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Disagreement between `PureWindowsPath.relative_to()` and `ntpath.relpath()` with rootless drives

barneygale

On Windows, the path C: means "the current directory on the C: drive".

But pathlib's relative_to() treats it as the immediate parent of C:\. This makes sense lexographically, but it's inconsistent with everything else:

C:\Users\Barney>python
>>> import ntpath, pathlib
>>> ntpath.relpath('C:/Windows', 'C:/Users/Barney')
'..\\..\\Windows'
>>> ntpath.relpath('C:/Windows', '.')
'..\\..\\Windows'
>>> ntpath.relpath('C:/Windows', 'C:')
'..\\..\\Windows'
>>> pathlib.Path('C:/Windows').relative_to('C:/Users/Barney', walk_up=True)
WindowsPath('../../Windows')
>>> pathlib.Path('C:/Windows').relative_to('.', walk_up=True)
ValueError: One path is relative and the other is absolute.
>>> pathlib.Path('C:/Windows').relative_to('C:')
WindowsPath('/Windows')  # should be ValueError, as we're mixing absolute + relative paths

This prevents us from using relpath() from pathlib, and renders the two implementations incompatible. Booo! Also prevents us from simplifying is_relative_to() down to other == self or other in self.parents

Special cases aren't special enough to break the rules. Let's make these compatible!

Previous discussion: #84538