Add F1 help keyword for default constraint

src/VisualStudio/CSharp/Impl/LanguageService/CSharpHelpContextService.cs

@@ -451,10 +451,31 @@ private static bool TryGetTextForKeyword(SyntaxToken token, [NotNullWhen(true)]
                 }
             }
 
-            if (token.IsKind(SyntaxKind.DefaultKeyword) && token.Parent is DefaultSwitchLabelSyntax)
+            if (token.IsKind(SyntaxKind.DefaultKeyword))
             {
-                text = Keyword("defaultcase");
-                return true;
+                if (token.Parent is DefaultSwitchLabelSyntax)
+                {
+                    text = Keyword("defaultcase");
+                    return true;
+                }
+
+                if (token.Parent is DefaultConstraintSyntax)
+                {
+                    text = Keyword("defaultconstraint");
+                    return true;
+                }
+
+                if (token.Parent is DefaultSwitchLabelSyntax or GotoStatementSyntax)
+                {
+                    text = Keyword("defaultcase");
+                    return true;
+                }
+
+                if (token.Parent is LineDirectiveTriviaSyntax)
+                {
+                    text = Keyword("defaultline");
+                    return true;
+                }
             }
 
             if (token.IsKind(SyntaxKind.ClassKeyword) && token.Parent is ClassOrStructConstraintSyntax)

src/VisualStudio/CSharp/Test/F1Help/F1HelpTests.cs

@@ -1739,5 +1739,68 @@ public class C
                 }
                 """, "required");
         }
+
+        [Fact]
+        public async Task TestDefaultConstraint()
+        {
+            await Test_KeywordAsync("""
+                public class Base
+                {
+                    virtual void M<T>(T? t) { }
+                }
+                public class C
+                {
+                    override void M<T>() where T : def[||]ault { }
+                }
+                """, expectedText: "defaultconstraint");
+        }
+
+        [Fact]
+        public async Task TestDefaultCase()
+        {
+            await Test_KeywordAsync("""
+                public class C
+                {
+                    void M(object o)
+                    {
+                        switch (o)
+                        {
+                            case 1:
+                                goto def[||]ault;
+                            default:
+                                return;
+                        }
+                    }
+                }
+                """, expectedText: "defaultcase");
+        }
+
+        [Fact]
+        public async Task TestGotoDefault()
+        {
+            await Test_KeywordAsync("""
+                public class C
+                {
+                    void M(object o)
+                    {
+                        switch (o)
+                        {
+                            case 1:
+                                goto default;
+                            def[||]ault:
+                                return;
+                        }
+                    }
+                }
+                """, expectedText: "defaultcase");
+        }
+
+        [Fact]
+        public async Task TestLineDefault()
+        {
+            await Test_KeywordAsync("""
+                #line def[||]ault
+                """, expectedText: "defaultline");
+        }
     }
 }