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.
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.
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>
This reverts commit ebc111b08bddca55d5f7e560a20bdb2c913d80cb.
Sorry, I forgot to append Phabricator reversion when land D140911, I try to revert this change and reland.
Reviewed By: #libc, ldionne, EricWF
Differential Revision: https://reviews.llvm.org/D150793
