Ensure values are decodable from nil

* Eliminate null checks from unboxing in the general case (so types
  can at least attempt to decode from nil)
* Add unit tests to confirm this behavior

Author: Itai Ferber

stdlib/public/SDK/Foundation/JSONEncoder.swift

@@ -1280,8 +1280,8 @@ fileprivate struct _JSONKeyedDecodingContainer<K : CodingKey> : KeyedDecodingCon
     }
 
     private func _superDecoder(forKey key: CodingKey) throws -> Decoder {
-        return try self.decoder.with(pushedKey: key) {
-            let value = self.container[key.stringValue]
+        return self.decoder.with(pushedKey: key) {
+            let value: Any = self.container[key.stringValue] ?? NSNull()
             return _JSONDecoder(referencing: value, at: self.decoder.codingPath, options: self.decoder.options)
         }
     }
@@ -2069,13 +2069,13 @@ extension _JSONDecoder {
     }
 
     fileprivate func unbox<T : Decodable>(_ value: Any, as type: T.Type) throws -> T? {
-        guard !(value is NSNull) else { return nil }
-
         let decoded: T
         if T.self == Date.self {
-            decoded = (try self.unbox(value, as: Date.self) as! T)
+            guard let date = try self.unbox(value, as: Date.self) else { return nil }
+            decoded = date as! T
         } else if T.self == Data.self {
-            decoded = (try self.unbox(value, as: Data.self) as! T)
+            guard let data = try self.unbox(value, as: Data.self) else { return nil }
+            decoded = data as! T
         } else if T.self == URL.self {
             guard let urlString = try self.unbox(value, as: String.self) else {
                 return nil
@@ -2088,7 +2088,8 @@ extension _JSONDecoder {
 
             decoded = (url as! T)
         } else if T.self == Decimal.self {
-            decoded = (try self.unbox(value, as: Decimal.self) as! T)
+            guard let decimal = try self.unbox(value, as: Decimal.self) else { return nil }
+            decoded = decimal as! T
         } else {
             self.storage.push(container: value)
             decoded = try T(from: self)

stdlib/public/SDK/Foundation/PlistEncoder.swift

@@ -1052,8 +1052,8 @@ fileprivate struct _PlistKeyedDecodingContainer<K : CodingKey> : KeyedDecodingCo
     }
 
     private func _superDecoder(forKey key: CodingKey) throws -> Decoder {
-        return try self.decoder.with(pushedKey: key) {
-            let value = self.container[key.stringValue]
+        return self.decoder.with(pushedKey: key) {
+            let value: Any = self.container[key.stringValue] ?? NSNull()
             return _PlistDecoder(referencing: value, at: self.decoder.codingPath, options: self.decoder.options)
         }
     }
@@ -1730,13 +1730,13 @@ extension _PlistDecoder {
     }
 
     fileprivate func unbox<T : Decodable>(_ value: Any, as type: T.Type) throws -> T? {
-        if let string = value as? String, string == _plistNull { return nil }
-
         let decoded: T
         if T.self == Date.self {
-            decoded = (try self.unbox(value, as: Date.self) as! T)
+            guard let date = try self.unbox(value, as: Date.self) else { return nil }
+            decoded = date as! T
         } else if T.self == Data.self {
-            decoded = (try self.unbox(value, as: Data.self) as! T)
+            guard let data = try self.unbox(value, as: Data.self) else { return nil }
+            decoded = data as! T
         } else {
             self.storage.push(container: value)
             decoded = try T(from: self)

test/stdlib/TestJSONEncoder.swift

@@ -92,6 +92,17 @@ class TestJSONEncoder : TestJSONEncoderSuper {
     _testRoundTrip(of: employee)
   }
 
+  func testEncodingTopLevelNullableType() {
+    // EnhancedBool is a type which encodes either as a Bool or as nil.
+    _testEncodeFailure(of: EnhancedBool.true)
+    _testEncodeFailure(of: EnhancedBool.false)
+    _testEncodeFailure(of: EnhancedBool.fileNotFound)
+
+    _testRoundTrip(of: TopLevelWrapper(EnhancedBool.true), expectedJSON: "{\"value\":true}".data(using: .utf8)!)
+    _testRoundTrip(of: TopLevelWrapper(EnhancedBool.false), expectedJSON: "{\"value\":false}".data(using: .utf8)!)
+    _testRoundTrip(of: TopLevelWrapper(EnhancedBool.fileNotFound), expectedJSON: "{\"value\":null}".data(using: .utf8)!)
+  }
+
   // MARK: - Output Formatting Tests
   func testEncodingOutputFormattingDefault() {
     let expectedJSON = "{\"name\":\"Johnny Appleseed\",\"email\":\"[email protected]\"}".data(using: .utf8)!
@@ -650,61 +661,30 @@ fileprivate struct Company : Codable, Equatable {
   }
 }
 
