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The detection logic added by commit v3.8.0-rc2~14^2 (VS2017: If Win 8.1 SDK is not available, use Win 10 SDK, 2017-02-20) was incomplete. It is possible for the Win 8.1 SDK registry entry to exist, and even the directory, but the header files to not actually be installed. Teach `cmGlobalVisualStudio15Generator::IsWin81SDKInstalled` to verify that the `windows.h` header actually exists in the SDK directory. We do this in `cmGlobalVisualStudio14Generator::GetWindows10SDKVersion` for the Windows 10 SDK already. Fixes: #16811
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