Join-Path doesn't handle provider prefixes correctly on Unix-like platforms

Steps to reproduce

On macOS or Linux:

Join-Path (Get-Item .).PSParentPath foo | Should -Be "$((Get-Item .).PSParentPath)/foo"

Expected behavior

The test should succeed.

Actual behavior

The test fails, because the existing provider prefix isn’t recognized as such, causing its \ separator to be normalized to / and another provider prefix to be prepended.

Expected strings to be the same, but they were different. Expected length: 65 Actual length:   103 Strings differ at index 38. Expected: '...tem::/User...' But was:  '...tem::Micro...'

That is, Join-Path’s output was something like (note the doubled provider prefix, with the 2nd one using /):

Microsoft.PowerShell.Core\FileSystem::Microsoft.PowerShell.Core/FileSystem::/Users/jdoe/Desktop/foo

Environment data

PowerShell Core 7.2.0-preview.2

[vexx32]

@octos4murai I haven’t seen anyone else wanting to take it up, and it seems a fairly clear case of the behaviour being bugged, so yeah, we may as well consider it up for grabs. 🙂

[mklement0]

@iSazonov, yes, because the problem is related to normalizing \ to /.