Support using non-nullable JsonConverter on nullable properties

json_serializable/CHANGELOG.md

@@ -1,3 +1,8 @@
+## 6.3.0-dev
+
+- Added support for using a `JsonConverter<MyClass, Object>` on properties
+  of type `MyClass?`. ([#822](https://github.com/google/json_serializable.dart/issues/822))
+
 ## 6.2.0
 
 - Added support for the new `FieldRename.screamingSnake` field in

json_serializable/lib/src/type_helpers/json_converter_helper.dart

@@ -32,6 +32,24 @@ class JsonConverterHelper extends TypeHelper {
       return null;
     }
 
+    if (!converter.fieldType.isNullableType && targetType.isNullableType) {
+      const converterToJsonName = r'_$JsonConverterToJson';
+      context.addMember('''
+Json? $converterToJsonName<Json, Value>(
+  Value? value,
+  Json? Function(Value value) toJson,
+) => ${ifNullOrElse('value', 'null', 'toJson(value)')};
+''');
+
+      return _nullableJsonConverterLambdaResult(
+        converter,
+        name: converterToJsonName,
+        targetType: targetType,
+        expression: expression,
+        callback: '${converter.accessString}.toJson',
+      );
+    }
+
     return LambdaResult(expression, '${converter.accessString}.toJson');
   }
 
@@ -49,6 +67,24 @@ class JsonConverterHelper extends TypeHelper {
 
     final asContent = asStatement(converter.jsonType);
 
+    if (!converter.jsonType.isNullableType && targetType.isNullableType) {
+      const converterFromJsonName = r'_$JsonConverterFromJson';
+      context.addMember('''
+Value? $converterFromJsonName<Json, Value>(
+  Object? json,
+  Value? Function(Json json) fromJson,
+) => ${ifNullOrElse('json', 'null', 'fromJson(json as Json)')};
+''');
+
+      return _nullableJsonConverterLambdaResult(
+        converter,
+        name: converterFromJsonName,
+        targetType: targetType,
+        expression: expression,
+        callback: '${converter.accessString}.fromJson',
+      );
+    }
+
     return LambdaResult(
       expression,
       '${converter.accessString}.fromJson',
@@ -57,24 +93,51 @@ class JsonConverterHelper extends TypeHelper {
   }
 }
 
+String _nullableJsonConverterLambdaResult(
+  _JsonConvertData converter, {
+  required String name,
+  required DartType targetType,
+  required String expression,
+  required String callback,
+}) {
+  final jsonDisplayString = typeToCode(converter.jsonType);
+  final fieldTypeDisplayString = converter.isGeneric
+      ? typeToCode(targetType)
+      : typeToCode(converter.fieldType);
+
+  return '$name<$jsonDisplayString, $fieldTypeDisplayString>('
+      '$expression, $callback)';
+}
+
 class _JsonConvertData {
   final String accessString;
   final DartType jsonType;
+  final DartType fieldType;
+  final bool isGeneric;
 
   _JsonConvertData.className(
     String className,
     String accessor,
     this.jsonType,
-  ) : accessString = 'const $className${_withAccessor(accessor)}()';
+    this.fieldType,
+  )   : accessString = 'const $className${_withAccessor(accessor)}()',
+        isGeneric = false;
 
   _JsonConvertData.genericClass(
     String className,
     String genericTypeArg,
     String accessor,
     this.jsonType,
-  ) : accessString = '$className<$genericTypeArg>${_withAccessor(accessor)}()';
+    this.fieldType,
+  )   : accessString =
+            '$className<$genericTypeArg>${_withAccessor(accessor)}()',
+        isGeneric = true;
 
-  _JsonConvertData.propertyAccess(this.accessString, this.jsonType);
+  _JsonConvertData.propertyAccess(
+    this.accessString,
+    this.jsonType,
+    this.fieldType,
+  ) : isGeneric = false;
 
   static String _withAccessor(String accessor) =>
       accessor.isEmpty ? '' : '.$accessor';
@@ -127,7 +190,11 @@ _JsonConvertData? _typeConverterFrom(
       accessString = '${enclosing.name}.$accessString';
     }
 
-    return _JsonConvertData.propertyAccess(accessString, match.jsonType);
+    return _JsonConvertData.propertyAccess(
+      accessString,
+      match.jsonType,
+      match.fieldType,
+    );
   }
 
   final reviver = ConstantReader(match.annotation).revive();
@@ -145,18 +212,21 @@ _JsonConvertData? _typeConverterFrom(
       match.genericTypeArg!,
       reviver.accessor,
       match.jsonType,
+      match.fieldType,
     );
   }
 
   return _JsonConvertData.className(
     match.annotation.type!.element!.name!,
     reviver.accessor,
     match.jsonType,
+    match.fieldType,
   );
 }
 
 class _ConverterMatch {
   final DartObject annotation;
+  final DartType fieldType;
   final DartType jsonType;
   final ElementAnnotation elementAnnotation;
   final String? genericTypeArg;
@@ -166,6 +236,7 @@ class _ConverterMatch {
     this.annotation,
     this.jsonType,
     this.genericTypeArg,
+    this.fieldType,
   );
 }
 
