Backwards compatible attempt

drift/lib/src/drift_dev_helper.dart

@@ -1,2 +1,3 @@
 // This field is analyzed by drift_dev to easily obtain common types.
-export 'runtime/types/converters.dart' show TypeConverter, JsonTypeConverter;
+export 'runtime/types/converters.dart'
+    show TypeConverter, JsonTypeConverter, JsonTypeConverterWithDifferentTypes;

drift/lib/src/runtime/types/converters.dart

@@ -33,13 +33,9 @@ abstract class TypeConverter<D, S> {
 /// to SQL (and vice-versa).
 /// When a [BuildGeneralColumn.map] column (or a `MAPPED BY` constraint in
 /// `.drift` files) refers to a type converter that inherits from
-/// [JsonTypeConverter], it will also be used for the conversion from and to
+/// [JsonTypeConverterWithDifferentTypes], it will also be used for the conversion from and to
 /// JSON.
-abstract class JsonTypeConverter<D, S, J> extends TypeConverter<D, S> {
+mixin JsonTypeConverterWithDifferentTypes<D, S, J> on TypeConverter<D, S> {
-  /// Empty constant constructor so that subclasses can have a constant
-  /// constructor.
-  const JsonTypeConverter();
-
   /// Map a value from the Data class to json.
   ///
   /// Defaults to doing the same conversion as for Dart -> SQL, [toSql].
@@ -56,9 +52,39 @@ abstract class JsonTypeConverter<D, S, J> extends TypeConverter<D, S> {
   /// The returned type converter will use the [inner] type converter for non-
   /// null values. Further, `null` is mapped to `null` in both directions (from
   /// Dart to SQL and vice-versa).
-  static JsonTypeConverter<D?, S?, J?>
+  static JsonTypeConverterWithDifferentTypes<D?, S?, J?>
       asNullable<D, S extends Object, J extends Object>(
-          JsonTypeConverter<D, S, J> inner) {
+          JsonTypeConverterWithDifferentTypes<D, S, J> inner) {
+    return _NullWrappingTypeConverterWithJson(inner);
+  }
+}
+
+/// A mixin for [TypeConverter]s that should also apply to drift's builtin
+/// JSON serialization of data classes.
+///
+/// By default, a [TypeConverter] only applies to the serialization from Dart
+/// to SQL (and vice-versa).
+/// When a [BuildGeneralColumn.map] column (or a `MAPPED BY` constraint in
+/// `.drift` files) refers to a type converter that inherits from
+/// [JsonTypeConverter], it will also be used for the conversion from and to
+/// JSON.
+mixin JsonTypeConverter<D, S>
+    implements JsonTypeConverterWithDifferentTypes<D, S, S> {
+  @override
+  S toJson(D value) => toSql(value);
+
+  @override
+  D fromJson(S json) => fromSql(json);
+
+  /// Wraps an [inner] type converter that only considers non-nullable values
+  /// as a type converter that handles null values too.
+  ///
+  /// The returned type converter will use the [inner] type converter for non-
+  /// null values. Further, `null` is mapped to `null` in both directions (from
+  /// Dart to SQL and vice-versa).
+  static JsonTypeConverterWithDifferentTypes<D?, S?, J?>
+      asNullable<D, S extends Object, J extends Object>(
+          JsonTypeConverterWithDifferentTypes<D, S, J> inner) {
     return _NullWrappingTypeConverterWithJson(inner);
   }
 }
@@ -157,8 +183,8 @@ class _NullWrappingTypeConverter<D, S extends Object>
 
 class _NullWrappingTypeConverterWithJson<D, S extends Object, J extends Object>
     extends NullAwareTypeConverter<D, S>
-    implements JsonTypeConverter<D?, S?, J?> {
+    implements JsonTypeConverterWithDifferentTypes<D?, S?, J?> {
-  final JsonTypeConverter<D, S, J> _inner;
+  final JsonTypeConverterWithDifferentTypes<D, S, J> _inner;
 
   const _NullWrappingTypeConverterWithJson(this._inner);
 

drift/test/generated/todos.dart

@@ -135,14 +135,8 @@ class CustomConverter extends TypeConverter<MyCustomObject, String> {
 }
 
 class CustomJsonConverter extends CustomConverter
-    implements JsonTypeConverter<MyCustomObject, String, String> {
+    with JsonTypeConverter<MyCustomObject, String> {
   const CustomJsonConverter();
-
-  @override
-  MyCustomObject fromJson(String json) => fromSql(json);
-
-  @override
-  String toJson(MyCustomObject value) => toSql(value);
 }
 
 abstract class CategoryTodoCountView extends View {

drift/test/generated/todos.g.dart

@@ -1373,10 +1373,11 @@ class $PureDefaultsTable extends PureDefaults
     return $PureDefaultsTable(attachedDatabase, alias);
   }
 
