feature: allow undefined types as long as they have a converter

json_serializable/lib/src/decode_helper.dart

@@ -4,7 +4,6 @@
 
 import 'package:analyzer/dart/element/element.dart';
 import 'package:json_annotation/json_annotation.dart';
-import 'package:source_gen/source_gen.dart';
 
 import 'helper_core.dart';
 import 'json_literal_generator.dart';
@@ -247,8 +246,8 @@ _ConstructorData _writeConstructorInvocation(
     usedCtorParamsAndFields.add(arg.name);
   }
 
-  _validateConstructorArguments(
-      classElement, constructorArguments.followedBy(namedConstructorArguments));
+  warnUndefinedElements(
+      constructorArguments.followedBy(namedConstructorArguments));
 
   // fields that aren't already set by the constructor and that aren't final
   var remainingFieldsForInvocationBody =
@@ -291,17 +290,3 @@ class _ConstructorData {
   _ConstructorData(
       this.content, this.fieldsToSet, this.usedCtorParamsAndFields);
 }
-
-void _validateConstructorArguments(
-    ClassElement element, Iterable<ParameterElement> constructorArguments) {
-  var undefinedArgs =
-      constructorArguments.where((pe) => pe.type.isUndefined).toList();
-  if (undefinedArgs.isNotEmpty) {
-    var description = undefinedArgs.map((fe) => '`${fe.name}`').join(', ');
-
-    throw new InvalidGenerationSourceError(
-        'At least one constructor argument has an invalid type: $description.',
-        todo: 'Check names and imports.',
-        element: element);
-  }
-}

json_serializable/lib/src/helper_core.dart

@@ -3,7 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:analyzer/dart/element/element.dart';
-
+import 'package:build/build.dart';
 import 'package:json_annotation/json_annotation.dart';
 import 'package:source_gen/source_gen.dart';
 
@@ -46,13 +46,24 @@ abstract class HelperCore {
 
 InvalidGenerationSourceError createInvalidGenerationError(
     String targetMember, FieldElement field, UnsupportedTypeError e) {
-  var extra = (field.type != e.type) ? ' because of type `${e.type}`' : '';
+  var message = 'Could not generate `$targetMember` code for `${field.name}`';
+
+  var todo = 'Make sure all of the types are serializable.';
 
-  var message = 'Could not generate `$targetMember` code for '
-      '`${field.name}`$extra.\n${e.reason}';
+  if (e.type.isUndefined) {
+    message = '$message because the type is undefined.';
+    todo = "Check your imports. If you're trying to generate code for a "
+        'Platform-provided type, you may have to specify a custom '
+        '`$targetMember` in the associated `@JsonKey` annotation.';
+  } else {
+    if (field.type != e.type) {
+      message = '$message because of type `${e.type}`';
+    }
+
+    message = '$message.\n${e.reason}';
+  }
 
-  return new InvalidGenerationSourceError(message,
-      todo: 'Make sure all of the types are serializable.', element: field);
+  return new InvalidGenerationSourceError(message, todo: todo, element: field);
 }
 
 /// Returns a [String] representing the type arguments that exist on
@@ -86,3 +97,13 @@ String genericClassArguments(ClassElement element, bool withConstraints) {
       .join(', ');
   return '<$values>';
 }
+
+void warnUndefinedElements(Iterable<VariableElement> elements) {
+  for (var element in elements.where((fe) => fe.type.isUndefined)) {
+    var span = spanForElement(element);
+    log.warning('''
+This element has an undefined type. It may causes issues when generated code.
+${span.start.toolString}
+${span.highlight()}''');
+  }
+}

json_serializable/lib/src/json_serializable_generator.dart

@@ -247,15 +247,7 @@ Set<FieldElement> _createSortedFieldSet(ClassElement element) {
     }
   }
 
