Handle unparsed non-optional complex property types (apple#554)

* Implement correct handling of unparsed property types which perform non-optional nested decoding.

Sources/ArgumentParser/Parsing/ArgumentDecoder.swift

@@ -103,6 +103,13 @@ final class ParsedArgumentsContainer<K>: KeyedDecodingContainerProtocol where K
   }
 
   func decode<T>(_ type: T.Type, forKey key: K) throws -> T where T : Decodable {
+    let parsedElement = element(forKey: key)
+    if parsedElement?.inputOrigin.isDefaultValue ?? false, let rawValue = parsedElement?.value {
+      guard let value = rawValue as? T else {
+        throw InternalParseError.wrongType(rawValue, forKey: parsedElement!.key)
+      }
+      return value
+    }
     let subDecoder = SingleValueDecoder(userInfo: decoder.userInfo, underlying: decoder, codingPath: codingPath + [key], key: InputKey(codingKey: key, path: codingPath), parsedElement: element(forKey: key))
     return try type.init(from: subDecoder)
   }

Tests/ArgumentParserEndToEndTests/UnparsedValuesEndToEndTest.swift

@@ -256,3 +256,72 @@ extension UnparsedValuesEndToEndTests {
     XCTAssertThrowsError(try Bar.parse(["--bar", "--bazz", "xyz", "--age", "None"]))
   }
 }
+
+// MARK: Value + unparsed dictionary
+
+fileprivate struct Bamf: ParsableCommand {
+  @Flag var bamph: Bool = false
+  var bop: [String: String] = [:]
+  var bopp: [String: [String]] = [:]
+}
+
+extension UnparsedValuesEndToEndTests {
+  func testUnparsedNestedDictionary() {
+    AssertParse(Bamf.self, []) { bamf in
+      XCTAssertFalse(bamf.bamph)
+      XCTAssertEqual(bamf.bop, [:])
+      XCTAssertEqual(bamf.bopp, [:])
+    }
+  }
+}
+
+// MARK: Value + unparsed enum with associated values
+
+fileprivate struct Qiqi: ParsableCommand {
+  @Flag var qiqiqi: Bool = false
+  var qiqii: Qiqii = .q("")
+}
+
+fileprivate enum Qiqii: Codable, Equatable {
+  // Enums with associated values generate a Codable conformance
+  // which calls `KeyedDecodingContainer.nestedContainer(keyedBy:)`.
+  //
+  // There is no known case of anything ever actually using the
+  // `.nestedUnkeyedContainer()` method.
+  case q(String)
+  case i(Int)
+}
+
+extension UnparsedValuesEndToEndTests {
+  func testUnparsedEnumWithAssociatedValues() {
+    AssertParse(Qiqi.self, []) { qiqi in
+      XCTAssertFalse(qiqi.qiqiqi)
+      XCTAssertEqual(qiqi.qiqii, .q(""))
+    }
+  }
+}
+
+// MARK: Value + nested decodable inheriting class type
+
+fileprivate struct Fry: ParsableCommand {
+  @Flag var c: Bool = false
+  var toksVig: Vig = .init()
+}
+
+fileprivate class Toks: Codable {
+  var a = "hello"
+}
+
+fileprivate final class Vig: Toks {
+  var b = "world"
+}
+
+extension UnparsedValuesEndToEndTests {
+  func testUnparsedNestedInheritingClassType() {
+    AssertParse(Fry.self, []) { fry in
+      XCTAssertFalse(fry.c)
+      XCTAssertEqual(fry.toksVig.a, "hello")
+      XCTAssertEqual(fry.toksVig.b, "world")
+    }
+  }
+}