Include nullability information for generic class

json_serializable/lib/src/decode_helper.dart

@@ -227,7 +227,11 @@ abstract class DecodeHelper implements HelperCore {
             'effect because both `disallowNullValue` and `required` are set to '
             '`true`.');
       }
-      if (contextHelper.deserializeConvertData != null) {
+
+      final deserializeConvertData = contextHelper.deserializeConvertData;
+
+      if (deserializeConvertData != null &&
+          !deserializeConvertData.nullableToAllowDefault) {
         log.warning('The field `${field.name}` has both `defaultValue` and '
             '`fromJson` defined which likely won\'t work for your scenario.\n'
             'Instead of using `defaultValue`, set `nullable: false` and handle '

json_serializable/lib/src/helper_core.dart

@@ -141,14 +141,12 @@ String typeToCode(
   if (type.isDynamic) {
     return 'dynamic';
   } else if (type is InterfaceType) {
-    final typeArguments = type.typeArguments;
-    if (typeArguments.isEmpty) {
-      final nullablePostfix = (type.isNullableType || forceNullable) ? '?' : '';
-      return '${type.element.name}$nullablePostfix';
-    } else {
-      final typeArgumentsCode = typeArguments.map(typeToCode).join(', ');
-      return '${type.element.name}<$typeArgumentsCode>';
-    }
+    return [
+      type.element.name,
+      if (type.typeArguments.isNotEmpty)
+        '<${type.typeArguments.map(typeToCode).join(', ')}>',
+      (type.isNullableType || forceNullable) ? '?' : '',
+    ].join();
   }
   throw UnimplementedError('(${type.runtimeType}) $type');
 }

json_serializable/lib/src/type_helper_ctx.dart

@@ -125,23 +125,34 @@ ConvertData? _convertData(DartObject obj, FieldElement element, bool isFrom) {
         'positional parameter.');
   }
 
+  var nullableToAllowDefault = false;
+
   final argType = executableElement.parameters.first.type;
   if (isFrom) {
+    final hasDefaultValue =
+        !jsonKeyAnnotation(element).read('defaultValue').isNull;
+
     final returnType = executableElement.returnType;
 
     if (returnType is TypeParameterType) {
       // We keep things simple in this case. We rely on inferred type arguments
       // to the `fromJson` function.
       // TODO: consider adding error checking here if there is confusion.
     } else if (!returnType.isAssignableTo(element.type)) {
-      final returnTypeCode = typeToCode(returnType);
-      final elementTypeCode = typeToCode(element.type);
-      throwUnsupported(
-          element,
-          'The `$paramName` function `${executableElement.name}` return type '
-          '`$returnTypeCode` is not compatible with field type '
-          '`$elementTypeCode`.');
+      if (returnType.promoteNonNullable().isAssignableTo(element.type) &&
+          hasDefaultValue) {
+        // noop
+      } else {
+        final returnTypeCode = typeToCode(returnType);
+        final elementTypeCode = typeToCode(element.type);
+        throwUnsupported(
+            element,
+            'The `$paramName` function `${executableElement.name}` return type '
+            '`$returnTypeCode` is not compatible with field type '
+            '`$elementTypeCode`.');
+      }
     }
+    nullableToAllowDefault = hasDefaultValue && returnType.isNullableType;
   } else {
     if (argType is TypeParameterType) {
       // We keep things simple in this case. We rely on inferred type arguments
@@ -164,5 +175,9 @@ ConvertData? _convertData(DartObject obj, FieldElement element, bool isFrom) {
     name = '${executableElement.enclosingElement.name}.$name';
   }
 
-  return ConvertData(name, argType);
+  return ConvertData(
+    name,
+    argType,
+    nullableToAllowDefault: nullableToAllowDefault,
+  );
 }

json_serializable/lib/src/type_helpers/convert_helper.dart

@@ -14,7 +14,15 @@ class ConvertData {
   final String name;
   final DartType paramType;
 
-  ConvertData(this.name, this.paramType);
+  /// `true` if the function returns a nullable value AND there is a default
+  /// value assigned.
+  final bool nullableToAllowDefault;
+
+  ConvertData(
+    this.name,
+    this.paramType, {
+    required this.nullableToAllowDefault,
+  });
 }
 
 abstract class TypeHelperContextWithConvert extends TypeHelperContext {

json_serializable/lib/src/utils.dart

@@ -202,3 +202,8 @@ Map<FieldElement, dynamic>? enumFieldsMap(DartType targetType) {
 /// Otherwise, `null`.
 Iterable<FieldElement>? iterateEnumFields(DartType targetType) =>
     enumFieldsMap(targetType)?.keys;
+
+extension DartTypeExtension on DartType {
+  DartType promoteNonNullable() =>
+      element?.library?.typeSystem.promoteToNonNull(this) ?? this;
+}

json_serializable/test/json_serializable_test.dart

@@ -94,6 +94,8 @@ const _expectedAnnotatedTests = {
   'OverrideGetterExampleI613',
   'PrivateFieldCtorClass',
   'PropInMixinI448Regression',
+  'Reproduce869NullableGenericType',
+  'Reproduce869NullableGenericTypeWithDefault',
   'SetSupport',
   'SubclassedJsonKey',
   'SubType',

json_serializable/test/src/to_from_json_test_input.dart

@@ -70,6 +70,64 @@ class BadToFuncReturnType {
   late String field;
 }
 
+@ShouldThrow(
+  'Error with `@JsonKey` on `values`. The `fromJson` function `_fromList` '
+  'return type `List<int>?` is not compatible with field type `List<int>`.',
+  element: 'values',
+)
+@JsonSerializable()
+class Reproduce869NullableGenericType {
+  @JsonKey(
+    toJson: _toList, // nullable
+    fromJson: _fromList, // nullable
+  )
+  late final List<int> values;
+}
+
+@ShouldGenerate(
+  r'''
+Reproduce869NullableGenericTypeWithDefault
+    _$Reproduce869NullableGenericTypeWithDefaultFromJson(
+            Map<String, dynamic> json) =>
+        Reproduce869NullableGenericTypeWithDefault()
+          ..values = _fromList(json['values'] as List?) ?? []
+          ..valuesNullable = _fromList(json['valuesNullable'] as List?) ?? [];
+
+Map<String, dynamic> _$Reproduce869NullableGenericTypeWithDefaultToJson(
+        Reproduce869NullableGenericTypeWithDefault instance) =>
+    <String, dynamic>{
+      'values': _toList(instance.values),
+      'valuesNullable': _toList(instance.valuesNullable),
+    };
+''',
+)
+@JsonSerializable()
+class Reproduce869NullableGenericTypeWithDefault {
+  @JsonKey(
+    toJson: _toList, // nullable
+    fromJson: _fromList, // nullable
+    defaultValue: <int>[],
+  )
+  late List<int> values;
+
+  @JsonKey(
+    toJson: _toList, // nullable
+    fromJson: _fromList, // nullable
+    defaultValue: <int>[],
+  )
+  List<int>? valuesNullable;
+}
+
+List<int>? _fromList(List? pairs) {
+  return pairs?.map((it) {
+    return it as int;
+  }).toList(growable: false);
+}
+
+List<List>? _toList(List<int>? pairs) {
+  return pairs?.map((it) => [it]).toList(growable: false);
+}
+
 @ShouldThrow(
   'Error with `@JsonKey` on `field`. The `toJson` function '
   '`_twoArgFunction` must have one positional parameter.',