As seen in the release build for 3.11a1, an assertion is raised when attempting to launch the debug build out of tree.

       _RegenTestFrozenmain:
         Regenerate test_frozenmain.h
         D:\a\1\b\bin\amd64\python_d.exe Programs\freeze_test_frozenmain.py Programs/test_frozenmain.h
         Assertion failed: stdlibdir[wcslen(prefix)] == SEP, file D:\a\1\s\PC\getpathp.c, line 300

This causes the release to fail.

I believe this is because getpath has hit its final fallback case of looking in ".\DLLs" and ".\Lib" for the standard library, and has not performed proper normalisation on these paths. So after removing the last segment, the result is "." which does not end in SEP.

So at least in this case, we can probably just ignore the assertion. However, I'm concerned that this would open up other cases where search paths may be added that are otherwise unintended.

The expected behaviour when the stdlib can't be found is a detailed error message, not a debug assertion/crash.

New changeset 5146877623ebe8a2806411703b0de9c0aba179a1 by Steve Dower in branch 'main':
bpo-45375: Fix assertion failure due to searching for stdlib in unnormalised paths (GH-28735)
https://github.com/python/cpython/commit/5146877623ebe8a2806411703b0de9c0aba179a1

I left a message on the PR a day ago about a one-off error in the allocation of `buff`. The size should be `MAXPATHLEN + 1`. That's what's used everywhere else in PC/getpathp.c and what gets passed in the PathCchCombineEx(buff, MAXPATHLEN + 1, ...) call.

Thanks for mentioning it! New PR to fix

New changeset 6c942a86a4fb4c8b731cb1bd2933dba554eb79cd by Steve Dower in branch 'main':
bpo-45375: Fix off by one error in buffer allocation (GH-28764)
https://github.com/python/cpython/commit/6c942a86a4fb4c8b731cb1bd2933dba554eb79cd