I've analyzed this PR's changes to understand their potential source-breaking impact.

• _Unfancy() is moving from <xstddef> all the way down to <xmemory>.
  • This is an internal function, so there's no source-breaking impact.
  • <xmemory> is a good location - it's appropriate for fancy pointer manipulation, and it's included by all usage sites.
• unary_function and binary_function are moving from <xstddef> all the way down to <functional>.
  • There's some potential source-breaking impact, as they're available in C++14 mode, and this is changing from "nearly ambiently available" to "must include the proper header". (In the entire STL at this time, only <future> includes <functional> as a whole.) I'm not too concerned, since they're removed by default in C++17 mode, so we've been discouraging use for a while. I think we can deal with the fallout of this change.
  • <functional> is the proper location, as no other header needs them.
• The _MEMBER_CALL macro family and _Fake_copy_init() are moving from <xstddef> slightly down to <type_traits>.
  • Internal machinery, no source-breaking impact.
  • <type_traits> is a good location; this machinery isn't needed by the 2 headers that care about the difference (see below).
• addressof(), less<_Ty>, and less<void> are moving from <xstddef> slightly down to <type_traits>.
  • This has very limited source-breaking impact - although less is extremely popular, this is still "nearly ambiently available". There are only 2 headers that included <xstddef> in main and don't include <type_traits> after this PR: <iosfwd> and <limits>. I think this is unlikely to be a problem.
  • <type_traits> is a good central location.
• The primary templates and void specializations of plus, minus, multiplies, equal_to, not_equal_to, greater, greater_equal, and less_equal are moving from <xstddef> down to <xutility>.
  • While <xutility> is fairly popular, this affects a lot of headers. Specifically, the following headers included <xstddef> in main and don't include <xutility> after this PR: <atomic>, <bit>, <compare>, <concepts>, <coroutine>, <exception>, <iosfwd>, <latch>, <limits>, <new>, <numbers>, <ratio>, <semaphore>, <tuple>, <type_traits>, <typeindex>, <typeinfo>, and <utility>.
  • <xutility> is a good location, as none of those affected headers need these operator function objects.
  • Although this is all available in C++14 mode, I hope/expect that the source-breaking impact will be low. These function objects are less popular than less (presumably greater is the second-most popular) and <xutility> is still included widely enough that it should be fairly uncommon for a TU to be including only the affected subset. I'm willing to risk it and see what the fallout is.
• <xstddef> no longer includes <cstdlib> and <initializer_list>.

In practice, the breaking change impact I've seen in the compiler's test suite has been:

• Need to include <functional> for unary_function and binary_function.
• Need to include <cstdlib> for things like exit and quick_exit.
• Need to include <initializer_list> for initializer_list, sometimes used implicitly.

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

Thanks for improving the STL's throughput and helping users to be more disciplined about inclusion! This ended up being more source-breaking than I originally expected, but hopefully we can power through the resulting breaks and ship this in VS 2022 17.7. 🚀 🚢 😹

Given that this breaks code when updating to VS17.7, could a note be included inside the release notes for VS17.7 discussing the breaking change?
Having to hunt down and find this when a team is broken (and it will take time to move code changes through repos etc) is not a good experience for a component that is usually treated as stable and is automatically updated.

Given that this breaks code when updating to VS17.7, could a note be included inside the release notes for VS17.7 discussing the breaking change?

This is already recorded in the Changelog, although it's not automatically merged into release notes.