diff --git a/Sources/ArgumentParser/Completions/BashCompletionsGenerator.swift b/Sources/ArgumentParser/Completions/BashCompletionsGenerator.swift
index 753ebcf51..1fea1b73e 100644
--- a/Sources/ArgumentParser/Completions/BashCompletionsGenerator.swift
+++ b/Sources/ArgumentParser/Completions/BashCompletionsGenerator.swift
@@ -132,7 +132,7 @@ struct BashCompletionsGenerator {
   ///
   /// These consist of completions that are defined as `.list` or `.custom`.
   fileprivate static func generateArgumentCompletions(_ commands: [ParsableCommand.Type]) -> [String] {
-    ArgumentSet(commands.last!, visibility: .default, parent: .root)
+    ArgumentSet(commands.last!, visibility: .default, parent: nil)
       .compactMap { arg -> String? in
         guard arg.isPositional else { return nil }
 
@@ -159,7 +159,7 @@ struct BashCompletionsGenerator {
 
   /// Returns the case-matching statements for supplying completions after an option or flag.
   fileprivate static func generateOptionHandlers(_ commands: [ParsableCommand.Type]) -> String {
-    ArgumentSet(commands.last!, visibility: .default, parent: .root)
+    ArgumentSet(commands.last!, visibility: .default, parent: nil)
       .compactMap { arg -> String? in
         let words = arg.bashCompletionWords()
         if words.isEmpty { return nil }
diff --git a/Sources/ArgumentParser/Parsable Properties/Flag.swift b/Sources/ArgumentParser/Parsable Properties/Flag.swift
index 334df8874..74a97f795 100644
--- a/Sources/ArgumentParser/Parsable Properties/Flag.swift	
+++ b/Sources/ArgumentParser/Parsable Properties/Flag.swift	
@@ -396,7 +396,7 @@ extension Flag where Value: EnumerableFlag {
       // flag, the default value to show to the user is the `--value-name`
       // flag that a user would provide on the command line, not a Swift value.
       let defaultValueFlag = initial.flatMap { value -> String? in
-        let defaultKey = InputKey(name: String(describing: value), parent: .key(key))
+        let defaultKey = InputKey(name: String(describing: value), parent: key)
         let defaultNames = Value.name(for: value).makeNames(defaultKey)
         return defaultNames.first?.synopsisString
       }
@@ -405,7 +405,7 @@ extension Flag where Value: EnumerableFlag {
       let hasCustomCaseHelp = caseHelps.contains(where: { $0 != nil })
       
       let args = Value.allCases.enumerated().map { (i, value) -> ArgumentDefinition in
-        let caseKey = InputKey(name: String(describing: value), parent: .key(key))
+        let caseKey = InputKey(name: String(describing: value), parent: key)
         let name = Value.name(for: value)
         
         let helpForCase = caseHelps[i] ?? help
@@ -519,7 +519,7 @@ extension Flag {
       let hasCustomCaseHelp = caseHelps.contains(where: { $0 != nil })
 
       let args = Element.allCases.enumerated().map { (i, value) -> ArgumentDefinition in
-        let caseKey = InputKey(name: String(describing: value), parent: .key(parentKey))
+        let caseKey = InputKey(name: String(describing: value), parent: parentKey)
         let name = Element.name(for: value)
         let helpForCase = hasCustomCaseHelp ? (caseHelps[i] ?? help) : help
 
@@ -552,7 +552,7 @@ extension Flag {
       let hasCustomCaseHelp = caseHelps.contains(where: { $0 != nil })
 
       let args = Element.allCases.enumerated().map { (i, value) -> ArgumentDefinition in
-        let caseKey = InputKey(name: String(describing: value), parent: .key(parentKey))
+        let caseKey = InputKey(name: String(describing: value), parent: parentKey)
         let name = Element.name(for: value)
         let helpForCase = hasCustomCaseHelp ? (caseHelps[i] ?? help) : help
         let help = ArgumentDefinition.Help(
diff --git a/Sources/ArgumentParser/Parsable Properties/NameSpecification.swift b/Sources/ArgumentParser/Parsable Properties/NameSpecification.swift
index bfdd48c4a..a71d9df6a 100644
--- a/Sources/ArgumentParser/Parsable Properties/NameSpecification.swift	
+++ b/Sources/ArgumentParser/Parsable Properties/NameSpecification.swift	
@@ -167,7 +167,7 @@ extension FlagInversion {
         case .short, .customShort:
           return includingShort ? element.name(for: key) : nil
         case .long:
-          let modifiedKey = key.with(newName: key.name.addingIntercappedPrefix(prefix))
+          let modifiedKey = InputKey(name: key.name.addingIntercappedPrefix(prefix), parent: key)
           return element.name(for: modifiedKey)
         case .customLong(let name, let withSingleDash):
           let modifiedName = name.addingPrefixWithAutodetectedStyle(prefix)
diff --git a/Sources/ArgumentParser/Parsable Properties/OptionGroup.swift b/Sources/ArgumentParser/Parsable Properties/OptionGroup.swift
index b74865b62..13192f8ed 100644
--- a/Sources/ArgumentParser/Parsable Properties/OptionGroup.swift	
+++ b/Sources/ArgumentParser/Parsable Properties/OptionGroup.swift	
@@ -78,7 +78,7 @@ public struct OptionGroup<Value: ParsableArguments>: Decodable, ParsedWrapper {
     visibility: ArgumentVisibility = .default
   ) {
     self.init(_parsedValue: .init { parentKey in
-      var args = ArgumentSet(Value.self, visibility: .private, parent: .key(parentKey))
+      var args = ArgumentSet(Value.self, visibility: .private, parent: parentKey)
       args.content.withEach {
         $0.help.parentTitle = title
       }
diff --git a/Sources/ArgumentParser/Parsable Types/ParsableArguments.swift b/Sources/ArgumentParser/Parsable Types/ParsableArguments.swift
index b2e572a6e..d85283c37 100644
--- a/Sources/ArgumentParser/Parsable Types/ParsableArguments.swift	
+++ b/Sources/ArgumentParser/Parsable Types/ParsableArguments.swift	
@@ -248,7 +248,7 @@ extension ArgumentSetProvider {
 }
 
