Don't throw DecodingError if superDecoder doesn't exist

Author: adam-fowler

Sources/Foundation/JSONDecoder.swift

@@ -750,11 +750,33 @@ extension JSONDecoderImpl {
         }
 
         func superDecoder() throws -> Decoder {
-            try decoderForKey(_JSONKey.super)
+            do {
+                return try decoderForKey(_JSONKey.super)
+            } catch DecodingError.keyNotFound {
+                var newPath = self.codingPath
+                newPath.append(_JSONKey.super)
+                return JSONDecoderImpl(
+                    userInfo: self.impl.userInfo,
+                    from: .null,
+                    codingPath: newPath,
+                    options: self.impl.options
+                )
+            }
         }
 
         func superDecoder(forKey key: K) throws -> Decoder {
-            try decoderForKey(key)
+            do {
+                return try decoderForKey(key)
+            } catch DecodingError.keyNotFound {
+                var newPath = self.codingPath
+                newPath.append(key)
+                return JSONDecoderImpl(
+                    userInfo: self.impl.userInfo,
+                    from: .null,
+                    codingPath: newPath,
+                    options: self.impl.options
+                )
+            }
         }
 
         private func decoderForKey<LocalKey: CodingKey>(_ key: LocalKey) throws -> JSONDecoderImpl {

Tests/Foundation/Tests/TestJSONEncoder.swift

@@ -456,6 +456,25 @@ class TestJSONEncoder : XCTestCase {
         }
     }
 
+    func test_notFoundSuperDecoder() {
+        struct NotFoundSuperDecoderTestType: Decodable {
+            init(from decoder: Decoder) throws {
+                let container = try decoder.container(keyedBy: CodingKeys.self)
+                _ = try container.superDecoder(forKey: .superDecoder)
+            }
+
+            private enum CodingKeys: String, CodingKey {
+                case superDecoder = "super"
+            }
+        }
+        let decoder = JSONDecoder()
+        do {
+            let _ = try decoder.decode(NotFoundSuperDecoderTestType.self, from: Data(#"{}"#.utf8))
+        } catch {
+            XCTFail("Caught error during decoding empty super decoder: \(error)")
+        }
+    }
+
     // MARK: - Test encoding and decoding of built-in Codable types
     func test_codingOfBool() {
         test_codingOf(value: Bool(true), toAndFrom: "true")