Handle Swift 6.2 sendability changes (#789)

SendableMetatype is a marker protocol, so while it doesn't have
runtime availability requirements, it's only available with >= 6.2
Swift compiler. This uses an underscored protocol to conditionally 
include conformance without repeating the entire declaration
(thanks @rauhul).

Author: natecook1000

Sources/ArgumentParser/CMakeLists.txt

@@ -46,6 +46,7 @@ add_library(ArgumentParser
   Utilities/Platform.swift
   Utilities/SequenceExtensions.swift
   Utilities/StringExtensions.swift
+  Utilities/SwiftExtensions.swift
   Utilities/Tree.swift
 
   Validators/CodingKeyValidator.swift

Sources/ArgumentParser/Parsable Types/ExpressibleByArgument.swift

@@ -10,7 +10,7 @@
 //===----------------------------------------------------------------------===//
 
 /// A type that can be expressed as a command-line argument.
-public protocol ExpressibleByArgument {
+public protocol ExpressibleByArgument: _SendableMetatype {
   /// Creates a new instance of this type from a command-line-specified
   /// argument.
   init?(argument: String)

Sources/ArgumentParser/Parsable Types/ParsableArguments.swift

@@ -13,7 +13,7 @@
 ///
 /// When you implement a `ParsableArguments` type, all properties must be declared with
 /// one of the four property wrappers provided by the `ArgumentParser` library.
-public protocol ParsableArguments: Decodable {
+public protocol ParsableArguments: Decodable, _SendableMetatype {
   /// Creates an instance of this parsable type using the definitions
   /// given by each property's wrapper.
   init()

Sources/ArgumentParser/Utilities/SwiftExtensions.swift

@@ -0,0 +1,17 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift Argument Parser open source project
+//
+// Copyright (c) 2020 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
+//
+//===----------------------------------------------------------------------===//
+
+#if compiler(>=6.2)
+/// Designates a type as having a sendable metatype.
+public protocol _SendableMetatype: SendableMetatype {}
+#else
+public protocol _SendableMetatype {}
+#endif

Tests/ArgumentParserEndToEndTests/CustomParsingEndToEndTests.swift

@@ -35,17 +35,19 @@ extension Array where Element == Name {
 // MARK: -
 
 private struct Foo: ParsableCommand {
-  enum Subgroup: Equatable {
+  enum Subgroup: Equatable, Sendable {
     case first(Int)
     case second(Int)
 
+    @Sendable
     static func makeFirst(_ str: String) throws -> Subgroup {
       guard let value = Int(str) else {
         throw ValidationError("Not a valid integer for 'first'")
       }
       return .first(value)
     }
 
+    @Sendable
     static func makeSecond(_ str: String) throws -> Subgroup {
       guard let value = Int(str) else {
         throw ValidationError("Not a valid integer for 'second'")

Tests/ArgumentParserUnitTests/HelpGenerationTests.swift

@@ -148,6 +148,8 @@ extension HelpGenerationTests {
   enum OptionFlags: String, EnumerableFlag { case optional, required }
   enum Degree {
     case bachelor, graduate, doctorate
+
+    @Sendable
     static func degreeTransform(_ string: String) throws -> Degree {
       switch string {
       case "bachelor":

Tests/ArgumentParserUnitTests/UsageGenerationTests.swift

@@ -132,6 +132,7 @@ extension UsageGenerationTests {
     enum Color {
       case red, blue
 
+      @Sendable
       static func transform(_ string: String) throws -> Color {
         switch string {
         case "red":