 extension ArgumentSet {
-  init(_ type: ParsableArguments.Type, visibility: ArgumentVisibility, parent: InputKey.Parent) {
+  init(_ type: ParsableArguments.Type, visibility: ArgumentVisibility, parent: InputKey?) {
     #if DEBUG
     do {
       try type._validate(parent: parent)
diff --git a/Sources/ArgumentParser/Parsable Types/ParsableArgumentsValidation.swift b/Sources/ArgumentParser/Parsable Types/ParsableArgumentsValidation.swift
index f633d91f7..c03e9d3af 100644
--- a/Sources/ArgumentParser/Parsable Types/ParsableArgumentsValidation.swift	
+++ b/Sources/ArgumentParser/Parsable Types/ParsableArgumentsValidation.swift	
@@ -10,7 +10,7 @@
 //===----------------------------------------------------------------------===//
 
 fileprivate protocol ParsableArgumentsValidator {
-  static func validate(_ type: ParsableArguments.Type, parent: InputKey.Parent) -> ParsableArgumentsValidatorError?
+  static func validate(_ type: ParsableArguments.Type, parent: InputKey?) -> ParsableArgumentsValidatorError?
 }
 
 enum ValidatorErrorKind {
@@ -37,7 +37,7 @@ struct ParsableArgumentsValidationError: Error, CustomStringConvertible {
 }
 
 extension ParsableArguments {
-  static func _validate(parent: InputKey.Parent) throws {
+  static func _validate(parent: InputKey?) throws {
     let validators: [ParsableArgumentsValidator.Type] = [
       PositionalArgumentsValidator.self,
       ParsableArgumentsCodingKeyValidator.self,
@@ -80,7 +80,7 @@ struct PositionalArgumentsValidator: ParsableArgumentsValidator {
     var kind: ValidatorErrorKind { .failure }
   }
   
-  static func validate(_ type: ParsableArguments.Type, parent: InputKey.Parent) -> ParsableArgumentsValidatorError? {
+  static func validate(_ type: ParsableArguments.Type, parent: InputKey?) -> ParsableArgumentsValidatorError? {
     let sets: [ArgumentSet] = Mirror(reflecting: type.init())
       .children
       .compactMap { child in
@@ -190,7 +190,7 @@ struct ParsableArgumentsCodingKeyValidator: ParsableArgumentsValidator {
     }
   }
   
-  static func validate(_ type: ParsableArguments.Type, parent: InputKey.Parent) -> ParsableArgumentsValidatorError? {
+  static func validate(_ type: ParsableArguments.Type, parent: InputKey?) -> ParsableArgumentsValidatorError? {
     let argumentKeys: [InputKey] = Mirror(reflecting: type.init())
       .children
       .compactMap { child in
@@ -235,7 +235,7 @@ struct ParsableArgumentsUniqueNamesValidator: ParsableArgumentsValidator {
     var kind: ValidatorErrorKind { .failure }
   }
 
-  static func validate(_ type: ParsableArguments.Type, parent: InputKey.Parent) -> ParsableArgumentsValidatorError? {
+  static func validate(_ type: ParsableArguments.Type, parent: InputKey?) -> ParsableArgumentsValidatorError? {
     let argSets: [ArgumentSet] = Mirror(reflecting: type.init())
       .children
       .compactMap { child in
@@ -283,7 +283,7 @@ struct NonsenseFlagsValidator: ParsableArgumentsValidator {
     var kind: ValidatorErrorKind { .warning }
   }
 