-  var undefinedFields = fieldsList.where((fe) => fe.type.isUndefined).toList();
-  if (undefinedFields.isNotEmpty) {
-    var description = undefinedFields.map((fe) => '`${fe.name}`').join(', ');
-
-    throw new InvalidGenerationSourceError(
-        'At least one field has an invalid type: $description.',
-        todo: 'Check names and imports.',
-        element: undefinedFields.first);
-  }
+  warnUndefinedElements(fieldsList);
 
   // Sort these in the order in which they appear in the class
   // Sadly, `classElement.fields` puts properties after fields

json_serializable/lib/src/type_helpers/value_helper.dart

@@ -14,6 +14,9 @@ class ValueHelper extends TypeHelper {
   @override
   String serialize(
       DartType targetType, String expression, SerializeContext context) {
+    if (targetType.isUndefined) {
+      return null;
+    }
     if (targetType.isDynamic ||
         targetType.isObject ||
         simpleJsonTypeChecker.isAssignableFromType(targetType)) {
@@ -26,6 +29,9 @@ class ValueHelper extends TypeHelper {
   @override
   String deserialize(
       DartType targetType, String expression, DeserializeContext context) {
+    if (targetType.isUndefined) {
+      return null;
+    }
     if (targetType.isDynamic || targetType.isObject) {
       // just return it as-is. We'll hope it's safe.
       return expression;

json_serializable/test/json_serializable_test.dart

@@ -168,19 +168,44 @@ class _$TrivialNestedNonNullableJsonMapWrapper extends $JsonMapWrapper {
           'Remove the JsonSerializable annotation from `annotatedMethod`.');
     });
   });
+
   group('unknown types', () {
-    test('in constructor arguments', () {
-      expectThrows(
-          'UnknownCtorParamType',
-          'At least one constructor argument has an invalid type: `number`.',
-          'Check names and imports.');
+    String flavorMessage(String flavor) =>
+        'Could not generate `$flavor` code for `number` '
+        'because the type is undefined.';
+
+    String flavorTodo(String flavor) =>
+        'Check your imports. If you\'re trying to generate code for a '
+        'Platform-provided type, you may have to specify a custom `$flavor` '
+        'in the associated `@JsonKey` annotation.';
+
+    group('fromJson', () {
+      var msg = flavorMessage('fromJson');
+      var todo = flavorTodo('fromJson');
+      test('in constructor arguments', () {
+        expectThrows('UnknownCtorParamType', msg, todo);
+      });
+
+      test('in fields', () {
+        expectThrows('UnknownFieldType', msg, todo);
+      });
     });
 
-    test('in fields', () {
-      expectThrows(
-          'UnknownFieldType',
-          'At least one field has an invalid type: `number`.',
-          'Check names and imports.');
+    group('toJson', () {
+      test('in fields', () {
+        expectThrows('UnknownFieldTypeToJsonOnly', flavorMessage('toJson'),
+            flavorTodo('toJson'));
+      });
+    });
+
+    test('with proper convert methods', () {
+      var output = runForElementNamed('UnknownFieldTypeWithConvert');
+      expect(output, contains("_everythingIs42(json['number'])"));
+      if (generator.useWrappers) {
+        expect(output, contains('_everythingIs42(_v.number)'));
+      } else {
+        expect(output, contains('_everythingIs42(number)'));
+      }
     });
   });
 

json_serializable/test/src/json_serializable_test_input.dart

@@ -104,6 +104,21 @@ class UnknownFieldType {
   Bob number;
 }
 
+@JsonSerializable(createFactory: false)
+class UnknownFieldTypeToJsonOnly {
+  // ignore: undefined_class
+  Bob number;
+}
+
+@JsonSerializable()
+class UnknownFieldTypeWithConvert {
+  @JsonKey(fromJson: _everythingIs42, toJson: _everythingIs42)
+  // ignore: undefined_class
+  Bob number;
+}
+
+dynamic _everythingIs42(Object input) => 42;
+
 @JsonSerializable(createFactory: false)
 class NoSerializeFieldType {
   Stopwatch watch;