Add specific error message that creating Junctions requires absolute path (#19409)

Author: SteveL-MSFT

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

@@ -2434,6 +2434,13 @@ protected override void NewItem(
 
                     bool exists = false;
 
+                   // junctions require an absolute path
+                    if (!Path.IsPathRooted(strTargetPath))
+                    {
+                        WriteError(new ErrorRecord(new ArgumentException(FileSystemProviderStrings.JunctionAbsolutePath), "NotAbsolutePath", ErrorCategory.InvalidArgument, strTargetPath));
+                        return;
+                    }
+
                     try
                     {
                         exists = GetFileSystemInfo(strTargetPath, out isDirectory) != null;

src/System.Management.Automation/resources/FileSystemProviderStrings.resx

@@ -345,4 +345,7 @@
   <data name="CopyingLocalBytesStatus" xml:space="preserve">
     <value>{0} of {1} ({2:0.0} MB/s)</value>
   </data>
+  <data name="JunctionAbsolutePath" xml:space="preserve">
+    <value>Creating a junction requires an absolute path for the target.</value>
+  </data>
 </root>

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

@@ -613,6 +613,17 @@ Describe "Hard link and symbolic link tests" -Tags "CI", "RequireAdminOnWindows"
             Test-Path $junctionToDir | Should -BeTrue
         }
 
+        It 'New-Item fails creating junction with relative path' -Skip:(!$IsWindows) {
+            try {
+                Push-Location $TestDrive
+                1 > 1.txt
+                { New-Item -ItemType Junction -Path 2.txt -Target 1.txt -ErrorAction Stop } | Should -Throw -ErrorId "NotAbsolutePath,Microsoft.PowerShell.Commands.NewItemCommand"
+            }
+            finally {
+                Pop-Location
+            }
+        }
+
         It 'New-Item can create hardlink with relative path' {
             try {
                 Push-Location $TestDrive