Enable bidirectional optimization for non-common ranges in std::ranges::partition

[mirion-dev]

Fixes #5886.

[frederick-vs-ja]

I think it's better to remove the issue number from the PR title. Things will be confusing if it goes into squashed commit message.

[vmichal]

Hi, just for completeness, this PR is not set to close the corresponding issue #5886 on merge.
I'd say it would help the PR process to indicate that properly (if you are certain the PR actually solves the issue, ofc. It looks good to me).

[mirion-dev]

Hi, just for completeness, this PR is not set to close the corresponding issue #5886 on merge. I'd say it would help the PR process to indicate that properly (if you are certain the PR actually solves the issue, ofc. It looks good to me).

Hi, thanks for the reminder! I've updated the description. I actually wasn't aware of the auto-close feature.

[StephanTLavavej]

Thanks! 😻 I verified with a static_assert that we have existing test coverage for bidi-but-not-common ranges in libcxx std/algorithms/alg.modifying.operations/alg.partitions/ranges_partition.pass.cpp and std P0896R4_ranges_alg_partition.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.