[llvm-branch-commits] [libcxx] release/21.x: [libc++] Ensure that we restore invariants in basic_filebuf::overflow (#147389) (PR #155712)

[Konstantin Varlamov via llvm-branch-commits]

var-const wrote:

@tru For context: we have originally requested a cherry-pick of this around the time it was merged to the main branch, but it looks like the bot wasn't triggered for some reason, and I don't see this commit in the release branch (see e.g. the [log for `<fstream>`](https://github.com/llvm/llvm-project/commits/release/21.x/libcxx/include/fstream)). This is an important bug fix for us and I hope this can still be cherry-picked into the upcoming release; sorry for missing the fact that this wasn't merged like we expected! (cc @ldionne)

https://github.com/llvm/llvm-project/pull/155712