-  static func validate(_ type: ParsableArguments.Type, parent: InputKey.Parent) -> ParsableArgumentsValidatorError? {
+  static func validate(_ type: ParsableArguments.Type, parent: InputKey?) -> ParsableArgumentsValidatorError? {
     let argSets: [ArgumentSet] = Mirror(reflecting: type.init())
       .children
       .compactMap { child in
diff --git a/Sources/ArgumentParser/Parsable Types/ParsableCommand.swift b/Sources/ArgumentParser/Parsable Types/ParsableCommand.swift
index 288ea7f6e..af835a007 100644
--- a/Sources/ArgumentParser/Parsable Types/ParsableCommand.swift	
+++ b/Sources/ArgumentParser/Parsable Types/ParsableCommand.swift	
@@ -166,7 +166,7 @@ extension ParsableCommand {
   /// `true` if this command contains any array arguments that are declared
   /// with `.unconditionalRemaining`.
   internal static var includesUnconditionalArguments: Bool {
-    ArgumentSet(self, visibility: .private, parent: .root).contains(where: {
+    ArgumentSet(self, visibility: .private, parent: nil).contains(where: {
       $0.isRepeatingPositional && $0.parsingStrategy == .allRemainingInput
     })
   }
diff --git a/Sources/ArgumentParser/Parsing/ArgumentDefinition.swift b/Sources/ArgumentParser/Parsing/ArgumentDefinition.swift
index bdbda9a26..d1036b39b 100644
--- a/Sources/ArgumentParser/Parsing/ArgumentDefinition.swift
+++ b/Sources/ArgumentParser/Parsing/ArgumentDefinition.swift
@@ -217,7 +217,7 @@ extension ArgumentDefinition {
   ///
   /// This initializer is used for any property defined on a `ParsableArguments`
   /// type that isn't decorated with one of ArgumentParser's property wrappers.
-  init(unparsedKey: String, default defaultValue: Any?, parent: InputKey.Parent) {
+  init(unparsedKey: String, default defaultValue: Any?, parent: InputKey?) {
     self.init(
       container: Bare<Any>.self,
       key: InputKey(name: unparsedKey, parent: parent),
diff --git a/Sources/ArgumentParser/Parsing/ArgumentSet.swift b/Sources/ArgumentParser/Parsing/ArgumentSet.swift
index 6adfebf99..dc689dc5e 100644
--- a/Sources/ArgumentParser/Parsing/ArgumentSet.swift
+++ b/Sources/ArgumentParser/Parsing/ArgumentSet.swift
@@ -438,7 +438,7 @@ extension ArgumentSet {
   func firstPositional(
     named name: String
   ) -> ArgumentDefinition? {
-    let key = InputKey(name: name, parent: .root)
+    let key = InputKey(name: name, parent: nil)
     return first(where: { $0.help.keys.contains(key) })
   }
   
diff --git a/Sources/ArgumentParser/Parsing/CommandParser.swift b/Sources/ArgumentParser/Parsing/CommandParser.swift
index ee327c41f..5c17cf228 100644
--- a/Sources/ArgumentParser/Parsing/CommandParser.swift
+++ b/Sources/ArgumentParser/Parsing/CommandParser.swift
@@ -140,7 +140,7 @@ extension CommandParser {
   /// possible.
   fileprivate mutating func parseCurrent(_ split: inout SplitArguments) throws -> ParsableCommand {
     // Build the argument set (i.e. information on how to parse):
-    let commandArguments = ArgumentSet(currentNode.element, visibility: .private, parent: .root)
+    let commandArguments = ArgumentSet(currentNode.element, visibility: .private, parent: nil)
     
     // Parse the arguments, ignoring anything unexpected
     let values = try commandArguments.lenientParse(
@@ -325,7 +325,7 @@ extension CommandParser {
     let completionValues = Array(args)
 
     // Generate the argument set and parse the argument to find in the set
-    let argset = ArgumentSet(current.element, visibility: .private, parent: .root)
+    let argset = ArgumentSet(current.element, visibility: .private, parent: nil)
     let parsedArgument = try! parseIndividualArg(argToMatch, at: 0).first!
     
     // Look up the specified argument and retrieve its custom completion function
diff --git a/Sources/ArgumentParser/Parsing/InputKey.swift b/Sources/ArgumentParser/Parsing/InputKey.swift
index 1d47d17cf..9837e41e0 100644
--- a/Sources/ArgumentParser/Parsing/InputKey.swift
+++ b/Sources/ArgumentParser/Parsing/InputKey.swift
@@ -9,122 +9,48 @@
 //
 //===----------------------------------------------------------------------===//
 
-/// Represents the path to a parsed field, annotated with ``Flag``, ``Option`` or
-/// ``Argument``. It has a parent, which will either be ``InputKey/Parent/root``
-/// if the field is on the root ``ParsableComand`` or ``AsyncParsableCommand``,
-/// or it will have a ``InputKey/Parent/key(InputKey)`` if it is defined in
-/// a ``ParsableArguments`` instance.
+/// Represents the path to a parsed field, annotated with ``Flag``, ``Option``
+/// or ``Argument``. Fields that are directly declared on a ``ParsableComand``
+/// have a path of length 1, while fields that are declared indirectly (and
+/// included via an option group) have longer paths.
 struct InputKey: Hashable {
-  /// Describes the parent of an ``InputKey``.
-  indirect enum Parent: Hashable {
-    /// There is no parent key.
-    case root
-    /// There is a parent key.
-    case key(InputKey)
-    
-    /// Initialises a parent depending on whether the key is provided.
-    init(_ key: InputKey?) {
-      if let key = key {
-        self = .key(key)
-      } else {
-        self = .root
-      }
-    }
-  }
-  
   /// The name of the input key.
-  let name: String
-  
-  /// The parent of this key.
-  let parent: Parent
+  var name: String
+
+  /// The path through the field's parents, if any.
+  var path: [String]
   