@@ -191,9 +262,15 @@ _ConverterMatch? _compatibleMatch(
 
   final fieldType = jsonConverterSuper.typeArguments[0];
 
-  if (fieldType == targetType) {
+  // Allow assigning T to T?
+  if (fieldType == targetType || fieldType == targetType.promoteNonNullable()) {
     return _ConverterMatch(
-        annotation, constantValue, jsonConverterSuper.typeArguments[1], null);
+      annotation,
+      constantValue,
+      jsonConverterSuper.typeArguments[1],
+      null,
+      fieldType,
+    );
   }
 
   if (fieldType is TypeParameterType && targetType is TypeParameterType) {
@@ -212,6 +289,7 @@ _ConverterMatch? _compatibleMatch(
       constantValue,
       jsonConverterSuper.typeArguments[1],
       '${targetType.element.name}${targetType.isNullableType ? '?' : ''}',
+      fieldType,
     );
   }
 

json_serializable/pubspec.yaml

@@ -1,5 +1,5 @@
 name: json_serializable
-version: 6.2.0
+version: 6.3.0-dev
 description: >-
   Automatically generate code for converting to and from JSON by annotating
   Dart classes.

json_serializable/test/json_serializable_test.dart

@@ -85,6 +85,7 @@ const _expectedAnnotatedTests = {
   'JsonConverterCtorParams',
   'JsonConverterDuplicateAnnotations',
   'JsonConverterNamedCtor',
+  'JsonConverterNullableToNonNullable',
   'JsonConverterOnGetter',
   'JsonConverterWithBadTypeArg',
   'JsonValueValid',

json_serializable/test/kitchen_sink/kitchen_sink.dart

@@ -66,9 +66,13 @@ class _Factory implements k.KitchenSinkFactory<String, dynamic> {
         [],
         BigInt.zero,
         {},
+        BigInt.zero,
+        {},
         TrivialNumber(0),
         {},
         DateTime.fromMillisecondsSinceEpoch(0),
+        TrivialNumber(0),
+        {},
       );
 
   k.JsonConverterTestClass jsonConverterFromJson(Map<String, dynamic> json) =>
@@ -203,9 +207,13 @@ class JsonConverterTestClass implements k.JsonConverterTestClass {
     this.durationList,
     this.bigInt,
     this.bigIntMap,
+    this.nullableBigInt,
+    this.nullableBigIntMap,
     this.numberSilly,
     this.numberSillySet,
     this.dateTime,
+    this.nullableNumberSilly,
+    this.nullableNumberSillySet,
   );
 
   factory JsonConverterTestClass.fromJson(Map<String, dynamic> json) =>
@@ -219,10 +227,16 @@ class JsonConverterTestClass implements k.JsonConverterTestClass {
   BigInt bigInt;
   Map<String, BigInt> bigIntMap;
 
+  BigInt? nullableBigInt;
+  Map<String, BigInt?> nullableBigIntMap;
+
   TrivialNumber numberSilly;
   Set<TrivialNumber> numberSillySet;
 
   DateTime? dateTime;
+
+  TrivialNumber? nullableNumberSilly;
+  Set<TrivialNumber?> nullableNumberSillySet;
 }
 
 @JsonSerializable()

json_serializable/test/kitchen_sink/kitchen_sink.g_any_map.dart

@@ -65,9 +65,13 @@ class _Factory implements k.KitchenSinkFactory<dynamic, dynamic> {
         [],
         BigInt.zero,
         {},
+        BigInt.zero,
+        {},
         TrivialNumber(0),
         {},
         DateTime.fromMillisecondsSinceEpoch(0),
+        TrivialNumber(0),
+        {},
       );
 
   k.JsonConverterTestClass jsonConverterFromJson(Map<String, dynamic> json) =>
@@ -205,9 +209,13 @@ class JsonConverterTestClass implements k.JsonConverterTestClass {
     this.durationList,
     this.bigInt,
     this.bigIntMap,
+    this.nullableBigInt,
+    this.nullableBigIntMap,
     this.numberSilly,
     this.numberSillySet,
     this.dateTime,
+    this.nullableNumberSilly,
+    this.nullableNumberSillySet,
   );
 
   factory JsonConverterTestClass.fromJson(Map<String, dynamic> json) =>
@@ -221,10 +229,16 @@ class JsonConverterTestClass implements k.JsonConverterTestClass {
   BigInt bigInt;
   Map<String, BigInt> bigIntMap;
 
+  BigInt? nullableBigInt;
+  Map<String, BigInt?> nullableBigIntMap;
+
   TrivialNumber numberSilly;
   Set<TrivialNumber> numberSillySet;
 
   DateTime? dateTime;
+
+  TrivialNumber? nullableNumberSilly;
+  Set<TrivialNumber?> nullableNumberSillySet;
 }
 
 @JsonSerializable(