Extensions: xml docs completion (#81274)

Fixes #78770

Author: jcouv

src/EditorFeatures/CSharpTest/Completion/CompletionProviders/XmlDocumentationCommentCompletionProviderTests.cs

@@ -1129,4 +1129,52 @@ await VerifyItemIsAbsentAsync("""
             "see",
             deletedCharTrigger: 'b');
     }
+
+    [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/78770")]
+    public Task ExtensionBlock_01()
+        => VerifyItemsExistAsync("""
+            public static class E
+            {
+                /// $$ 
+                extension<T>(int i) { }
+            }
+            """, """
+            typeparam name="T"
+            """, """
+            param name="i"
+            """,
+            "summary");
+
+    [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/78770")]
+    public Task ExtensionBlock_02()
+        => VerifyItemsExistAsync("""
+            public static class E
+            {
+                /// <summary> $$ </summary>
+                extension<T>(int i)
+                {
+                } 
+            }
+            """, """
+            paramref name="i"
+            """, """
+            typeparamref name="T"
+            """);
+
+    [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/78770")]
+    public Task ExtensionBlock_03()
+        => VerifyItemsExistAsync("""
+            public static class E
+            {
+                extension<T>(int i)
+                {
+                    /// <summary> $$ </summary>
+                    void M() { }
+                } 
+            }
+            """, """
+            paramref name="i"
+            """, """
+            typeparamref name="T"
+            """);
 }

src/EditorFeatures/CSharpTest/DocumentationComments/DocumentationCommentTests.cs

@@ -33,6 +33,26 @@ class C
             }
             """);
 
+    [WpfFact, WorkItem("https://github.com/dotnet/roslyn/issues/78770")]
+    public void TypingCharacter_Extension()
+        => VerifyTypingCharacter("""
+            static class C
+            {
+                //$$
+                extension<T>(int i) { }
+            }
+            """, """
+            static class C
+            {
+                /// <summary>
+                /// $$
+                /// </summary>
+                /// <typeparam name="T"></typeparam>
+                /// <param name="i"></param>
+                extension<T>(int i) { }
+            }
+            """);
+
     [WpfFact]
     public void TypingCharacter_Record()
         => VerifyTypingCharacter("""

src/Features/CSharp/Portable/Completion/CompletionProviders/XmlDocCommentCompletionProvider.cs

@@ -399,6 +399,10 @@ protected override ImmutableArray<IParameterSymbol> GetParameters(ISymbol declar
             {
                 declaredParameters = delegateInvokeParameters;
             }
+            else if (namedTypeSymbol.IsExtension && namedTypeSymbol.ExtensionParameter is { } extensionParameter)
+            {
+                declaredParameters = [extensionParameter];
+            }
         }
 
         return declaredParameters;

src/Features/CSharp/Portable/DocumentationComments/CSharpDocumentationCommentSnippetService.cs

@@ -56,6 +56,7 @@ protected override bool SupportsDocumentationComments(MemberDeclarationSyntax me
             case SyntaxKind.EventFieldDeclaration:
             case SyntaxKind.OperatorDeclaration:
             case SyntaxKind.ConversionOperatorDeclaration:
+            case SyntaxKind.ExtensionBlockDeclaration:
                 return true;
 
             default:

src/Features/Core/Portable/Completion/Providers/AbstractDocCommentCompletionProvider.cs

@@ -8,6 +8,7 @@
 using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
+using Microsoft.CodeAnalysis.CodeFixes;
 using Microsoft.CodeAnalysis.PooledObjects;
 using Microsoft.CodeAnalysis.Shared.Extensions;
 using Microsoft.CodeAnalysis.Text;
@@ -150,6 +151,11 @@ protected IEnumerable<CompletionItem> GetNestedItems(ISymbol? symbol, bool inclu
                          .Concat(GetTypeParamRefItems(symbol));
         }
 
+        if (symbol is { ContainingSymbol: INamedTypeSymbol { IsExtension: true } extension })
+        {
+            items = items.Concat(GetParamRefItems(extension));
+        }
+
         if (includeKeywords)
         {
             items = items.Concat(GetKeywordNames().Select(CreateLangwordCompletionItem));