Allow params keyword completion in lambdas

src/EditorFeatures/CSharpTest2/Recommendations/ParamsKeywordRecommenderTests.cs

@@ -213,6 +213,20 @@ await VerifyKeywordAsync(
 @"delegate void D(int i, [Goo]$$");
         }
 
+        [Fact]
+        public async Task TestAfterLambdaOpenParen()
+        {
+            await VerifyKeywordAsync(
+@"var lam = ($$");
+        }
+
+        [Fact]
+        public async Task TestAfterLambdaComma()
+        {
+            await VerifyKeywordAsync(
+@"var lam = (int i, $$");
+        }
+
         [Fact]
         public async Task TestNotAfterOperator()
         {

src/EditorFeatures/Test2/IntelliSense/CSharpCompletionCommandHandlerTests.vb

@@ -10151,6 +10151,86 @@ class C
             End Using
         End Function
 
+        <WpfTheory, CombinatorialData>
+        <Trait(Traits.Feature, Traits.Features.Completion)>
+        Public Async Function CompletionForLambdaParamsArray(showCompletionInArgumentLists As Boolean) As Task
+            Using state = TestStateFactory.CreateCSharpTestState(
+                <Document>
+class C
+{
+    void M()
+    {
+        var lam = ($$
+    }
+}
+                </Document>,
+                showCompletionInArgumentLists:=showCompletionInArgumentLists, languageVersion:=LanguageVersion.Preview)
+
+                state.SendTypeChars("p")
+                Await state.AssertSelectedCompletionItem(displayText:="params")
+            End Using
+        End Function
+
+        <WpfTheory, CombinatorialData>
+        <Trait(Traits.Feature, Traits.Features.Completion)>
+        Public Async Function CompletionForLambdaParamsArray_BeforeParameter(showCompletionInArgumentLists As Boolean) As Task
+            Using state = TestStateFactory.CreateCSharpTestState(
+                <Document>
+class C
+{
+    void M()
+    {
+        var lam = ($$ int[] xs) => { };
+    }
+}
+                </Document>,
+                showCompletionInArgumentLists:=showCompletionInArgumentLists, languageVersion:=LanguageVersion.Preview)
+
+                state.SendTypeChars("p")
+                Await state.AssertSelectedCompletionItem(displayText:="params")
+            End Using
+        End Function
+
+        <WpfTheory, CombinatorialData>
+        <Trait(Traits.Feature, Traits.Features.Completion)>
+        Public Async Function CompletionForLambdaParamsArray_AfterParameter(showCompletionInArgumentLists As Boolean) As Task
+            Using state = TestStateFactory.CreateCSharpTestState(
+                <Document>
+class C
+{
+    void M()
+    {
+        var lam = (int x, $$
+    }
+}
+                </Document>,
+                showCompletionInArgumentLists:=showCompletionInArgumentLists, languageVersion:=LanguageVersion.Preview)
+
+                state.SendTypeChars("p")
+                Await state.AssertSelectedCompletionItem(displayText:="params")
+            End Using
+        End Function
+
+        <WpfTheory, CombinatorialData>
+        <Trait(Traits.Feature, Traits.Features.Completion)>
+        Public Async Function CompletionForLambdaParamsArray_AfterOptionalParameter(showCompletionInArgumentLists As Boolean) As Task
+            Using state = TestStateFactory.CreateCSharpTestState(
+                <Document>
+class C
+{
+    void M()
+    {
+        var lam = (int x = 1, $$) => { };
+    }
+}
+                </Document>,
+                showCompletionInArgumentLists:=showCompletionInArgumentLists, languageVersion:=LanguageVersion.Preview)
+
+                state.SendTypeChars("p")
+                Await state.AssertSelectedCompletionItem(displayText:="params")
+            End Using
+        End Function
+
         ' Simulate the situation that some provider (e.g. IntelliCode) provides items with higher match priority that only match case-insensitively.
         <ExportCompletionProvider(NameOf(PreselectionProvider), LanguageNames.CSharp)>
         <[Shared]>

src/Features/CSharp/Portable/Completion/KeywordRecommenders/ParamsKeywordRecommender.cs

@@ -15,6 +15,6 @@ public ParamsKeywordRecommender()
         }
 
         protected override bool IsValidContext(int position, CSharpSyntaxContext context, CancellationToken cancellationToken)
-            => context.SyntaxTree.IsParamsModifierContext(context.Position, context.LeftToken);
+            => context.SyntaxTree.IsParamsModifierContext(position, context.LeftToken, cancellationToken);
     }
 }

src/Workspaces/SharedUtilitiesAndExtensions/Workspace/CSharp/Extensions/ContextQuery/SyntaxTreeExtensions.cs

@@ -1099,14 +1099,20 @@ parameter3.Parent is ParameterListSyntax parameterList3 &&
         public static bool IsParamsModifierContext(
             this SyntaxTree syntaxTree,
             int position,
-            SyntaxToken tokenOnLeftOfPosition)
+            SyntaxToken tokenOnLeftOfPosition,
+            CancellationToken cancellationToken)
         {
             if (syntaxTree.IsParameterModifierContext(position, tokenOnLeftOfPosition, includeOperators: false, out _, out var previousModifier) &&
                 previousModifier == SyntaxKind.None)
             {
                 return true;
             }
 
+            if (syntaxTree.IsPossibleLambdaParameterModifierContext(position, tokenOnLeftOfPosition, cancellationToken))
+            {
+                return true;
+            }
+
             var token = tokenOnLeftOfPosition;
             token = token.GetPreviousTokenIfTouchingWord(position);