fix help message with @OptionGroup ParsableCommand

Sources/ArgumentParser/Parsing/ArgumentDecoder.swift

@@ -17,13 +17,16 @@
 struct DecodedArguments {
   var type: ParsableArguments.Type
   var value: ParsableArguments
+  var isOptionGroup: Bool = false
 
   var commandType: ParsableCommand.Type? {
-    type as? ParsableCommand.Type
+    if isOptionGroup { return nil }
+    return type as? ParsableCommand.Type
   }
 
   var command: ParsableCommand? {
-    value as? ParsableCommand
+    if isOptionGroup { return nil }
+    return value as? ParsableCommand
   }
 }
 
@@ -182,7 +185,7 @@ struct SingleValueDecoder: Decoder {
   }
 
   func saveValue<T: ParsableArguments>(_ value: T, type: T.Type = T.self) {
-    underlying.previouslyDecoded.append(DecodedArguments(type: type, value: value))
+    underlying.previouslyDecoded.append(DecodedArguments(type: type, value: value, isOptionGroup: true))
   }
 
   struct SingleValueContainer: SingleValueDecodingContainer {

Tests/ArgumentParserUnitTests/HelpGenerationTests.swift

@@ -514,6 +514,31 @@ extension HelpGenerationTests {
 
     """)
   }
+
+  struct ParsableCommandOption : ParsableCommand {
+    struct Group: ParsableCommand {
+      @Option
+      var opt: Int = 0
+    }
+    @OptionGroup
+    var options: Group
+    @Argument
+    var arg: String = ""
+  }
+
+  func testHelpWithParsableCommandOption() {
+    AssertHelp(.default, for: ParsableCommandOption.self, equals: """
+    USAGE: parsable-command-option [--opt <opt>] [<arg>]
+
+    ARGUMENTS:
+      <arg>
+
+    OPTIONS:
+      --opt <opt>             (default: 0)
+      -h, --help              Show help information.
+
+    """)
+  }
 }
 
 extension HelpGenerationTests {