Fix AsyncParseableCommand hierarchy (#436)

Sources/ArgumentParser/Parsable Types/AsyncParsableCommand.swift

@@ -73,4 +73,3 @@ extension AsyncMainProtocol {
     }
   }
 }
-

Sources/ArgumentParser/Parsable Types/ParsableCommand.swift

@@ -84,7 +84,7 @@ extension ParsableCommand {
   public static func helpMessage(
     for subcommand: ParsableCommand.Type,
     columns: Int? = nil
-  ) -> String { 
+  ) -> String {
     helpMessage(for: subcommand, includeHidden: false, columns: columns)
   }
 
@@ -124,6 +124,13 @@ extension ParsableCommand {
   /// - Parameter arguments: An array of arguments to use for parsing. If
   ///   `arguments` is `nil`, this uses the program's command-line arguments.
   public static func main(_ arguments: [String]?) {
+
+#if DEBUG
+    if #available(macOS 10.15, macCatalyst 13, iOS 13, tvOS 13, watchOS 6, *) {
+      checkAsyncHierarchy(self, root: "\(self)")
+    }
+#endif
+
     do {
       var command = try parseAsRoot(arguments)
       try command.run()
@@ -164,4 +171,28 @@ extension ParsableCommand {
   internal static var defaultIncludesUnconditionalArguments: Bool {
     configuration.defaultSubcommand?.includesUnconditionalArguments == true
   }
+
+#if DEBUG
+  @available(macOS 10.15, macCatalyst 13, iOS 13, tvOS 13, watchOS 6, *)
+  internal static func checkAsyncHierarchy(_ command: ParsableCommand.Type, root: String) {
+    for sub in command.configuration.subcommands {
+      checkAsyncHierarchy(sub, root: root)
+
+      guard sub.configuration.subcommands.isEmpty else { continue }
+      guard sub is AsyncParsableCommand.Type else { continue }
+
+      fatalError("""
+
+      --------------------------------------------------------------------
+      Asynchronous subcommand of a synchronous root.
+
+      The asynchronous command `\(sub)` is declared as a subcommand of the synchronous root command `\(root)`.
+
+      With this configuration, your asynchronous `run()` method will not be called. To fix this issue, change `\(root)`'s `ParsableCommand` conformance to `AsyncParsableCommand`.
+      --------------------------------------------------------------------
+
+      """.wrapped(to: 70))
+    }
+  }
+#endif
 }