-  static JsonTypeConverter<MyCustomObject, String, String> $converter0 =
+  static JsonTypeConverterWithDifferentTypes<MyCustomObject, String, String>
-      const CustomJsonConverter();
+      $converter0 = const CustomJsonConverter();
-  static JsonTypeConverter<MyCustomObject?, String?, String?> $converter0n =
+  static JsonTypeConverterWithDifferentTypes<MyCustomObject?, String?, String?>
-      JsonTypeConverter.asNullable($converter0);
+      $converter0n =
+      JsonTypeConverterWithDifferentTypes.asNullable($converter0);
 }
 
 class CategoryTodoCountViewData extends DataClass {

drift_dev/lib/src/analyzer/dart_types.dart

@@ -181,7 +181,7 @@ UsedTypeConverter? readTypeConverter(
           "potentially map to `null` which can't be stored in the database.");
     } else if (!canBeSkippedForNulls) {
       final alternative = appliesToJsonToo
-          ? 'JsonTypeConverter.asNullable'
+          ? 'JsonTypeConverterWithDifferentTypes.asNullable'
           : 'NullAwareTypeConverter.wrap';
 
       reportError('This column is nullable, but the type converter has a non-'

drift_dev/lib/src/analyzer/helper.dart

@@ -27,18 +27,18 @@ class HelperLibrary {
   }
 
   /// Converts the given Dart [type] into an instantiation of the
-  /// `TypeConverter` class from drift.
+  /// `JsonTypeConverter` class from drift.
   ///
   /// Returns `null` if [type] is not a subtype of `TypeConverter`.
   InterfaceType? asJsonTypeConverter(DartType type) {
-    final converter = helperLibrary.exportNamespace.get('JsonTypeConverter')
+    final converter = helperLibrary.exportNamespace
-        as InterfaceElement;
+        .get('JsonTypeConverterWithDifferentTypes') as InterfaceElement;
     return type.asInstanceOf(converter);
   }
 
   bool isJsonAwareTypeConverter(DartType? type, LibraryElement context) {
-    final jsonMixin = helperLibrary.exportNamespace.get('JsonTypeConverter')
+    final jsonMixin = helperLibrary.exportNamespace
-        as InterfaceElement;
+        .get('JsonTypeConverterWithDifferentTypes') as InterfaceElement;
     final jsonConverterType = jsonMixin.instantiate(
       typeArguments: [
         context.typeProvider.dynamicType,

drift_dev/lib/src/model/used_type_converter.dart

@@ -143,7 +143,7 @@ class UsedTypeConverter {
     if (makeNullable) jsonDartType += '?';
 
     if (alsoAppliesToJsonConversion) {
-      return 'JsonTypeConverter<${dartTypeCode(makeNullable)}, $sqlDartType, $jsonDartType>';
+      return 'JsonTypeConverterWithDifferentTypes<${dartTypeCode(makeNullable)}, $sqlDartType, $jsonDartType>';
     }
     return 'TypeConverter<${dartTypeCode(makeNullable)}, $sqlDartType>';
   }

drift_dev/lib/src/writer/tables/table_writer.dart

@@ -336,7 +336,7 @@ class TableWriter extends TableOrViewWriter {
             converter.converterNameInCode(makeNullable: true);
 
         final wrap = converter.alsoAppliesToJsonConversion
-            ? 'JsonTypeConverter.asNullable'
+            ? 'JsonTypeConverterWithDifferentTypes.asNullable'
             : 'NullAwareTypeConverter.wrap';
 
         final code = '$wrap(${converter.fieldName})';

drift_dev/test/analyzer/dart/type_converter_test.dart

@@ -15,7 +15,7 @@ void main() {
 import 'package:drift/drift.dart';
 
 TypeConverter<String, String> withoutJson() => throw 'stub';
-JsonTypeConverter<String, String, String> withJson() => throw 'stub';
+JsonTypeConverter<String, String> withJson() => throw 'stub';
 
 class Users extends Table {
   TextColumn get foo => text().map(withoutJson())();