I pushed the following additional changes:

• Increase _STL_WIN32_WINNT default to Win7.
  • Note that the DLL build still overrides this to XP.
• Remove __stl_sync_api_impl_mode.
  • This is now dead - its value is never read. We need to keep __set_stl_sync_api_mode() as a no-op for the DLL's export surface, appropriately commented. (Long ago, this function was intended to allow testing of downlevel OS fallback codepaths on modern OS machines; it had no other purpose during normal operation.)
  • Note that __stl_sync_api_impl_mode wasn't mentioned outside of
    stl/src, so we don't have to worry about previously compiled user
    code looking for it.
• Remove stl_critical_section_vista, stl_condition_variable_vista.
  • Like Remove unused ConcRT wrappers #1325, we can replace those classes with their size/alignment values. I used temporary static_asserts (passing for x86, x64, ARM, and ARM64) to verify the values.
  • We can eliminate the __max() usage because the concrt sizes are greater than (or equal to) the vista sizes, which in turn are greater than (or equal to) the win7 sizes.
  • The alignments are all identical; I extracted them for clarity. (I felt that using alignof(win7) was clearer than alignof(void*).)

I believe that it makes sense to do this simultaneously, but these commits could be done as a followup PR if anyone is concerned about the complexity.

Labeling as blocked only because we need to wait to merge this until we're certain that changes are flowing into 17.4 Preview 2 specifically (right now it might be Preview 1).

@barcharcraz mentioned that we've likely already dropped Vista support by merging #2654 previously, which is yet another reason that this PR is okay to merge.

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

Thank you for improving library performance by eradicating this ancient machinery! I never thought I'd see the day! 😻 🐴 🦄