[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>

1 files changed, 2 insertions, 2 deletions

diff --git a/libcxx/test/std/utilities/expected/expected.expected/monadic/transform_error.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/monadic/transform_error.pass.cpp
index 84b57ae..ec55f63 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/monadic/transform_error.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/monadic/transform_error.pass.cpp
@@ -9,8 +9,8 @@
 // UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
 
 // GCC has a issue for `Guaranteed copy elision for potentially-overlapping non-static data members`,
-// please refer to: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108333, but we have a workaround to
-// avoid this issue.
+// please refer to: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108333.
+// XFAIL: gcc-13
 
 // <expected>