Fix splatting being treated as positional parameter (#14623)

src/System.Management.Automation/engine/CommandCompletion/PseudoParameterBinder.cs

@@ -1862,6 +1862,13 @@ namespace System.Management.Automation.Language
             while (unboundArgumentsIndex < unboundArgumentsCollection.Count)
             {
                 AstParameterArgumentPair argument = unboundArgumentsCollection[unboundArgumentsIndex++];
+                if (argument is AstPair astPair
+                    && astPair.Argument is VariableExpressionAst argumentVariable
+                    && argumentVariable.Splatted)
+                {
+                    continue;
+                }
+
                 if (!argument.ParameterSpecified)
                 {
                     result = argument;

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

@@ -1022,6 +1022,13 @@ dir -Recurse `
             $res.CompletionMatches | Should -HaveCount 4
             [string]::Join(',', ($res.CompletionMatches.completiontext | Sort-Object)) | Should -BeExactly "-wa,-Wait,-WarningAction,-WarningVariable"
         }
+
+        It "Test completion with splatted variable" {
+            $inputStr = 'Get-Content @Splat -P'
+            $res = TabExpansion2 -inputScript $inputStr -cursorColumn $inputStr.Length
+            $res.CompletionMatches | Should -HaveCount 4
+            [string]::Join(',', ($res.CompletionMatches.completiontext | Sort-Object)) | Should -BeExactly "-Path,-PipelineVariable,-PSPath,-pv"
+        }
     }
 
     Context "Module completion for 'using module'" {