+  /// The full path of the field.
+  var fullPath: [String] { path + [name] }
   
-  /// Constructs a new ``InputKey``, cleaing the `name`, with the specified ``InputKey/Parent``.
+  /// Constructs a new input key, cleaning the name, with the specified parent.
   ///
   /// - Parameter name: The name of the key.
-  /// - Parameter parent: The ``InputKey/Parent`` of the key.
-  init(name: String, parent: Parent) {
-    self.name = Self.clean(codingKey: name)
-    self.parent = parent
-  }
-  
-  @inlinable
-  init?(path: [CodingKey]) {
-    var parentPath = path
-    guard let key = parentPath.popLast() else {
-      return nil
-    }
-    self.name = Self.clean(codingKey: key)
-    self.parent = Parent(InputKey(path: parentPath))
-  }
-
-  /// Constructs a new ``InputKey``, "cleaning the `value` and `path` if necessary.
-  ///
-  /// - Parameter value: The base value of the key.
-  /// - Parameter path: The list of ``CodingKey`` values that lead to this one. May be empty.
-  @inlinable
-  init(name: String, path: [CodingKey]) {
-    self.init(name: name, parent: Parent(InputKey(path: path)))
+  /// - Parameter parent: The input key of the parent.
+  init(name: String, parent: InputKey?) {
+    // Property wrappers have underscore-prefixed names, so we remove the
+    // leading `_`, if present.
+    self.name = name.first == "_"
+      ? String(name.dropFirst(1))
+      : name
+    self.path = parent?.fullPath ?? []
   }
   
-  /// Constructs a new ``InputKey``, "cleaning the `value` and `path` if necessary.
+  /// Constructs a new input key from the given coding key and parent path.
   ///
-  /// - Parameter codingKey: The base ``CodingKey``
-  /// - Parameter path: The list of ``CodingKey`` values that lead to this one. May be empty.
+  /// - Parameter codingKey: The base ``CodingKey``. Leading underscores in
+  ///   `codingKey` is preserved.
+  /// - Parameter path: The list of ``CodingKey`` values that lead to this one.
+  ///   `path` may be empty.
   @inlinable
   init(codingKey: CodingKey, path: [CodingKey]) {
-    self.init(name: codingKey.stringValue, parent: Parent(InputKey(path: path)))
-  }
-  
-  /// The full path, including the ``parent`` and the ``name``.
-  var fullPath: [String] {
-    switch parent {
-    case .root:
-      return [name]
-    case .key(let key):
-      var parentPath = key.fullPath
-      parentPath.append(name)
-      return parentPath
-    }
-  }
-  
-  /// Returns a new ``InputKey`` with the same ``path`` and a new ``name``.
-  /// The new value will be cleaned.
-  ///
-  /// - Parameter newName: The new ``String`` value.
-  /// - Returns: A new ``InputKey`` with the cleaned value and the same ``path``.
-  func with(newName: String) -> InputKey {
-    return .init(name: Self.clean(codingKey: newName), parent: self.parent)
-  }
-}
-
-extension InputKey {
-  /// Property wrappers have underscore-prefixed names, so this returns a "clean"
-  /// version of the `codingKey`, which has the leading `'_'` removed, if present.
-  ///
-  /// - Parameter codingKey: The key to clean.
-  /// - Returns: The cleaned key.
-  static func clean(codingKey: String) -> String {
-    String(codingKey.first == "_" ? codingKey.dropFirst(1) : codingKey.dropFirst(0))
-  }
-  
-  /// Property wrappers have underscore-prefixed names, so this returns a "clean"
-  /// version of the `codingKey`, which has the leading `'_'` removed, if present.
-  ///
-  /// - Parameter codingKey: The key to clean.
-  /// - Returns: The cleaned key.
-  static func clean(codingKey: CodingKey) -> String {
-    clean(codingKey: codingKey.stringValue)
+    self.name = codingKey.stringValue
+    self.path = path.map { $0.stringValue }
   }
 }
 
 extension InputKey: CustomStringConvertible {
   var description: String {
-    switch parent {
-    case .key(let parent):
-      return "\(parent).\(name)"
-    case .root:
-      return name
-    }
+    fullPath.joined(separator: ".")
   }
 }
diff --git a/Sources/ArgumentParser/Usage/DumpHelpGenerator.swift b/Sources/ArgumentParser/Usage/DumpHelpGenerator.swift
index 658fc9efe..efd0442ab 100644
--- a/Sources/ArgumentParser/Usage/DumpHelpGenerator.swift
+++ b/Sources/ArgumentParser/Usage/DumpHelpGenerator.swift
@@ -38,7 +38,7 @@ fileprivate extension BidirectionalCollection where Element == ParsableCommand.T
   /// Returns the ArgumentSet for the last command in this stack, including
   /// help and version flags, when appropriate.
   func allArguments() -> ArgumentSet {
-    guard var arguments = self.last.map({ ArgumentSet($0, visibility: .private, parent: .root) })
+    guard var arguments = self.last.map({ ArgumentSet($0, visibility: .private, parent: nil) })
     else { return ArgumentSet() }
     self.versionArgumentDefinition().map { arguments.append($0) }
     self.helpArgumentDefinition().map { arguments.append($0) }
diff --git a/Sources/ArgumentParser/Usage/HelpGenerator.swift b/Sources/ArgumentParser/Usage/HelpGenerator.swift
index 7bc6020c6..440f73f3e 100644
--- a/Sources/ArgumentParser/Usage/HelpGenerator.swift
+++ b/Sources/ArgumentParser/Usage/HelpGenerator.swift
@@ -97,7 +97,7 @@ internal struct HelpGenerator {
       fatalError()
     }
     
-    let currentArgSet = ArgumentSet(currentCommand, visibility: visibility, parent: .root)
+    let currentArgSet = ArgumentSet(currentCommand, visibility: visibility, parent: nil)
     self.commandStack = commandStack
 