-// MARK: - Helper Types
-
-/// Wraps a type T so that it can be encoded at the top level of a payload.
-fileprivate struct TopLevelWrapper<T> : Codable, Equatable where T : Codable, T : Equatable {
-  let value: T
-
-  init(_ value: T) {
-    self.value = value
-  }
-
-  static func ==(_ lhs: TopLevelWrapper<T>, _ rhs: TopLevelWrapper<T>) -> Bool {
-    return lhs.value == rhs.value
-  }
-}
-
-fileprivate struct FloatNaNPlaceholder : Codable, Equatable {
-  init() {}
-
-  func encode(to encoder: Encoder) throws {
-    var container = encoder.singleValueContainer()
-    try container.encode(Float.nan)
-  }
+/// An enum type which decodes from Bool?.
+fileprivate enum EnhancedBool : Codable {
+  case `true`
+  case `false`
+  case fileNotFound
 
   init(from decoder: Decoder) throws {
     let container = try decoder.singleValueContainer()
-    let float = try container.decode(Float.self)
-    if !float.isNaN {
-      throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: decoder.codingPath, debugDescription: "Couldn't decode NaN."))
+    if container.decodeNil() {
+      self = .fileNotFound
+    } else {
+      let value = try container.decode(Bool.self)
+      self = value ? .true : .false
     }
   }
 
-  static func ==(_ lhs: FloatNaNPlaceholder, _ rhs: FloatNaNPlaceholder) -> Bool {
-    return true
-  }
-}
-
-fileprivate struct DoubleNaNPlaceholder : Codable, Equatable {
-  init() {}
-
   func encode(to encoder: Encoder) throws {
     var container = encoder.singleValueContainer()
-    try container.encode(Double.nan)
-  }
-
-  init(from decoder: Decoder) throws {
-    let container = try decoder.singleValueContainer()
-    let double = try container.decode(Double.self)
-    if !double.isNaN {
-      throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: decoder.codingPath, debugDescription: "Couldn't decode NaN."))
+    switch self {
+    case .true: try container.encode(true)
+    case .false: try container.encode(false)
+    case .fileNotFound: try container.encodeNil()
     }
   }
-
-  static func ==(_ lhs: DoubleNaNPlaceholder, _ rhs: DoubleNaNPlaceholder) -> Bool {
-    return true
-  }
 }
 
 /// A type which encodes as an array directly through a single value container.
@@ -853,6 +833,63 @@ struct NestedContainersTestType : Encodable {
   }
 }
 
+// MARK: - Helper Types
+
+/// Wraps a type T so that it can be encoded at the top level of a payload.
+fileprivate struct TopLevelWrapper<T> : Codable, Equatable where T : Codable, T : Equatable {
+  let value: T
+
+  init(_ value: T) {
+    self.value = value
+  }
+
+  static func ==(_ lhs: TopLevelWrapper<T>, _ rhs: TopLevelWrapper<T>) -> Bool {
+    return lhs.value == rhs.value
+  }
+}
+
+fileprivate struct FloatNaNPlaceholder : Codable, Equatable {
+  init() {}
+
+  func encode(to encoder: Encoder) throws {
+    var container = encoder.singleValueContainer()
+    try container.encode(Float.nan)
+  }
+
+  init(from decoder: Decoder) throws {
+    let container = try decoder.singleValueContainer()
+    let float = try container.decode(Float.self)
+    if !float.isNaN {
+      throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: decoder.codingPath, debugDescription: "Couldn't decode NaN."))
+    }
+  }
+
+  static func ==(_ lhs: FloatNaNPlaceholder, _ rhs: FloatNaNPlaceholder) -> Bool {
+    return true
+  }
+}
+
+fileprivate struct DoubleNaNPlaceholder : Codable, Equatable {
+  init() {}
+
+  func encode(to encoder: Encoder) throws {
+    var container = encoder.singleValueContainer()
+    try container.encode(Double.nan)
+  }
+
+  init(from decoder: Decoder) throws {
+    let container = try decoder.singleValueContainer()
+    let double = try container.decode(Double.self)
+    if !double.isNaN {
+      throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: decoder.codingPath, debugDescription: "Couldn't decode NaN."))
+    }
+  }
+
+  static func ==(_ lhs: DoubleNaNPlaceholder, _ rhs: DoubleNaNPlaceholder) -> Bool {
+    return true
+  }
+}
+
 // MARK: - Run Tests
 
