VS 2019 16.9 Preview 1 toolset update

[StephanTLavavej]

This updates the toolset to VS 2019 16.9 Preview 1 and removes a bunch of compiler bug workarounds.

• .github/ISSUE_TEMPLATE/cxx23-feature.md
  • Transform the issue template for C++20 features to C++23. This adds a note that we aren't ready to begin work on C++23.
• CMakeLists.txt
  • VS 2019 16.9 Preview 1 ships CMake 3.18; make that our minimum required version.
• README.md
  • Mention the new VS and CMake versions.
• azure-devops/create-vmss.ps1
  • Increase $TotalProgress and add an initial step to Set-AzContext -SubscriptionName CPP_STL_GitHub. (For unknown reasons, I now have access to a bunch of subscriptions, and Connect-AzAccount selects the first one, which isn't CPP_STL_GitHub. This Set-AzContext step allows the following steps to work, and should have no harmful effects if a maintainer has access to only CPP_STL_GitHub.)
  • When the script is done, don't print AdminUser and $AdminPW. We don't need these credentials.
• azure-devops/provision-image.ps1
  • Install the Spectre-mitigated libraries and the Windows Driver Kit, for later PRs addressing Create and use test harness for the Windows kernel (freestanding) #1289.
• azure-pipelines.yml
  • Use the new Virtual Machine Scale Set pool.
• stl/inc/filesystem
  • Remove the workaround for the internal compiler bug VSO-571749.
• stl/inc/memory_resource
  • Remove the workaround for the compiler bug DevCom-1159869 that affected Standard Library Header Units.
• stl/inc/tuple
• stl/inc/utility
  • Work around the internal compiler bug VSO-1236041 affecting pair's piecewise constructor with Standard Library Header Units, by defining that constructor in <utility> instead of <tuple>. To do so, we need to define an internal _Tuple_get helper for modifiable rvalues only. This mitigates the throughput impact for <utility>, and should avoid disrupting conformance tests that are overly sensitive to the order of get declarations. As a bonus, we can now define pair's constructors entirely within the class definition.
• stl/inc/yvals_core.h
  • MSVC and EDG now define __cpp_impl_coroutine to its C++20 value, so we don't need the 197000L workaround. (We still need #ifdef __cpp_impl_coroutine for Clang.)
• tests/libcxx/expected_results.txt
  • The MSVC configuration of allocator.members/allocate.pass.cpp now passes, so mark it as FAIL for Clang only. (No change to skipped_tests.txt.)
• tests/std/tests/P0433R2_deduction_guides/test.cpp
  • Remove the workaround for the internal compiler bug VSO-587956.
• tests/std/tests/P0896R4_ranges_range_machinery/test.cpp
  • Unrelated to the toolset update, I noticed that Implement ranges::ref_view #1132 implemented ref_view so we can test_borrowed_range here.
• tests/std/tests/P0896R4_views_common/test.cpp
• tests/std/tests/P0896R4_views_drop_while/test.cpp
• tests/std/tests/P0896R4_views_filter/test.cpp
• tests/std/tests/P0896R4_views_reverse/test.cpp
• tests/std/tests/P0896R4_views_take_while/test.cpp
• tests/std/tests/P0896R4_views_transform/test.cpp
  • Remove the (hilariously bizarre 🤪) workaround for the compiler bug DevCom-1159442.
• tests/std/tests/P1502R1_standard_library_header_units/test.cpp
  • Remove many compiler bug workarounds.
• tests/std/tests/VSO_0157762_feature_test_macros/test.compile.pass.cpp
  • Remove the __cpp_impl_coroutine 197000L workaround; see yvals_core.h above.

---

Note: The _MSC_VER check is not changing, as its value will remain 1928 for 16.9 (due to running out of one's digits). The difference is detectable through _MSC_FULL_VER, but this check doesn't need to be strict - it merely diagnoses egregious compiler version mismatch.

STL/stl/inc/yvals_core.h

Lines 515 to 516 in 9959929

	#if _MSC_VER < 1928 // Coarse-grained, not inspecting _MSC_FULL_VER
	#error STL1001: Unexpected compiler version, expected MSVC 19.28 or newer.

[StephanTLavavej]

(I'm trying to get the internal test suite to pass, hence the back-and-forth commits)