | +- expected "has value" flag This is problematic because `emplace()`ing the "inner" expected can not only overwrite the "inner" expected "has value" flag (issue #68552) but also the tail padding where other objects might live. This patch fixes the problem by ensuring that std::expected has no tail padding, which is achieved by conditional usage of [[no_unique_address]] based on the tail padding that this would create. This is an ABI breaking change because the following property changes: sizeof(std::expected<std::optional<int>, bool>) < sizeof(std::expected<std::expected<std::optional<int>, bool>, bool>) Before the change, this relation didn't hold. After the change, the relation does hold, which means that the size of std::expected in these cases increases after this patch. The data layout will change in the following cases where tail padding can be reused by other objects: class foo : std::expected<std::optional<int>, bool> { bool b; }; or using [[no_unique_address]]: struct foo { [[no_unique_address]] std::expected<std::optional<int>, bool> e; bool b; }; The vendor communication is handled in #70820. Fixes: #70494 Co-authored-by: philnik777 <nikolasklauser@berlin.de> Co-authored-by: Louis Dionne <ldionne.2@gmail.com>
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