[libcxx][test] Silence nodiscard warnings for `std::expected` (#119174)

I'm exploring marking microsoft/STL's std::expected as [[nodiscard]],
which affects all functions returning std::expected, in

[libcxx][test] Silence nodiscard warnings for `std::expected` (#119174)

I'm exploring marking microsoft/STL's std::expected as [[nodiscard]],
which affects all functions returning std::expected, including its
own monadic member functions.

As usual, libc++'s test suite contains calls to these member functions
to make sure they compile, but it's discarding the returns. I'm adding
void casts to silence the [[nodiscard]] warnings without altering
what the test is covering.

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[libc++] Remove annotations for GCC 13 and update the documentation (#97744)

GCC 14 has been released a while ago. We've updated the CI to use GCC 14
now. This removes any old annotations in the te

[libc++] Remove annotations for GCC 13 and update the documentation (#97744)

GCC 14 has been released a while ago. We've updated the CI to use GCC 14
now. This removes any old annotations in the tests and updates the
documentation to reflect the updated version requirements.

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[libc++] Fix failures with GCC 14 (#92663)

Fixes #91831

[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 othe

[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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[libc++] Implement LWG3938 (Cannot use std::expected monadic ops with move-only error_type)

Implement LWG3938 (Cannot use std::expected monadic ops with move-only error_type)
https://wg21.link/LWG39

[libc++] Implement LWG3938 (Cannot use std::expected monadic ops with move-only error_type)

Implement LWG3938 (Cannot use std::expected monadic ops with move-only error_type)
https://wg21.link/LWG3938

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D154116

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[libc++][NFC] Fix typos in comments

[libc++] Implement P2505R5(Monadic operations for std::expected).

Implement P2505R5(https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2505r5.html)

Reviewed By: #libc, philnik, ldionne

Diff

[libc++] Implement P2505R5(Monadic operations for std::expected).

Implement P2505R5(https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2505r5.html)

Reviewed By: #libc, philnik, ldionne

Differential Revision: https://reviews.llvm.org/D140911

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[libc++] Implement P2505R5(Monadic operations for std::expected).

Signed-off-by: yronglin <yronglin777@gmail.com>