Prevent serialization of non-nullable enums using _$xxxEnumMap from i…

…ncorrectly producing

nullable string types.

For example, before this change serializing a class member of type Map<enum, String>
produced a type Map<String?, dynamic> where Map<String, dynamic> should have been produced.

Nullable enums should still produce nullable types, eg. serializing List<enum?> should produce
List<String?>

fixes 1145

json_serializable/lib/src/type_helpers/enum_helper.dart

@@ -29,7 +29,11 @@ class EnumHelper extends TypeHelper<TypeHelperContextWithConfig> {
 
     context.addMember(memberContent);
 
-    return '${constMapName(targetType)}[$expression]';
+    if (targetType.isNullableType) {
+      return '${constMapName(targetType)}[$expression]';
+    } else {
+      return '${constMapName(targetType)}[$expression]!';
+    }
   }
 
   @override

json_serializable/test/integration/integration_test.dart

@@ -304,6 +304,23 @@ void main() {
     expect(dayTypeEnumValues, ['no-good', 'rotten', 'very-bad']);
   });
 
+  test(
+      'serializing a non-nullable enum as a key in a map should produce a '
+      'non-nullable string key', () {
+    final cls =
+        Issue1145RegressionA(status: {Issue1145RegressionEnum.gamma: true});
+    // Prior to issue 1145 resulted in Map<String?, dynamic>
+    expect(cls.toJson()['status'], const TypeMatcher<Map<String, dynamic>>());
+  });
+
+  test(
+      'serializing a nullable enum in a list should produce a list with'
+      ' nullable entries', () {
+    final cls = Issue1145RegressionB(status: [Issue1145RegressionEnum.gamma]);
+    // Prior to issue 1145 resulted in Map<String?, dynamic>
+    expect(cls.toJson()['status'], const TypeMatcher<List<String?>>());
+  });
+
   test('unknown as null for enum', () {
     expect(
       () => Issue559Regression.fromJson({}).status,

json_serializable/test/integration/json_enum_example.dart

@@ -49,3 +49,35 @@ enum Issue559RegressionEnum {
   beta,
   gamma,
 }
+
+enum Issue1145RegressionEnum {
+  alpha,
+  beta,
+  gamma,
+}
+
+@JsonSerializable(
+  createFactory: false,
+)
+class Issue1145RegressionA {
+  Issue1145RegressionA({
+    required this.status,
+  });
+
+  Map<String, dynamic> toJson() => _$Issue1145RegressionAToJson(this);
+
+  final Map<Issue1145RegressionEnum, bool> status;
+}
+
+@JsonSerializable(
+  createFactory: false,
+)
+class Issue1145RegressionB {
+  Issue1145RegressionB({
+    required this.status,
+  });
+
+  Map<String, dynamic> toJson() => _$Issue1145RegressionBToJson(this);
+
+  final List<Issue1145RegressionEnum?> status;
+}