enable using filesystem from a UNC location (#4998)

* enable using filesystem from a UNC location

* [feature]
address PR feedback

* [feature]
removed changed in NavigationProviderBase and made change in FileSystemProvider where it should belong

* added variations of tests for set-location and push-location
no need to run [feature] anymore since it passed previously and the test case added is CI

* [feature]
move code to reproduce UNC path to GetParentPath()

src/System.Management.Automation/namespaces/FileSystemProvider.cs

@@ -4817,6 +4817,13 @@ namespace Microsoft.PowerShell.Commands
             {
                 parentPath = EnsureDriveIsRooted(parentPath);
             }
+#if !UNIX
+            else if (parentPath.Equals(StringLiterals.DefaultPathSeparatorString, StringComparison.Ordinal))
+            {
+                // make sure we return two backslashes so it still results in a UNC path
+                parentPath = "\\\\";
+            }
+#endif
             return parentPath;
         } // GetParentPath
 

test/powershell/Modules/Microsoft.PowerShell.Management/FileSystem.Tests.ps1

@@ -1368,3 +1368,24 @@ Describe "Extended FileSystem Path/Location Cmdlet Provider Tests" -Tags "Featur
         }
     }
 }
+
+Describe "UNC paths" -Tags 'CI' {
+    It "Can Get-ChildItems from a UNC location using <cmdlet>" -Skip:(!$IsWindows) -TestCases @(
+        @{cmdlet="Push-Location"},
+        @{cmdlet="Set-Location"}
+    ) {
+        param($cmdlet)
+        $originalLocation = Get-Location
+        try {
+            $systemDrive = ($env:SystemDrive).Replace(":","$")
+            $testPath = Join-Path "\\localhost" $systemDrive
+            & $cmdlet $testPath
+            Get-Location | Should BeExactly "Microsoft.PowerShell.Core\FileSystem::$testPath"
+            $children = { Get-ChildItem -ErrorAction Stop } | Should Not Throw
+            $children.Count | Should BeGreaterThan 0
+        }
+        finally {
+            Set-Location $originalLocation
+        }
+    }
+}