Allow normal Swift default property initialization syntax

Sources/ArgumentParser/Parsable Properties/Argument.swift

@@ -68,19 +68,85 @@ extension Argument: DecodableParsedWrapper where Value: Decodable {}
 // MARK: Property Wrapper Initializers
 
 extension Argument where Value: ExpressibleByArgument {
+  /// 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.
+  private init(
+    initial: Value?,
+    help: ArgumentHelp?
+  ) {
+    self.init(_parsedValue: .init { key in
+      ArgumentSet(key: key, kind: .positional, parseType: Value.self, name: NameSpecification.long, default: initial, help: help)
+      })
+  }
+
   /// Creates a property that reads its value from an argument.
   ///
+  /// This method is deprecated, with usage split into two other methods below:
+  /// - `init(wrappedValue:help:)` for properties with a default value
+  /// - `init(help:)` for properties with no default value
+  ///
+  /// Existing usage of the `default` parameter should be replaced such as follows:
+  /// ```diff
+  /// -@Argument(default: "bar")
+  /// -var foo: String
+  /// +@Argument()
+  /// +var foo: String = "bar"
+  /// ```
+  ///
   /// - Parameters:
   ///   - initial: A default value to use for this property. If `initial` is
   ///     `nil`, the user must supply a value for this argument.
   ///   - help: Information about how to use this argument.
+  @available(*, deprecated, message: "Use regular property initialization for default values (`var foo: String = \"bar\"`)")
   public init(
-    default initial: Value? = nil,
+    default initial: Value?,
     help: ArgumentHelp? = nil
   ) {
-    self.init(_parsedValue: .init { key in
-      ArgumentSet(key: key, kind: .positional, parseType: Value.self, name: NameSpecification.long, default: initial, help: help)
-      })
+    self.init(
+      initial: initial,
+      help: help
+    )
+  }
+
+  /// Creates a property with a default value provided by standard Swift default value syntax.
+  ///
+  /// This method is called to initialize an `Argument` with a default value such as:
+  /// ```swift
+  /// @Argument()
+  /// var foo: String = "bar"
+  /// ```
+  ///
+  /// - Parameters:
+  ///   - wrappedValue: A default value to use for this property, provided implicitly by the compiler during propery wrapper initialization.
+  ///   - help: Information about how to use this argument.
+  public init(
+    wrappedValue: Value,
+    help: ArgumentHelp? = nil
+  ) {
+    self.init(
+      initial: wrappedValue,
+      help: help
+    )
+  }
+
+  /// Creates a property with no default value.
+  ///
+  /// This method is called to initialize an `Argument` without a default value such as:
+  /// ```swift
+  /// @Argument()
+  /// var foo: String
+  /// ```
+  ///
+  /// - Parameters:
+  ///   - help: Information about how to use this argument.
+  public init(
+    help: ArgumentHelp? = nil
+  ) {
+    self.init(
+      initial: nil,
+      help: help
+    )
   }
 }
 
@@ -169,18 +235,13 @@ extension Argument {
         help: help)
     })
   }
-
-  /// Creates a property that reads its value from an argument, parsing with
-  /// the given closure.
+
+  /// Creates a property with an optional default value, intended to be called by other constructors to centralize logic.
   ///
-  /// - Parameters:
-  ///   - initial: A default value to use for this property.
-  ///   - help: Information about how to use this argument.
-  ///   - transform: A closure that converts a string into this property's
-  ///     type or throws an error.
-  public init(
-    default initial: Value? = nil,
-    help: ArgumentHelp? = nil,
+  /// This private `init` allows us to expose multiple other similar constructors to allow for standard default property initialization while reducing code duplication.
+  private init(
+    initial: Value?,
+    help: ArgumentHelp?,
     transform: @escaping (String) throws -> Value
   ) {
     self.init(_parsedValue: .init { key in
@@ -201,7 +262,86 @@ extension Argument {
       return ArgumentSet(alternatives: [arg])
       })
   }
-
+
+  /// Creates a property that reads its value from an argument, parsing with
+  /// the given closure.
+  ///
+  /// This method is deprecated, with usage split into two other methods below:
+  /// - `init(wrappedValue:help:transform:)` for properties with a default value
+  /// - `init(help:transform:)` for properties with no default value
+  ///
+  /// Existing usage of the `default` parameter should be replaced such as follows:
+  /// ```diff
+  /// -@Argument(default: "bar", transform: baz)
+  /// -var foo: String
+  /// +@Argument(transform: baz)
+  /// +var foo: String = "bar"
+  /// ```
+  ///
+  /// - Parameters:
+  ///   - initial: A default value to use for this property.
+  ///   - help: Information about how to use this argument.
+  ///   - transform: A closure that converts a string into this property's
+  ///     type or throws an error.
+  @available(*, deprecated, message: "Use regular property initialization for default values (`var foo: String = \"bar\"`)")
+  public init(
+    default initial: Value?,
+    help: ArgumentHelp? = nil,
+    transform: @escaping (String) throws -> Value
+  ) {
+    self.init(
+      initial: initial,
+      help: help,
+      transform: transform
+    )
+  }
+
+  /// Creates a property with a default value provided by standard Swift default value syntax, parsing with the given closure.
+  ///
+  /// This method is called to initialize an `Argument` with a default value such as:
+  /// ```swift
+  /// @Argument(transform: baz)
+  /// var foo: String = "bar"
+  /// ```
+  ///
+  /// - Parameters:
+  ///   - wrappedValue: A default value to use for this property, provided implicitly by the compiler during property wrapper initialization.
+  ///   - help: Information about how to use this argument.
+  ///   - transform: A closure that converts a string into this property's type or throws an error.
+  public init(
+    wrappedValue: Value,
+    help: ArgumentHelp? = nil,
+    transform: @escaping (String) throws -> Value
+  ) {
+    self.init(
+      initial: wrappedValue,
+      help: help,
+      transform: transform
+    )
+  }
+
+  /// Creates a property with no default value, parsing with the given closure.
+  ///
+  /// This method is called to initialize an `Argument` with no default value such as:
+  /// ```swift
+  /// @Argument(transform: baz)
+  /// var foo: String
+  /// ```
+  ///
+  /// - Parameters:
+  ///   - help: Information about how to use this argument.
+  ///   - transform: A closure that converts a string into this property's type or throws an error.
+  public init(
+    help: ArgumentHelp? = nil,
+    transform: @escaping (String) throws -> Value
+  ) {
+    self.init(
+      initial: nil,
+      help: help,
+      transform: transform
+    )
+  }
+
   /// Creates a property that reads an array from zero or more arguments.
   ///
   /// The property has an empty array as its default value.