Prevent empty dictionary with non-String keys from encoding as a JSON object.

Darwin encodes these as an empty array (`[]`), but SCF was encoding an empty JSON object instead (`{}`). Fixes https://bugs.swift.org/browse/SR-15781.

Author: millenomi

Sources/Foundation/JSONEncoder.swift

@@ -504,8 +504,8 @@ extension _SpecialTreatmentEncoder {
             return .string(url.absoluteString)
         case let decimal as Decimal:
             return .number(decimal.description)
-        case let object as [String: Encodable]:
-            return try self.wrapObject(object, for: additionalKey)
+        case let object as _JSONStringDictionaryEncodableMarker:
+            return try self.wrapObject(object as! [String: Encodable], for: additionalKey)
         default:
             let encoder = self.getEncoder(for: additionalKey)
             try encodable.encode(to: encoder)

Tests/Foundation/Tests/TestJSONEncoder.swift

@@ -869,6 +869,39 @@ class TestJSONEncoder : XCTestCase {
         XCTAssertEqual(JSONEncoder.OutputFormatting.withoutEscapingSlashes.rawValue, 8)
     }
 
+    func test_SR17581_codingEmptyDictionaryWithNonstringKeyDoesRoundtrip() throws {
+        struct Something: Codable {
+            struct Key: Codable, Hashable {
+                var x: String
+            }
+
+            var dict: [Key: String]
+
+            enum CodingKeys: String, CodingKey {
+                case dict
+            }
+
+            init(from decoder: Decoder) throws {
+                let container = try decoder.container(keyedBy: CodingKeys.self)
+                self.dict = try container.decode([Key: String].self, forKey: .dict)
+            }
+
+            func encode(to encoder: Encoder) throws {
+                var container = encoder.container(keyedBy: CodingKeys.self)
+                try container.encode(dict, forKey: .dict)
+            }
+
+            init(dict: [Key: String]) {
+                self.dict = dict
+            }
+        }
+
+        let toEncode = Something(dict: [:])
+        let data = try JSONEncoder().encode(toEncode)
+        let result = try JSONDecoder().decode(Something.self, from: data)
+        XCTAssertEqual(result.dict.count, 0)
+    }
+
     // MARK: - Helper Functions
     private var _jsonEmptyDictionary: Data {
         return "{}".data(using: .utf8)!
@@ -1471,6 +1504,7 @@ extension TestJSONEncoder {
             ("test_dictionary_snake_case_decoding", test_dictionary_snake_case_decoding),
             ("test_dictionary_snake_case_encoding", test_dictionary_snake_case_encoding),
             ("test_OutputFormattingValues", test_OutputFormattingValues),
+            ("test_SR17581_codingEmptyDictionaryWithNonstringKeyDoesRoundtrip", test_SR17581_codingEmptyDictionaryWithNonstringKeyDoesRoundtrip),
         ]
     }
 }