Fixing #1507 AddDelegate uses an abstract type when used in a branch (#1509)

src/Spectre.Console.Cli/ConfiguratorExtensions.cs

@@ -324,11 +324,16 @@ public static class ConfiguratorExtensions
     /// <param name="func">The delegate to execute as part of command execution.</param>
     /// <returns>A command configurator that can be used to configure the command further.</returns>
     public static ICommandConfigurator AddDelegate<TSettings>(
-        this IConfigurator<TSettings> configurator,
+        this IConfigurator<TSettings>? configurator,
         string name,
         Func<CommandContext, int> func)
         where TSettings : CommandSettings
     {
+        if (typeof(TSettings).IsAbstract)
+        {
+            AddDelegate(configurator as IConfigurator<EmptyCommandSettings>, name, func);
+        }
+
         if (configurator == null)
         {
             throw new ArgumentNullException(nameof(configurator));

test/Spectre.Console.Cli.Tests/Unit/CommandAppTests.cs

@@ -1136,6 +1136,55 @@ public sealed partial class CommandAppTests
             data.ShouldBe(2);
         }
 
+        [Fact]
+        public void Should_Execute_Nested_Delegate_Empty_Command()
+        {
+            // Given
+            var app = new CommandAppTester();
+            app.Configure(cfg =>
+            {
+                cfg.AddBranch("a", d =>
+                {
+                    d.AddDelegate("b", _ =>
+                    {
+                        AnsiConsole.MarkupLine("[red]Complete[/]");
+                        return 0;
+                    });
+                });
+            });
+
+            // When
+            var result = app.Run([
+                "a", "b"
+            ]);
+
+            // Then
+            result.ExitCode.ShouldBe(0);
+        }
+
+        [Fact]
+        public void Should_Execute_Delegate_Empty_Command_At_Root_Level()
+        {
+            // Given
+            var app = new CommandAppTester();
+            app.Configure(cfg =>
+            {
+                cfg.AddDelegate("a", _ =>
+                {
+                    AnsiConsole.MarkupLine("[red]Complete[/]");
+                    return 0;
+                });
+            });
+
+            // When
+            var result = app.Run([
+                "a"
+            ]);
+
+            // Then
+            result.ExitCode.ShouldBe(0);
+        }
+
         [Fact]
         public async void Should_Execute_Async_Delegate_Command_At_Root_Level()
         {