     // Build the tool name and subcommand name from the command configuration
@@ -292,7 +292,7 @@ fileprivate extension NameSpecification {
   /// step, the name are returned in descending order.
   func generateHelpNames(visibility: ArgumentVisibility) -> [Name] {
     self
-      .makeNames(InputKey(name: "help", parent: .root))
+      .makeNames(InputKey(name: "help", parent: nil))
       .compactMap { name in
         guard visibility.base != .default else { return name }
         switch name {
@@ -333,7 +333,7 @@ internal extension BidirectionalCollection where Element == ParsableCommand.Type
         options: [.isOptional],
         help: "Show the version.",
         defaultValue: nil,
-        key: InputKey(name: "", parent: .root),
+        key: InputKey(name: "", parent: nil),
         isComposite: false),
       completion: .default,
       update: .nullary({ _, _, _ in })
@@ -350,7 +350,7 @@ internal extension BidirectionalCollection where Element == ParsableCommand.Type
         options: [.isOptional],
         help: "Show help information.",
         defaultValue: nil,
-        key: InputKey(name: "", parent: .root),
+        key: InputKey(name: "", parent: nil),
         isComposite: false),
       completion: .default,
       update: .nullary({ _, _, _ in })
@@ -365,7 +365,7 @@ internal extension BidirectionalCollection where Element == ParsableCommand.Type
         options: [.isOptional],
         help: ArgumentHelp("Dump help information as JSON."),
         defaultValue: nil,
-        key: InputKey(name: "", parent: .root),
+        key: InputKey(name: "", parent: nil),
         isComposite: false),
       completion: .default,
       update: .nullary({ _, _, _ in })
@@ -375,7 +375,7 @@ internal extension BidirectionalCollection where Element == ParsableCommand.Type
   /// Returns the ArgumentSet for the last command in this stack, including
   /// help and version flags, when appropriate.
   func argumentsForHelp(visibility: ArgumentVisibility) -> ArgumentSet {
-    guard var arguments = self.last.map({ ArgumentSet($0, visibility: visibility, parent: .root) })
+    guard var arguments = self.last.map({ ArgumentSet($0, visibility: visibility, parent: nil) })
       else { return ArgumentSet() }
     self.versionArgumentDefinition().map { arguments.append($0) }
     self.helpArgumentDefinition().map { arguments.append($0) }
diff --git a/Sources/ArgumentParser/Usage/MessageInfo.swift b/Sources/ArgumentParser/Usage/MessageInfo.swift
index 68d4a01cd..896c565b7 100644
--- a/Sources/ArgumentParser/Usage/MessageInfo.swift
+++ b/Sources/ArgumentParser/Usage/MessageInfo.swift
@@ -122,7 +122,7 @@ enum MessageInfo {
         guard case ParserError.noArguments = parserError else { return usage }
         return "\n" + HelpGenerator(commandStack: [type.asCommand], visibility: .default).rendered()
       }()
-      let argumentSet = ArgumentSet(commandStack.last!, visibility: .default, parent: .root)
+      let argumentSet = ArgumentSet(commandStack.last!, visibility: .default, parent: nil)
       let message = argumentSet.errorDescription(error: parserError) ?? ""
       let helpAbstract = argumentSet.helpDescription(error: parserError) ?? ""
       self = .validation(message: message, usage: usage, help: helpAbstract)
diff --git a/Sources/ArgumentParser/Usage/UsageGenerator.swift b/Sources/ArgumentParser/Usage/UsageGenerator.swift
index b80bdee15..baf46c044 100644
--- a/Sources/ArgumentParser/Usage/UsageGenerator.swift
+++ b/Sources/ArgumentParser/Usage/UsageGenerator.swift
@@ -22,7 +22,7 @@ extension UsageGenerator {
     self.init(toolName: toolName, definition: definition)
   }
   
-  init(toolName: String, parsable: ParsableArguments, visibility: ArgumentVisibility, parent: InputKey.Parent) {
+  init(toolName: String, parsable: ParsableArguments, visibility: ArgumentVisibility, parent: InputKey?) {
     self.init(
       toolName: toolName,
       definition: ArgumentSet(type(of: parsable), visibility: visibility, parent: parent))
diff --git a/Tests/ArgumentParserEndToEndTests/DefaultsEndToEndTests.swift b/Tests/ArgumentParserEndToEndTests/DefaultsEndToEndTests.swift
index 4c396c581..4a8e7b7df 100644
--- a/Tests/ArgumentParserEndToEndTests/DefaultsEndToEndTests.swift
+++ b/Tests/ArgumentParserEndToEndTests/DefaultsEndToEndTests.swift
@@ -828,3 +828,33 @@ extension DefaultsEndToEndTests {
     }
   }
 }
+
+extension DefaultsEndToEndTests {
+  private struct UnderscoredOptional: ParsableCommand {
+    @Option(name: .customLong("arg"))
+    var _arg: String?
+  }
+
+  private struct UnderscoredArray: ParsableCommand {
+    @Option(name: .customLong("columns"), parsing: .upToNextOption)
+    var _columns: [String] = []
+  }
+
+  func testUnderscoredOptional() throws {
+    AssertParse(UnderscoredOptional.self, []) { parsed in
+      XCTAssertNil(parsed._arg)
+    }
+    AssertParse(UnderscoredOptional.self, ["--arg", "foo"]) { parsed in
+      XCTAssertEqual(parsed._arg, "foo")
+    }
+  }
+
+  func testUnderscoredArray() throws {
+    AssertParse(UnderscoredArray.self, []) { parsed in
+      XCTAssertEqual(parsed._columns, [])
+    }
+    AssertParse(UnderscoredArray.self, ["--columns", "foo", "bar", "baz"]) { parsed in
+      XCTAssertEqual(parsed._columns, ["foo", "bar", "baz"])
+    }
+  }
+}
diff --git a/Tests/ArgumentParserEndToEndTests/OptionGroupEndToEndTests.swift b/Tests/ArgumentParserEndToEndTests/OptionGroupEndToEndTests.swift
index c47ac2776..0968157c8 100644
--- a/Tests/ArgumentParserEndToEndTests/OptionGroupEndToEndTests.swift
+++ b/Tests/ArgumentParserEndToEndTests/OptionGroupEndToEndTests.swift
@@ -109,52 +109,66 @@ extension OptionGroupEndToEndTests {
   }
 }
 
