Adding to Vajrasky's report, the same commit also adds 3 warnings when building on Windows:

..\Objects\unicodeobject.c(10588): warning C4018: '>' : signed/unsigned mismatch [P:\Projects\OSS\Python\cpython\PCbuild\pythoncore.vcxproj]
..\Objects\unicodeobject.c(10592): warning C4018: '>' : signed/unsigned mismatch [P:\Projects\OSS\Python\cpython\PCbuild\pythoncore.vcxproj]
..\Objects\unicodeobject.c(10594): warning C4018: '>' : signed/unsigned mismatch [P:\Projects\OSS\Python\cpython\PCbuild\pythoncore.vcxproj]

Vajrasky's patch doesn't fix the Windows warnings (but doesn't create any new ones either); the attached patch does (but likely doesn't do anything for the other).

I don't know nearly enough C to say whether my patch is any good or not, just enough to say it compiles, doesn't break anything immediately obvious, and removes the warnings on Windows.