Merged PR 3115: Use Directory.CreateDirectory instead of DirectoryInfo.CreateSubDirectory

Change to use Directory.CreateDirectory because of regression introduced by https://github.com/dotnet/corefx/pull/27810 in creating a subdirectory under root path.

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

@@ -2599,10 +2599,7 @@ namespace Microsoft.PowerShell.Commands
 
                 if (ShouldProcess(resource, action))
                 {
-                    // Use the parent directory to create the sub-directory
-
-                    DirectoryInfo parentDirectory = new DirectoryInfo(parentPath);
-                    DirectoryInfo result = parentDirectory.CreateSubdirectory(childName);
+                    var result = Directory.CreateDirectory(Path.Combine(parentPath, childName));
 
                     if (streamOutput)
                     {

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

@@ -1396,3 +1396,21 @@ Describe "Remove-Item UnAuthorized Access" -Tags "CI", "RequireAdminOnWindows" {
         Get-Content $errorFile | Should -BeExactly 'RemoveItemUnauthorizedAccessError,Microsoft.PowerShell.Commands.RemoveItemCommand'
     }
 }
+
+Describe "Verify sub-directory creation under root" -Tag 'CI','RequireSudoOnUnix' {
+    BeforeAll {
+        $dirPath = "\TestDir-$((New-Guid).Guid)"
+    }
+
+    AfterAll {
+        if(Test-Path $dirPath)
+        {
+            Remove-Item -Path $dirPath -Force -ErrorAction SilentlyContinue
+        }
+    }
+
+    It "Can create a sub directory under root path" {
+        New-Item -Path $dirPath -ItemType Directory -Force > $null
+        $dirPath | Should -Exist
+    }
+}