-fileprivate struct DuplicatedFlagOption: ParsableArguments {
+fileprivate struct DuplicatedFlagGroupCustom: ParsableArguments {
   @Flag(name: .customLong("duplicated-option"))
   var duplicated: Bool = false
-  
-  enum CodingKeys: CodingKey {
-    case duplicated
-  }
 }
 
-fileprivate struct DuplicatedFlagCommand: ParsableCommand {
+fileprivate struct DuplicatedFlagGroupCustomCommand: ParsableCommand {
+  @Flag var duplicated: Bool = false
+  @OptionGroup var option: DuplicatedFlagGroupCustom
+}
+
+fileprivate struct DuplicatedFlagGroupLong: ParsableArguments {
+  @Flag var duplicated: Bool = false
+}
 
-  @Flag
+fileprivate struct DuplicatedFlagGroupLongCommand: ParsableCommand {
+  @Flag(name: .customLong("duplicated-option"))
   var duplicated: Bool = false
-  
-  @OptionGroup var option: DuplicatedFlagOption
-  
-  enum CodingKeys: CodingKey {
-    case duplicated
-    case option
-  }
+  @OptionGroup var option: DuplicatedFlagGroupLong
 }
 
 extension OptionGroupEndToEndTests {
   func testUniqueNamesForDuplicatedFlag_NoFlags() throws {
-    AssertParseCommand(DuplicatedFlagCommand.self, DuplicatedFlagCommand.self, []) { command in
+    AssertParse(DuplicatedFlagGroupCustomCommand.self, []) { command in
+      XCTAssertFalse(command.duplicated)
+      XCTAssertFalse(command.option.duplicated)
+    }
+    AssertParse(DuplicatedFlagGroupLongCommand.self, []) { command in
       XCTAssertFalse(command.duplicated)
       XCTAssertFalse(command.option.duplicated)
     }
   }
   
   func testUniqueNamesForDuplicatedFlag_RootOnly() throws {
-    AssertParseCommand(DuplicatedFlagCommand.self, DuplicatedFlagCommand.self, ["--duplicated"]) { command in
+    AssertParse(DuplicatedFlagGroupCustomCommand.self, ["--duplicated"]) { command in
       XCTAssertTrue(command.duplicated)
       XCTAssertFalse(command.option.duplicated)
     }
+    AssertParse(DuplicatedFlagGroupLongCommand.self, ["--duplicated"]) { command in
+      XCTAssertFalse(command.duplicated)
+      XCTAssertTrue(command.option.duplicated)
+    }
   }
   
   func testUniqueNamesForDuplicatedFlag_OptionOnly() throws {
-    AssertParseCommand(DuplicatedFlagCommand.self, DuplicatedFlagCommand.self, ["--duplicated-option"]) { command in
+    AssertParse(DuplicatedFlagGroupCustomCommand.self, ["--duplicated-option"]) { command in
       XCTAssertFalse(command.duplicated)
       XCTAssertTrue(command.option.duplicated)
     }
+    AssertParse(DuplicatedFlagGroupLongCommand.self, ["--duplicated-option"]) { command in
+      XCTAssertTrue(command.duplicated)
+      XCTAssertFalse(command.option.duplicated)
+    }
   }
   
   func testUniqueNamesForDuplicatedFlag_RootAndOption() throws {
-    AssertParseCommand(DuplicatedFlagCommand.self, DuplicatedFlagCommand.self, ["--duplicated", "--duplicated-option"]) { command in
+    AssertParse(DuplicatedFlagGroupCustomCommand.self, ["--duplicated", "--duplicated-option"]) { command in
+      XCTAssertTrue(command.duplicated)
+      XCTAssertTrue(command.option.duplicated)
+    }
+    AssertParse(DuplicatedFlagGroupLongCommand.self, ["--duplicated", "--duplicated-option"]) { command in
       XCTAssertTrue(command.duplicated)
       XCTAssertTrue(command.option.duplicated)
     }
diff --git a/Tests/ArgumentParserUnitTests/HelpGenerationTests.swift b/Tests/ArgumentParserUnitTests/HelpGenerationTests.swift
index 1b3d7435c..476eb67a8 100644
--- a/Tests/ArgumentParserUnitTests/HelpGenerationTests.swift
+++ b/Tests/ArgumentParserUnitTests/HelpGenerationTests.swift
@@ -638,7 +638,7 @@ extension HelpGenerationTests {
   }
 
