Fix the regression when doing type inference for $_ (#21223)

src/System.Management.Automation/engine/parser/TypeInferenceVisitor.cs

@@ -1983,8 +1983,19 @@ namespace System.Management.Automation
                     {
                         if (switchErrorStatement.Conditions?.Count > 0)
                         {
-                            parent = switchErrorStatement.Conditions[0];
+                            if (switchErrorStatement.Conditions[0].Extent.EndOffset < variableExpressionAst.Extent.StartOffset)
+                            {
+                                parent = switchErrorStatement.Conditions[0];
+                                break;
+                            }
+                            else
+                            {
+                                // $_ is inside the condition that is being declared, eg: Get-Process | Sort-Object -Property {switch ($_.Proc<Tab>
+                                parent = switchErrorStatement.Parent;
+                                continue;
+                            }
                         }
+
                         break;
                     }
                     else if (parent is ScriptBlockExpressionAst)

test/powershell/Host/TabCompletion/TabCompletion.Tests.ps1

@@ -1860,6 +1860,11 @@ class InheritedClassTest : System.Attribute
             $res.CompletionMatches[0].CompletionText | Should -BeExactly 'Black' # 0 = NonPositive
         }
 
+        It 'Tab completion of $_ inside incomplete switch condition' {
+            $res = TabExpansion2 -inputScript 'Get-PSDrive | Sort-Object -Property {switch ($_.nam'
+            $res.CompletionMatches[0].CompletionText | Should -Be 'Name'
+        }
+
         It "Test [CommandCompletion]::GetNextResult" {
             $inputStr = "Get-Command -Type Alias,c"
             $res = TabExpansion2 -inputScript $inputStr -cursorColumn $inputStr.Length