Allow default nil values for optional properties (#480)

This adds underscored initializers that let library users add `= nil` to
declarations of optional `@Option` and `@Argument` properties. Previously,
default values have been available for properties of non-optional types
only.

These new initializers use `_OptionalNilComparisonType` as the wrapped
value parameter, so only a `nil` literal is acceptable in the default
value position. This avoids the problem of declaring an optional property
with a non-`nil` default, which ends up negating the purpose of an optional.

Author: natecook1000

Examples/count-lines/CountLines.swift

@@ -18,10 +18,10 @@ struct CountLines: AsyncParsableCommand {
     @Argument(
         help: "A file to count lines in. If omitted, counts the lines of stdin.",
         completion: .file(), transform: URL.init(fileURLWithPath:))
-    var inputFile: URL?
+    var inputFile: URL? = nil
 
     @Option(help: "Only count lines with this prefix.")
-    var prefix: String?
+    var prefix: String? = nil
 
     @Flag(help: "Include extra information in the output.")
     var verbose = false

Examples/repeat/Repeat.swift

@@ -14,7 +14,7 @@ import ArgumentParser
 @main
 struct Repeat: ParsableCommand {
     @Option(help: "The number of times to repeat 'phrase'.")
-    var count: Int?
+    var count: Int? = nil
 
     @Flag(help: "Include a counter with each repetition.")
     var includeCounter = false

Examples/roll/main.swift

@@ -22,7 +22,7 @@ struct RollOptions: ParsableArguments {
     var sides = 6
 
     @Option(help: "A seed to use for repeatable random generation.")
-    var seed: Int?
+    var seed: Int? = nil
 
     @Flag(name: .shortAndLong, help: "Show all roll results.")
     var verbose = false

README.md

@@ -17,7 +17,7 @@ struct Repeat: ParsableCommand {
     var includeCounter = false
 
     @Option(name: .shortAndLong, help: "The number of times to repeat 'phrase'.")
-    var count: Int?
+    var count: Int? = nil
 
     @Argument(help: "The phrase to repeat.")
     var phrase: String

Sources/ArgumentParser/Documentation.docc/ArgumentParser.md

@@ -21,7 +21,7 @@ struct Repeat: ParsableCommand {
     var phrase: String
 
     @Option(help: "The number of times to repeat 'phrase'.")
-    var count: Int?
+    var count: Int? = nil
 
     mutating func run() throws {
         let repeatCount = count ?? 2

Sources/ArgumentParser/Documentation.docc/Articles/CustomizingCommandHelp.md

@@ -22,7 +22,7 @@ struct Repeat: ParsableCommand {
     var phrase: String
 
     @Option(help: "How many times to repeat.")
-    var count: Int?
+    var count: Int? = nil
 
     mutating func run() throws {
         for _ in 0..<(count ?? 2) {

Sources/ArgumentParser/Documentation.docc/Extensions/ParsableCommand.md

@@ -9,7 +9,7 @@ struct Repeat: ParsableCommand {
     var phrase: String
 
     @Option(help: "The number of times to repeat 'phrase'.")
-    var count: Int?
+    var count: Int? = nil
 
     mutating func run() throws {
         let repeatCount = count ?? 2

Sources/ArgumentParser/Parsable Properties/Argument.swift

@@ -234,6 +234,19 @@ extension Argument {
     })
   }
 
+  /// This initializer allows a user to provide a `nil` default value for an
+  /// optional `@Argument`-marked property without allowing a non-`nil` default
+  /// value.
+  public init<T: ExpressibleByArgument>(
+    wrappedValue _value: _OptionalNilComparisonType,
+    help: ArgumentHelp? = nil,
+    completion: CompletionKind? = nil
+  ) where Value == T? {
+    self.init(
+      help: help,
+      completion: completion)
+  }
+  
   /// Creates a property with an optional default value, intended to be called by other constructors to centralize logic.
   ///
   /// This private `init` allows us to expose multiple other similar constructors to allow for standard default property initialization while reducing code duplication.

Sources/ArgumentParser/Parsable Properties/Flag.swift

@@ -219,6 +219,24 @@ extension Flag where Value == Optional<Bool> {
         help: help)
     })
   }
+  
+  /// This initializer allows a user to provide a `nil` default value for
+  /// `@Flag`-marked `Optional<Bool>` property without allowing a non-`nil`
+  /// default value.
+  public init(
+    wrappedValue _value: _OptionalNilComparisonType,
+    name: NameSpecification = .long,
+    inversion: FlagInversion,
+    exclusivity: FlagExclusivity = .chooseLast,
+    help: ArgumentHelp? = nil
+  ) {
+    self.init(
+      name: name,
+      inversion: inversion,
+      exclusivity: exclusivity,
+      help: help)
+  }
+
 }
 
 extension Flag where Value == Bool {

Sources/ArgumentParser/Parsable Properties/Option.swift

@@ -25,7 +25,7 @@
 ///     @main
 ///     struct Greet: ParsableCommand {
 ///         @Option var greeting = "Hello"
-///         @Option var age: Int?
+///         @Option var age: Int? = nil
 ///         @Option var name: String
 ///
 ///         mutating func run() {
@@ -361,6 +361,24 @@ extension Option {
     })
   }
 
+  /// This initializer allows a user to provide a `nil` default value for an
+  /// optional `@Option`-marked property without allowing a non-`nil` default
+  /// value.
+  public init<T: ExpressibleByArgument>(
+    wrappedValue _value: _OptionalNilComparisonType,
+    name: NameSpecification = .long,
+    parsing parsingStrategy: SingleValueParsingStrategy = .next,
+    help: ArgumentHelp? = nil,
+    completion: CompletionKind? = nil
+  ) where Value == T? {
+    self.init(
+      name: name,
+      parsing: parsingStrategy,
+      help: help,
+      completion: completion
+    )
+  }
+
   /// Creates a property with an optional default value, intended to be called by other constructors to centralize logic.
   ///
   /// This private `init` allows us to expose multiple other similar constructors to allow for standard default property initialization while reducing code duplication.