Extended Description

Here's how I reproed this (there may be a simpler way). Install VS 2022 17.0 Preview 2. After choosing the "Desktop development with C++" workload, customize it:

• Select the "C++ Clang tools for Windows" (containing Clang 12)
• Select the "MSVC v142 - VS 2019 C++ ARM64 build tools (Latest)" (slightly misnamed; this is the VS 2022 compiler)
• Unselect the default WinSDK 10.0.19041.0
• Select the latest "Windows 10 SDK (10.0.20348.0)"

This setup (Clang 12 + latest WinSDK) works for x86, x64, and ARM (32-bit) in the microsoft/STL test suite; however, we encountered an incompatibility on ARM64 due to the new WinSDK attempting to use the __umulh intrinsic, which works for MSVC but not Clang 12. Self-contained test case, starting from a clean command prompt:

C:\Temp>"C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Auxiliary\Build\vcvarsall.bat" x64_arm64

** Visual Studio 2022 Developer Command Prompt v17.0.0-pre.2.0
** Copyright (c) 2021 Microsoft Corporation

[vcvarsall.bat] Environment initialized for: 'x64_arm64'

C:\Temp>type meow.cpp
#include <Windows.h>

C:\Temp>cl
Microsoft (R) C/C++ Optimizing Compiler Version 19.30.30401 for ARM64
Copyright (C) Microsoft Corporation. All rights reserved.

usage: cl [ option... ] filename... [ /link linkoption... ]

C:\Temp>cl /EHsc /nologo /W4 /c meow.cpp
meow.cpp

C:\Temp>clang-cl --target=arm64-pc-windows-msvc --version
clang version 12.0.0
Target: arm64-pc-windows-msvc
Thread model: posix
InstalledDir: C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\Llvm\x64\bin

C:\Temp>clang-cl --target=arm64-pc-windows-msvc /EHsc /nologo /W4 /c meow.cpp
In file included from meow.cpp:1:
In file included from C:\Program Files (x86)\Windows Kits\10\include\10.0.20348.0\um\Windows.h:171:
In file included from C:\Program Files (x86)\Windows Kits\10\include\10.0.20348.0\shared\windef.h:24:
In file included from C:\Program Files (x86)\Windows Kits\10\include\10.0.20348.0\shared\minwindef.h:182:
C:\Program Files (x86)\Windows Kits\10\include\10.0.20348.0\um\winnt.h(6370,20): error: use of undeclared identifier
'__umulh'
*HighProduct = UnsignedMultiplyHigh(Multiplier, Multiplicand);
^
C:\Program Files (x86)\Windows Kits\10\include\10.0.20348.0\um\winnt.h(6236,30): note: expanded from macro
'UnsignedMultiplyHigh'
#define UnsignedMultiplyHigh __umulh
^
1 error generated.