   func testAllValues() {
-    let opts = ArgumentSet(AllValues.self, visibility: .private, parent: .root)
+    let opts = ArgumentSet(AllValues.self, visibility: .private, parent: nil)
     XCTAssertEqual(AllValues.Manual.allValueStrings, opts[0].help.allValues)
     XCTAssertEqual(AllValues.Manual.allValueStrings, opts[1].help.allValues)
 
diff --git a/Tests/ArgumentParserUnitTests/NameSpecificationTests.swift b/Tests/ArgumentParserUnitTests/NameSpecificationTests.swift
index b0b6c905d..328b77afb 100644
--- a/Tests/ArgumentParserUnitTests/NameSpecificationTests.swift
+++ b/Tests/ArgumentParserUnitTests/NameSpecificationTests.swift
@@ -17,7 +17,7 @@ final class NameSpecificationTests: XCTestCase {
 
 extension NameSpecificationTests {
   func testFlagNames_withNoPrefix() {
-    let key = InputKey(name: "index", parent: .root)
+    let key = InputKey(name: "index", parent: nil)
     
     XCTAssertEqual(FlagInversion.prefixedNo.enableDisableNamePair(for: key, name: .customLong("foo")).1, [.long("no-foo")])
     XCTAssertEqual(FlagInversion.prefixedNo.enableDisableNamePair(for: key, name: .customLong("foo-bar-baz")).1, [.long("no-foo-bar-baz")])
@@ -26,7 +26,7 @@ extension NameSpecificationTests {
   }
   
   func testFlagNames_withEnableDisablePrefix() {
-    let key = InputKey(name: "index", parent: .root)
+    let key = InputKey(name: "index", parent: nil)
     XCTAssertEqual(FlagInversion.prefixedEnableDisable.enableDisableNamePair(for: key, name: .long).0, [.long("enable-index")])
     XCTAssertEqual(FlagInversion.prefixedEnableDisable.enableDisableNamePair(for: key, name: .long).1, [.long("disable-index")])
     
@@ -42,7 +42,7 @@ extension NameSpecificationTests {
   }
 }
 
-fileprivate func Assert(nameSpecification: NameSpecification, key: String, parent: InputKey.Parent = .root, makeNames expected: [Name], file: StaticString = #file, line: UInt = #line) {
+fileprivate func Assert(nameSpecification: NameSpecification, key: String, parent: InputKey? = nil, makeNames expected: [Name], file: StaticString = #file, line: UInt = #line) {
   let names = nameSpecification.makeNames(InputKey(name: key, parent: parent))
   Assert(names: names, expected: expected, file: file, line: line)
 }
diff --git a/Tests/ArgumentParserUnitTests/ParsableArgumentsValidationTests.swift b/Tests/ArgumentParserUnitTests/ParsableArgumentsValidationTests.swift
index cb2ba5f6e..16a6ecfb4 100644
--- a/Tests/ArgumentParserUnitTests/ParsableArgumentsValidationTests.swift
+++ b/Tests/ArgumentParserUnitTests/ParsableArgumentsValidationTests.swift
@@ -81,7 +81,7 @@ final class ParsableArgumentsValidationTests: XCTestCase {
   }
 
   func testCodingKeyValidation() throws {
-    let parent = InputKey.Parent.key(InputKey(name: "parentKey", parent: .root))
+    let parent = InputKey(name: "parentKey", parent: nil)
     XCTAssertNil(ParsableArgumentsCodingKeyValidator.validate(A.self, parent: parent))
     XCTAssertNil(ParsableArgumentsCodingKeyValidator.validate(B.self, parent: parent))
 
@@ -130,7 +130,7 @@ final class ParsableArgumentsValidationTests: XCTestCase {
   }
 
   func testCustomDecoderValidation() throws {
-    let parent = InputKey.Parent(InputKey(name: "foo", parent: .root))
+    let parent = InputKey(name: "foo", parent: nil)
         if let error = ParsableArgumentsCodingKeyValidator.validate(TypeWithInvalidDecoder.self, parent: parent)
       as? ParsableArgumentsCodingKeyValidator.InvalidDecoderError
     {
@@ -211,7 +211,7 @@ final class ParsableArgumentsValidationTests: XCTestCase {
   }
 
   func testPositionalArgumentsValidation() throws {
-    let parent = InputKey.Parent(InputKey(name: "foo", parent: .root))
+    let parent = InputKey(name: "foo", parent: nil)
     XCTAssertNil(PositionalArgumentsValidator.validate(A.self, parent: parent))
     XCTAssertNil(PositionalArgumentsValidator.validate(F.self, parent: parent))
     XCTAssertNil(PositionalArgumentsValidator.validate(H.self, parent: parent))
@@ -246,7 +246,7 @@ final class ParsableArgumentsValidationTests: XCTestCase {
   }
 
   func testUniqueNamesValidation_NoViolation() throws {
-    let parent = InputKey.Parent(InputKey(name: "foo", parent: .root))
+    let parent = InputKey(name: "foo", parent: nil)
     XCTAssertNil(ParsableArgumentsUniqueNamesValidator.validate(DifferentNames.self, parent: parent))
   }
 
