InterfaceType is not a subtype of Class in VariableGet

Question

Getting the following error when trying to build my app after upgrading to Flutter 1.20.1.

Unhandled exception:
Crash when compiling null,
at character offset null:
InterfaceType(PagingResponse<Assignment*>*) is not a subtype of Class(Response) in VariableGet(response{PagingResponse<Assignment*>*})
#0      TypeEnvironment.typeError  (package:kernel/type_environment.dart:164:7)
#1      Expression.getStaticTypeAsInstanceOf  (package:kernel/ast.dart:3037:10)
#2      PropertyGet.getStaticType  (package:kernel/ast.dart:3200:20)
#3      transformAsExpression  (package:kernel/transformations/type_casts_optimizer.dart:22:45)
#4      _Lowering.visitAsExpression  (package:vm/transformations/lowering.dart:65:12)
#5      AsExpression.accept  (package:kernel/ast.dart:4913:44)
#6      VariableSet.transformChildren  (package:kernel/ast.dart:3154:21)
#7      Transformer.defaultTreeNode  (package:kernel/visitor.dart:653:10)
#8      TreeVisitor.defaultExpression  (package:kernel/visitor.dart:144:43)
#9      TreeVisitor.visitVariableSet  (package:kernel/visitor.dart:148:43)
#10     VariableSet.accept  (package:kernel/ast.dart:3144:44)
#11     ExpressionStatement.transformChildren  (package:kernel/ast.dart:5794:31)

// bunch of stack that doesn't look important

#133    KernelTarget.buildComponent  (package:front_end/src/fasta/kernel/kernel_target.dart:355:12)
#134    IncrementalCompiler.computeDelta.<anonymous closure>  (package:front_end/src/fasta/incremental_compiler.dart:274:28)
<asynchronous suspension>
#135    IncrementalCompiler.computeDelta.<anonymous closure> (package:front_end/src/fasta/incremental_compiler.dart)
#136    CompilerContext.runInContext.<anonymous closure>.<anonymous closure>  (package:front_end/src/fasta/compiler_context.dart:123:46)
#137    new Future.sync  (dart:async/future.dart:223:31)
#138    CompilerContext.runInContext.<anonymous closure>  (package:front_end/src/fasta/compiler_context.dart:123:19)
#139    _rootRun  (dart:async/zone.dart:1190:13)
#140    _CustomZone.run  (dart:async/zone.dart:1093:19)
#141    _runZoned  (dart:async/zone.dart:1630:10)
#142    runZoned  (dart:async/zone.dart:1550:10)

Assignment is just a json_serializable model.

Here's what PagingResponse looks like:

@JsonSerializable()
class PagingResponse<T> {
  PagingResponse(
    this.status,
    this.message,
    this.code,
    this.dataJson,
  );

  String status;
  String message;
  String code;
  @JsonKey(ignore: true)
  PagingResponseData<T> data;
  @JsonKey(name: 'data')
  Map<String, dynamic> dataJson;

  factory PagingResponse.fromJson(
    Map<String, dynamic> json,
    PagingResponseDataDataGeneratorFunction<T> rowsFromJson,
  ) {
    PagingResponse<T> response = _$PagingResponseFromJson<T>(json);

    // Our API sometimes returns data directly and at other times nested under
    // another object. Check if data directly has a rows field. If not, this
    // probably means that data is nested inside an object.
    if (response.dataJson?.containsKey('rows') ?? false) {
      response.data = new PagingResponseData<T>.fromJson(response.dataJson);
      response.data.rows = rowsFromJson(response.data.rowsJson);
    } else if (response.dataJson?.isNotEmpty ?? false) {
      response.data =
          new PagingResponseData<T>.fromJson(response.dataJson.values.first);
      response.data.rows = rowsFromJson(response.data.rowsJson);
    }

    return response;
  }

  Map<String, dynamic> toJson() => _$PagingResponseToJson(this);
}

typedef List<T> PagingResponseDataDataGeneratorFunction<T>(List<dynamic> json);

Here's various versions:

[✓] Flutter (Channel stable, 1.20.1, on Mac OS X 10.15.4 19E287, locale en-US)
    • Flutter version 1.20.1 at /Users/------/dev/tools/flutter
    • Framework revision 2ae34518b8 (9 days ago), 2020-08-05 19:53:19 -0700
    • Engine revision c8e3b94853
    • Dart version 2.9.0

I'm omitting the rest of the doctor output as this happens for both Android and iOS builds.

Looking around on the internet, I wasn't able to find anyone else with this issue who had a solution that applied. Here's some though:

• https://github.com/flutter/flutter/issues/48071
• https://github.com/dart-lang/build/issues/2478
• https://github.com/flutter/flutter/issues/33013

I tried:

• deleting every use of PagingResponse<Assignment> and running a build - got the same error message with a different model class;
• flutter clean
• deleting ~/.pub-cache
• various other stuff I don't remember at this point

If anyone has as much as an idea on what this could be even related to, I would appreciate it, as currently I'm looking at code and making blind changes in hope I get a hit. Especially some hints on what this Dart stack is about would be appreciated, if some more knowledgeable folks stumble upon this question.

UPDATE: Ended up opening an issue for this and we are heading towards a resolution there: https://github.com/flutter/flutter/issues/64155

Answer 1

I have updated the flutter version to 1.20.2, and I have not encountered any problems, knowing that I use JsonSerializable as well, try this:

Remove these files:

pubspec.lock

.packages

.flutter-plugins

.flutter-plugins-dependencies

then apply flutter pub get command