WIP: add test coverage and fixes for #466

- Start of a fix for issue 466. After an initial read through the code
  it feels like there more @argument initializers than needed and this
  logic can probably be reudced and simplified.
- Fixes isOptional derivision for non-Optional @arguments with default
  values, however the simple solution doesn't work properly for Optional
  @arguments without a non-nil default value.

Sources/ArgumentParser/Parsable Properties/Flag.swift

@@ -382,7 +382,14 @@ extension Flag where Value: EnumerableFlag {
         let caseKey = InputKey(rawValue: String(describing: value))
         let name = Value.name(for: value)
         let helpForCase = hasCustomCaseHelp ? (caseHelps[i] ?? help) : help
-        let help = ArgumentDefinition.Help(options: initial != nil ? .isOptional : [], help: helpForCase, defaultValue: defaultValue, key: key, isComposite: !hasCustomCaseHelp)
+        let help = ArgumentDefinition.Help(
+          // TODO: (feedback from Nate) maybe this needs to be derived from allCases?
+          allValues: [],
+          options: initial != nil ? [.isOptional] : [],
+          help: helpForCase,
+          defaultValue: defaultValue,
+          key: key,
+          isComposite: !hasCustomCaseHelp)
         return ArgumentDefinition.flag(name: name, key: key, caseKey: caseKey, help: help, parsingStrategy: .default, initialValue: initial, update: .nullary({ (origin, name, values) in
           hasUpdated = try ArgumentSet.updateFlag(key: key, value: value, origin: origin, values: &values, hasUpdated: hasUpdated, exclusivity: exclusivity)
         }))
@@ -471,7 +478,17 @@ extension Flag {
         let caseKey = InputKey(rawValue: String(describing: value))
         let name = Element.name(for: value)
         let helpForCase = hasCustomCaseHelp ? (caseHelps[i] ?? help) : help
-        let help = ArgumentDefinition.Help(options: .isOptional, help: helpForCase, key: key, isComposite: !hasCustomCaseHelp)
+
+        let help = ArgumentDefinition.Help(
+          // TODO: (feedback from Nate) maybe this needs to be derived from allCases?
+          allValues: [],
+          options: [.isOptional],
+          help: helpForCase,
+          // TODO: (feedback from Nate) maybe this needs to be derived from allCases?
+          defaultValue: nil,
+          key: key,
+          isComposite: !hasCustomCaseHelp)
+
         return ArgumentDefinition.flag(name: name, key: key, caseKey: caseKey, help: help, parsingStrategy: .default, initialValue: nil as Element?, update: .nullary({ (origin, name, values) in
           hasUpdated = try ArgumentSet.updateFlag(key: key, value: value, origin: origin, values: &values, hasUpdated: hasUpdated, exclusivity: exclusivity)
         }))
@@ -496,7 +513,16 @@ extension Flag {
         let caseKey = InputKey(rawValue: String(describing: value))
         let name = Element.name(for: value)
         let helpForCase = hasCustomCaseHelp ? (caseHelps[i] ?? help) : help
-        let help = ArgumentDefinition.Help(options: .isOptional, help: helpForCase, key: key, isComposite: !hasCustomCaseHelp)
+        let help = ArgumentDefinition.Help(
+          // TODO: (feedback from Nate) maybe this needs to be derived from allCases?
+          allValues: [],
+          options: [.isOptional],
+          help: helpForCase,
+          // TODO: (feedback from Nate) maybe this needs to be derived from allCases?
+          defaultValue: nil,
+          key: key,
+          isComposite: !hasCustomCaseHelp)
+
         return ArgumentDefinition.flag(name: name, key: key, caseKey: caseKey, help: help, parsingStrategy: .default, initialValue: initial, update: .nullary({ (origin, name, values) in
           values.update(forKey: key, inputOrigin: origin, initial: [Element](), closure: {
             $0.append(value)