Recommend "when" keyword after case-pattern-switch-label (#24131)

src/EditorFeatures/CSharpTest2/Recommendations/RecommenderTests.cs

@@ -83,7 +83,7 @@ private async Task CheckResultAsync(bool absent, int position, CSharpSyntaxConte
                 else
                 {
                     var result = (await RecommendKeywordsAsync(position, context)).Single();
-                    Assert.NotNull(result);
+                    Assert.True(result != null, "No recommended keywords");
                     Assert.Equal(keywordText, result.Keyword);
                     if (matchPriority != null)
                     {

src/EditorFeatures/CSharpTest2/Recommendations/WhenKeywordRecommenderTests.cs

@@ -63,5 +63,13 @@ public async Task TestNotAfterFilter3()
             await VerifyAbsenceAsync(AddInsideMethod(
 @"try {} catch (Exception e) when (true) $$"));
         }
+
+        [Fact, Trait(Traits.Feature, Traits.Features.KeywordRecommending)]
+        [WorkItem(24113, "https://github.com/dotnet/roslyn/issues/24113")]
+        public async Task TestInCasePattern()
+        {
+            await VerifyKeywordAsync(AddInsideMethod(
+@"switch (1) { case int i $$ }"));
+        }
     }
 }

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

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
 using System.Threading;
+using Microsoft.CodeAnalysis.CSharp.Extensions;
 using Microsoft.CodeAnalysis.CSharp.Extensions.ContextQuery;
 
 namespace Microsoft.CodeAnalysis.CSharp.Completion.KeywordRecommenders
@@ -14,7 +15,26 @@ public WhenKeywordRecommender()
 
         protected override bool IsValidContext(int position, CSharpSyntaxContext context, CancellationToken cancellationToken)
         {
-            return context.IsCatchFilterContext;
+            if (context.IsCatchFilterContext)
+            {
+                return true;
+            }
+
+            // case int i $$
+            // case int i w$$
+            if (IsIdentifierInCasePattern(context.TargetToken.Parent))
+            {
+                return true;
+            }
+
+            return false;
+        }
+
+        private static bool IsIdentifierInCasePattern(SyntaxNode node)
+        {
+            return node.IsKind(SyntaxKind.SingleVariableDesignation)
+                && node.IsParentKind(SyntaxKind.DeclarationPattern)
+                && node.Parent.IsParentKind(SyntaxKind.CasePatternSwitchLabel);
         }
     }
 }