Fix dynamic parameter completion (#19510)

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

@@ -708,7 +708,18 @@ namespace System.Management.Automation
                     break;
             }
 
-            Diagnostics.Assert(matchedParameterName != null, "we should find matchedParameterName from the BoundArguments");
+            if (matchedParameterName is null)
+            {
+                // The pseudo binder has skipped a parameter
+                // This will happen when completing parameters for commands with dynamic parameters.
+                result = GetParameterCompletionResults(
+                    parameterName,
+                    bindingInfo.ValidParameterSetsFlags,
+                    bindingInfo.UnboundParameters,
+                    withColon);
+                return result;
+            }
+
             MergedCompiledCommandParameter param = bindingInfo.BoundParameters[matchedParameterName];
 
             WildcardPattern pattern = WildcardPattern.Get(parameterName + "*", WildcardOptions.IgnoreCase);

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

@@ -982,7 +982,7 @@ namespace System.Management.Automation.Language
                             executionContext.LanguageMode = PSLanguageMode.ConstrainedLanguage;
                         }
 
-                        _bindingEffective = PrepareCommandElements(executionContext);
+                        _bindingEffective = PrepareCommandElements(executionContext, paramAstAtCursor);
                     }
                     finally
                     {
@@ -1189,7 +1189,7 @@ namespace System.Management.Automation.Language
             _duplicateParameters.Clear();
         }
 
-        private bool PrepareCommandElements(ExecutionContext context)
+        private bool PrepareCommandElements(ExecutionContext context, CommandParameterAst paramAtCursor)
         {
             int commandIndex = 0;
             bool dotSource = _commandAst.InvocationOperator == TokenKind.Dot;
@@ -1216,6 +1216,13 @@ namespace System.Management.Automation.Language
                 // Pre-processing the arguments -- command arguments
                 for (commandIndex++; commandIndex < _commandElements.Count; commandIndex++)
                 {
+                    if (implementsDynamicParameters && _commandElements[commandIndex] == paramAtCursor)
+                    {
+                        // Commands with dynamic parameters will try to bind the command elements.
+                        // A partially complete parameter will most likely cause a binding error and negatively affect the results.
+                        continue;
+                    }
+
                     var parameter = _commandElements[commandIndex] as CommandParameterAst;
                     if (parameter != null)
                     {

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

@@ -992,6 +992,19 @@ class InheritedClassTest : System.Attribute
         $res.CompletionMatches.CompletionText | Should -Contain '-ProgressAction'
     }
 
+    It 'Should complete dynamic parameters with partial input' {
+        # See issue: #19498
+        try
+        {
+            Push-Location function:
+            $res = TabExpansion2 -inputScript 'Get-ChildItem -LiteralPath $PSHOME -Fi'
+            $res.CompletionMatches[1].CompletionText | Should -Be '-File'
+        }
+        finally
+        {
+            Pop-Location
+        }
+    }
     it 'Should complete enum class members for Enums in script text' {
         $res = TabExpansion2 -inputScript 'enum Test1 {Val1};([Test1]"").'
         $res.CompletionMatches.CompletionText[0] | Should -Be 'value__'