Add a parent command property wrapper to gain access to parent state

Sources/ArgumentParser/Parsable Properties/ParentCommand.swift

@@ -0,0 +1,91 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift Argument Parser open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+//
+//===----------------------------------------------------------------------===//
+
+/// A wrapper that adds a reference to a parent command.
+///
+/// Use the parent command wrapper to gain access to a parent command's state.
+///
+/// ```swift
+/// struct SuperCommand: ParsableCommand {
+///     static let configuration = CommandConfiguration(
+///         subcommands: [SubCommand.self]
+///     )
+///
+///     @Flag(name: .shortAndLong)
+///     var verbose: Bool = false
+/// }
+///
+/// struct SubCommand: ParsableCommand {
+///     @ParentCommand var parent: SuperCommand
+///
+///     mutating func run() throws {
+///         if self.parent.verbose {
+///             print("Verbose")
+///         }
+///     }
+/// }
+/// ```
+@propertyWrapper
+public struct ParentCommand<Value: ParsableCommand>: Decodable, ParsedWrapper {
+  internal var _parsedValue: Parsed<Value>
+  internal var _visibility: ArgumentVisibility
+
+  internal init(_parsedValue: Parsed<Value>) {
+    self._parsedValue = _parsedValue
+    self._visibility = .default
+  }
+
+  public init(from _decoder: Decoder) throws {
+    if let d = _decoder as? SingleValueDecoder,
+      let value = try? d.previousValue(Value.self)
+    {
+      self.init(_parsedValue: .value(value))
+    } else {
+      // TODO produce a specialized error in the case where the parent is not in fact a parent of this command
+      throw ParserError.notParentCommand("\(Value.self)")
+    }
+  }
+
+  public init() {
+    self.init(
+      _parsedValue: .init { _ in
+        .init()
+      }
+    )
+  }
+
+  public var wrappedValue: Value {
+    get {
+      switch _parsedValue {
+      case .value(let v):
+        return v
+      case .definition:
+        configurationFailure(directlyInitializedError)
+      }
+    }
+    set {
+      _parsedValue = .value(newValue)
+    }
+  }
+}
+
+extension ParentCommand: Sendable where Value: Sendable {}
+
+extension ParentCommand: CustomStringConvertible {
+  public var description: String {
+    switch _parsedValue {
+    case .value(let v):
+      return String(describing: v)
+    case .definition:
+      return "ParentCommand(*definition*)"
+    }
+  }
+}

Sources/ArgumentParser/Parsing/ParserError.swift

@@ -38,6 +38,7 @@ enum ParserError: Error {
   case missingSubcommand
   case userValidationError(Error)
   case noArguments(Error)
+  case notParentCommand(String)
 }
 
 /// These are errors used internally to the parsing, and will not be exposed to the help generation.

Sources/ArgumentParser/Usage/UsageGenerator.swift

@@ -226,6 +226,8 @@ extension ErrorMessageGenerator {
       default:
         return error.describe()
       }
+    case .notParentCommand(let parent):
+      return "Command '\(parent)' is not a parent of the current command."
     }
   }
 

Sources/ArgumentParserTestHelpers/TestHelpers.swift

@@ -154,6 +154,26 @@ public func AssertParseCommand<A: ParsableCommand>(
   }
 }
 
+// swift-format-ignore: AlwaysUseLowerCamelCase
+public func AssertParseCommandErrorMessage<A: ParsableCommand>(
+  _ rootCommand: ParsableCommand.Type, _ type: A.Type, _ arguments: [String],
+  _ errorMessage: String,
+  file: StaticString = #filePath, line: UInt = #line
+) {
+  do {
+    let command = try rootCommand.parseAsRoot(arguments)
+    guard (command as? A) != nil else {
+      XCTFail(
+        "Command is of unexpected type: \(command)", file: (file), line: line)
+      return
+    }
+    XCTFail("Parsing as root should have failed.", file: file, line: line)
+  } catch {
+    let message = rootCommand.message(for: error)
+    XCTAssertEqual(message, errorMessage, file: file, line: line)
+  }
+}
+
 // swift-format-ignore: AlwaysUseLowerCamelCase
 public func AssertEqualStrings(
   actual: String,

Tests/ArgumentParserEndToEndTests/DefaultSubcommandEndToEndTests.swift

@@ -10,6 +10,7 @@
 //===----------------------------------------------------------------------===//
 
 import ArgumentParserTestHelpers
+import ArgumentParserToolInfo
 import XCTest
 
 @testable import ArgumentParser
@@ -75,10 +76,14 @@ extension DefaultSubcommandEndToEndTests {
 extension DefaultSubcommandEndToEndTests {
   fileprivate struct MyCommand: ParsableCommand {
     static let configuration = CommandConfiguration(
-      subcommands: [Plugin.self, NonDefault.self, Other.self],
+      subcommands: [
+        Plugin.self, NonDefault.self, Other.self, Child.self, BadParent.self,
+      ],
       defaultSubcommand: Plugin.self
     )
 
+    @Option var foo: String?
+
     @OptionGroup
     var options: CommonOptions
   }
@@ -110,6 +115,53 @@ extension DefaultSubcommandEndToEndTests {
     @OptionGroup var options: CommonOptions
   }
 
+  fileprivate struct Child: ParsableCommand {
+    @ParentCommand var parent: MyCommand
+  }
+
+  fileprivate struct BadParent: ParsableCommand {
+    @ParentCommand var notMyParent: Other
+  }
+
+  func testAccessToParent() throws {
+    AssertParseCommand(
+      MyCommand.self, Child.self, ["--verbose", "--foo=bar", "child"]
+    ) { child in
+      XCTAssertEqual(child.parent.foo, "bar")
+      XCTAssertEqual(child.parent.options.verbose, true)
+    }
+  }
+
+  func testNotMyParent() throws {
+    AssertParseCommandErrorMessage(
+      MyCommand.self, BadParent.self, ["--verbose", "bad-parent"],
+      "Command 'Other' is not a parent of the current command.")
+  }
+
+  func testNotLeakingParentOptions() throws {
+    // Verify that the help for the child command doesn't leak the parent command's options in the help
+    let childHelp = MyCommand.message(for: CleanExit.helpRequest(Child.self))
+    XCTAssertEqual(
+      childHelp,
+      """
+      USAGE: my-command child
+
+      OPTIONS:
+        -h, --help              Show help information.
+
+      """)
+
+    // Now check that the foo option doesn't leak into the JSON dump
+    let toolInfo = ToolInfoV0(commandStack: [MyCommand.self.asCommand])
+    // It's there in the parent
+    XCTAssertNotNil(toolInfo.command.arguments!.first { $0.valueName == "foo" })
+    let childInfo = toolInfo.command.subcommands!.first { cmd in
+      cmd.commandName == "child"
+    }
+    // It's not there in the child subcommand
+    XCTAssertNil(childInfo!.arguments!.first { $0.valueName == "foo" })
+  }
+
   func testRemainingDefaultImplicit() throws {
     AssertParseCommand(MyCommand.self, Plugin.self, ["my-plugin"]) { plugin in
       XCTAssertEqual(plugin.pluginName, "my-plugin")