I'm getting an "error MSB4086: A numeric comparison was attempted ..." while attempting to build Python 3.8.3 (branch `3.8` from github) on Windows 8.1 (64-bit) with VC++ Build Tools 14.0 (2015 R3), at the command line.

I'm guessing that I might have an incorrect (ie. too old) version of the .NET tools?

I've attach the `PCBuild\clean.bat -vv` output.

I can provide many more details but I didn't want to overwhelm.

Haven't looked at your logs yet (on my phone), but make sure you have as recent an install of the Windows SDK as you can handle. That's typically what our build files check version numbers on.

Using Windows 10 SDK (from late 2019).  It's essential for the 2015r3 tools (since the build tools don't include the standard lib headers! --Gimme a break Microsoft!)

Here's the TL;DR from the log:

     2>Building with tools version "4.0".
     2>C:\src\cpython\PCbuild\python.props(111,31): error MSB4086: A numeric comparison was attempted on "$([System.Version]::Parse($(_RegistryVersion)))" that evaluates to "10.0.18362.0" instead of a number, in condition "$([System.Version]::Parse($(_RegistryVersion))) > $([System.Version]::Parse($(DefaultWindowsSDKVersion)))". [C:\src\cpython\PCbuild\_freeze_importlib.vcxproj]

Probably a result of pulling in `msbuild` from .net 4.0 tools.

I'm setting up a dedicated virtual machine.

Yeah, that looks like a version of .NET that pre-dates Version being made comparable, which probably means you're not getting the MSBuild installed with VS.

Check out "where MSBuild" or %MSBUILD% to see if there's something obvious overriding it, and use the MSBUILD variable to override it yourself if necessary.

Good luck! Would love to hear if you find an approach that doesn't involve just setting up a whole machine!