 #if !FOUNDATION_XCTEST
@@ -870,6 +907,7 @@ JSONEncoderTests.test("testEncodingTopLevelDeepStructuredType") { TestJSONEncode
 JSONEncoderTests.test("testEncodingTopLevelStructuredClass") { TestJSONEncoder().testEncodingTopLevelStructuredClass() }
 JSONEncoderTests.test("testEncodingTopLevelDeepStructuredType") { TestJSONEncoder().testEncodingTopLevelDeepStructuredType()}
 JSONEncoderTests.test("testEncodingClassWhichSharesEncoderWithSuper") { TestJSONEncoder().testEncodingClassWhichSharesEncoderWithSuper() }
+JSONEncoderTests.test("testEncodingTopLevelNullableType") { TestJSONEncoder().testEncodingTopLevelNullableType() }
 JSONEncoderTests.test("testEncodingOutputFormattingDefault") { TestJSONEncoder().testEncodingOutputFormattingDefault() }
 JSONEncoderTests.test("testEncodingOutputFormattingPrettyPrinted") { TestJSONEncoder().testEncodingOutputFormattingPrettyPrinted() }
 JSONEncoderTests.test("testEncodingOutputFormattingSortedKeys") { TestJSONEncoder().testEncodingOutputFormattingSortedKeys() }

test/stdlib/TestPlistEncoder.swift

@@ -111,6 +111,24 @@ class TestPropertyListEncoder : TestPropertyListEncoderSuper {
     _testRoundTrip(of: employee, in: .xml)
   }
 
+  func testEncodingTopLevelNullableType() {
+    // EnhancedBool is a type which encodes either as a Bool or as nil.
+    _testEncodeFailure(of: EnhancedBool.true, in: .binary)
+    _testEncodeFailure(of: EnhancedBool.true, in: .xml)
+    _testEncodeFailure(of: EnhancedBool.false, in: .binary)
+    _testEncodeFailure(of: EnhancedBool.false, in: .xml)
+    _testEncodeFailure(of: EnhancedBool.fileNotFound, in: .binary)
+    _testEncodeFailure(of: EnhancedBool.fileNotFound, in: .xml)
+
+    _testRoundTrip(of: TopLevelWrapper(EnhancedBool.true), in: .binary)
+    _testRoundTrip(of: TopLevelWrapper(EnhancedBool.true), in: .xml)
+    _testRoundTrip(of: TopLevelWrapper(EnhancedBool.false), in: .binary)
+    _testRoundTrip(of: TopLevelWrapper(EnhancedBool.false), in: .xml)
+    _testRoundTrip(of: TopLevelWrapper(EnhancedBool.fileNotFound), in: .binary)
+    _testRoundTrip(of: TopLevelWrapper(EnhancedBool.fileNotFound), in: .xml)
+  }
+
+
   // MARK: - Encoder Features
   func testNestedContainerCodingPaths() {
     let encoder = JSONEncoder()
@@ -438,6 +456,32 @@ fileprivate struct Company : Codable, Equatable {
   }
 }
 
+/// An enum type which decodes from Bool?.
+fileprivate enum EnhancedBool : Codable {
+  case `true`
+  case `false`
+  case fileNotFound
+
+  init(from decoder: Decoder) throws {
+    let container = try decoder.singleValueContainer()
+    if container.decodeNil() {
+      self = .fileNotFound
+    } else {
+      let value = try container.decode(Bool.self)
+      self = value ? .true : .false
+    }
+  }
+
+  func encode(to encoder: Encoder) throws {
+    var container = encoder.singleValueContainer()
+    switch self {
+    case .true: try container.encode(true)
+    case .false: try container.encode(false)
+    case .fileNotFound: try container.encodeNil()
+    }
+  }
+}
+
 /// A type which encodes as an array directly through a single value container.
 struct Numbers : Codable, Equatable {
   let values = [4, 8, 15, 16, 23, 42]
@@ -614,6 +658,7 @@ PropertyListEncoderTests.test("testEncodingTopLevelStructuredSingleStruct") { Te
 PropertyListEncoderTests.test("testEncodingTopLevelStructuredSingleClass") { TestPropertyListEncoder().testEncodingTopLevelStructuredSingleClass() }
 PropertyListEncoderTests.test("testEncodingTopLevelDeepStructuredType") { TestPropertyListEncoder().testEncodingTopLevelDeepStructuredType() }
 PropertyListEncoderTests.test("testEncodingClassWhichSharesEncoderWithSuper") { TestPropertyListEncoder().testEncodingClassWhichSharesEncoderWithSuper() }
+PropertyListEncoderTests.test("testEncodingTopLevelNullableType") { TestPropertyListEncoder().testEncodingTopLevelNullableType() }
 PropertyListEncoderTests.test("testNestedContainerCodingPaths") { TestPropertyListEncoder().testNestedContainerCodingPaths() }
 PropertyListEncoderTests.test("testSuperEncoderCodingPaths") { TestPropertyListEncoder().testSuperEncoderCodingPaths() }
 runAllTests()