Split-Path -Parent / currently broken on Unix

Steps to reproduce

Split-Path -Parent /

Expected behavior

The empty string (current behavior on Windows), but, ideally, / - see #4131

Actual behavior

An error occurs.

Split-Path : Cannot process argument because the value of argument "path" is not valid. Change the value of the "path" argument and run the operation again.

Environment data

PowerShell Core v6.0.0-beta.3 on macOS 10.12.5
PowerShell Core v6.0.0-beta.3 on Ubuntu 16.04.1 LTS

[SteveL-MSFT]

@PowerShell/powershell-committee reviewed this. Expectation is that scripts using the current Windows PowerShell behavior to parse a file path would need to special case Unix if we have non-symmetric behavior. For example:

$path = "/"; while ($path -ne "") { $path = split-path -parent $path;$path }

This loops forever since the ending state is never hit. To support the same scripts working on Windows and Unix, we need to fix the behavior on Unix so that:

split-path -leaf / => returns / split-path -parent / => returns “”

[SteveL-MSFT]

I think it’s fine to not have symmetry for the sake of symmetry. On Unix, if the expectation is that the -Leaf and -Parent of / is /, then I think that would be ok and just documented. On Windows, I don’t think we’ll change it as it would be a breaking change w/o clear benefit to warrant the change. Will have @PowerShell/powershell-committee review.