@@ -260,7 +260,7 @@ final class ParsableArgumentsValidationTests: XCTestCase {
   }
 
   func testUniqueNamesValidation_TwoOfSameName() throws {
-    if let error = ParsableArgumentsUniqueNamesValidator.validate(TwoOfTheSameName.self, parent: .root)
+    if let error = ParsableArgumentsUniqueNamesValidator.validate(TwoOfTheSameName.self, parent: nil)
       as? ParsableArgumentsUniqueNamesValidator.Error
     {
       XCTAssertEqual(error.description, "Multiple (2) `Option` or `Flag` arguments are named \"--foo\".")
@@ -288,7 +288,7 @@ final class ParsableArgumentsValidationTests: XCTestCase {
   }
 
   func testUniqueNamesValidation_TwoDuplications() throws {
-    let parent = InputKey.Parent(InputKey(name: "option", parent: .root))
+    let parent = InputKey(name: "option", parent: nil)
     if let error = ParsableArgumentsUniqueNamesValidator.validate(MultipleUniquenessViolations.self, parent: parent)
       as? ParsableArgumentsUniqueNamesValidator.Error
     {
@@ -324,7 +324,7 @@ final class ParsableArgumentsValidationTests: XCTestCase {
   }
 
   func testUniqueNamesValidation_ArgumentHasMultipleNames() throws {
-    if let error = ParsableArgumentsUniqueNamesValidator.validate(MultipleNamesPerArgument.self, parent: .root)
+    if let error = ParsableArgumentsUniqueNamesValidator.validate(MultipleNamesPerArgument.self, parent: nil)
       as? ParsableArgumentsUniqueNamesValidator.Error
     {
       XCTAssertEqual(error.description, "Multiple (2) `Option` or `Flag` arguments are named \"-v\".")
@@ -355,7 +355,7 @@ final class ParsableArgumentsValidationTests: XCTestCase {
   }
 
   func testUniqueNamesValidation_MoreThanTwoDuplications() throws {
-    if let error = ParsableArgumentsUniqueNamesValidator.validate(FourDuplicateNames.self, parent: .root)
+    if let error = ParsableArgumentsUniqueNamesValidator.validate(FourDuplicateNames.self, parent: nil)
       as? ParsableArgumentsUniqueNamesValidator.Error
     {
       XCTAssertEqual(error.description, "Multiple (4) `Option` or `Flag` arguments are named \"--foo\".")
@@ -397,7 +397,7 @@ final class ParsableArgumentsValidationTests: XCTestCase {
   }
 
   func testUniqueNamesValidation_DuplicatedFlagFirstLetters_ShortNames() throws {
-    if let error = ParsableArgumentsUniqueNamesValidator.validate(DuplicatedFirstLettersShortNames.self, parent: .root)
+    if let error = ParsableArgumentsUniqueNamesValidator.validate(DuplicatedFirstLettersShortNames.self, parent: nil)
       as? ParsableArgumentsUniqueNamesValidator.Error
     {
       XCTAssertEqual(error.description, "Multiple (3) `Option` or `Flag` arguments are named \"-f\".")
@@ -407,7 +407,7 @@ final class ParsableArgumentsValidationTests: XCTestCase {
   }
 
   func testUniqueNamesValidation_DuplicatedFlagFirstLetters_LongNames() throws {
-    XCTAssertNil(ParsableArgumentsUniqueNamesValidator.validate(DuplicatedFirstLettersLongNames.self, parent: .root))
+    XCTAssertNil(ParsableArgumentsUniqueNamesValidator.validate(DuplicatedFirstLettersLongNames.self, parent: nil))
   }
     
   fileprivate struct HasOneNonsenseFlag: ParsableCommand {
@@ -439,7 +439,7 @@ final class ParsableArgumentsValidationTests: XCTestCase {
   }
 
   func testNonsenseFlagsValidation_OneFlag() throws {
-    if let error = NonsenseFlagsValidator.validate(HasOneNonsenseFlag.self, parent: .root)
+    if let error = NonsenseFlagsValidator.validate(HasOneNonsenseFlag.self, parent: nil)
       as? NonsenseFlagsValidator.Error
     {
       XCTAssertEqual(
@@ -476,7 +476,7 @@ final class ParsableArgumentsValidationTests: XCTestCase {
   }
 
   func testNonsenseFlagsValidation_MultipleFlags() throws {
-    if let error = NonsenseFlagsValidator.validate(MultipleNonsenseFlags.self, parent: .root)
+    if let error = NonsenseFlagsValidator.validate(MultipleNonsenseFlags.self, parent: nil)
         as? NonsenseFlagsValidator.Error
     {
       XCTAssertEqual(
diff --git a/Tests/ArgumentParserUnitTests/UsageGenerationTests.swift b/Tests/ArgumentParserUnitTests/UsageGenerationTests.swift
index 7f57f6102..2c605aab8 100644
--- a/Tests/ArgumentParserUnitTests/UsageGenerationTests.swift
+++ b/Tests/ArgumentParserUnitTests/UsageGenerationTests.swift
@@ -22,7 +22,7 @@ func _testSynopsis<T: ParsableArguments>(
   file: StaticString = #file,
   line: UInt = #line
 ) {
-  let help = UsageGenerator(toolName: "example", parsable: T(), visibility: visibility, parent: .root)
+  let help = UsageGenerator(toolName: "example", parsable: T(), visibility: visibility, parent: nil)
   XCTAssertEqual(help.synopsis, expected, file: file, line: line)
 }