Fix various completion issues inside the param block (#17489)

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

@@ -155,6 +155,14 @@ namespace System.Management.Automation
                 ast => IsCursorWithinOrJustAfterExtent(positionForAstSearch, ast.Extent),
                 searchNestedScriptBlocks: true).ToList();
 
+            // If the last ast is an unnamed block that starts with "param" the cursor is inside a param block.
+            // To avoid adding special handling to all the completers that look at the last ast, we remove it here because it's not useful for completion. 
+            if (relatedAsts[^1].Extent.Text.StartsWith("param", StringComparison.OrdinalIgnoreCase)
+                && relatedAsts[^1] is NamedBlockAst namedBlock && namedBlock.Unnamed)
+            {
+                relatedAsts.RemoveAt(relatedAsts.Count - 1);
+            }
+
             Diagnostics.Assert(tokenAtCursor == null || tokenBeforeCursor == null, "Only one of these tokens can be non-null");
 
             return new AstAnalysisContext(tokenAtCursor, tokenBeforeCursor, relatedAsts, replacementIndex);

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

@@ -5529,7 +5529,7 @@ namespace System.Management.Automation
             }
 
             Ast lastAst = context.RelatedAsts[^1];
-            Ast firstAstAfterComment = lastAst.Find(ast => ast.Extent.StartOffset >= context.TokenAtCursor.Extent.EndOffset, searchNestedScriptBlocks: false);
+            Ast firstAstAfterComment = lastAst.Find(ast => ast.Extent.StartOffset >= context.TokenAtCursor.Extent.EndOffset && ast is not NamedBlockAst, searchNestedScriptBlocks: false);
 
             // Comment-based help can apply to a following function definition if it starts within 2 lines
             int commentEndLine = context.TokenAtCursor.Extent.EndLineNumber + 2;
@@ -5556,9 +5556,7 @@ namespace System.Management.Automation
                 // Helpblock before function
                 if (firstAstAfterComment is not null
                     && firstAstAfterComment.Extent.StartLineNumber <= commentEndLine
-                    && firstAstAfterComment is NamedBlockAst block
-                    && block.Statements.Count > 0
-                    && block.Statements[0] is FunctionDefinitionAst statement)
+                    && firstAstAfterComment is FunctionDefinitionAst statement)
                 {
                     return statement;
                 }

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

@@ -454,6 +454,19 @@ switch ($x)
         $actual | Should -BeExactly $expected
     }
 
+    It 'Should complete parameter in param block' {
+        $res = TabExpansion2 -inputScript 'Param($Param1=(Get-ChildItem -))' -cursorColumn 30
+        $res.CompletionMatches[0].CompletionText | Should -BeExactly '-Path'
+    }
+    It 'Should complete member in param block' {
+        $res = TabExpansion2 -inputScript 'Param($Param1=($PSVersionTable.))' -cursorColumn 31
+        $res.CompletionMatches[0].CompletionText | Should -BeExactly 'Count'
+    }
+    It 'Should complete attribute argument in param block' {
+        $res = TabExpansion2 -inputScript 'Param([Parameter()]$Param1)' -cursorColumn 17
+        $res.CompletionMatches[0].CompletionText | Should -BeExactly 'Position'
+    }
+
     Context "Format cmdlet's View paramter completion" {
         BeforeAll {
             $viewDefinition = @'
@@ -2053,6 +2066,30 @@ function MyFunction ($param1, $param2)
     {
     }
 }
+'@
+            }
+            @{
+                Intent = 'Complete help keyword PARAMETER argument for function inside advanced function'
+                Expected = 'param1','param2'
+                TestString = @'
+function Verb-Noun
+{
+    Param
+    (
+        [Parameter()]
+        [string[]]
+        $ParamA
+    )
+    Begin
+    {
+        <#
+            .Parameter ^
+        #>
+        function MyFunction ($param1, $param2)
+        {
+        }
+    }
+}
 '@
             }
             @{