dart:convert jsonEncode() throws an exception if the method toJson() is defined in an extension.

[simphotonics]

My SDK version is:

Dart VM version: 2.9.0-14.1.beta (beta) (Tue Jun 9 10:52:57 2020 +0200) on "linux_x64"

When attempting to move the method toJson() to an extension I noticed that jsonEncode() does not behave as expected.
If the method toJson() is defined within the class A (see below) the program produces the expected output:

{"index":0}

However, if toJson() is defined within the extension JsonA dart:convert does not seem to have access to toJson and a call to jsonEncode results in this unhandled exception:

Unhandled exception:
Converting object to an encodable object failed: Instance of 'A'
#0      _JsonStringifier.writeObject (dart:convert/json.dart:687:7)
#1      _JsonStringStringifier.printOn (dart:convert/json.dart:876:17)
#2      _JsonStringStringifier.stringify (dart:convert/json.dart:861:5)
#3      JsonEncoder.convert (dart:convert/json.dart:261:30)
#4      JsonCodec.encode (dart:convert/json.dart:171:45)
#5      jsonEncode (dart:convert/json.dart:81:10)
#6      main (file:///home/dan/WORK/RemoteFlutterProjects/generic_enum/generic_enum/example/bin/example.dart:20:9)
#7      _startIsolate.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:301:19)
#8      _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:168:12)

import 'dart:convert';

class A {
  const A(this.a);
  final int a;
  static const values = [A(1), A(2)];

  // Map<String, dynamic> toJson() => {
  //       'index': A.values.indexOf(this),
  //     };
}

extension JsonA on A {
  Map<String, dynamic> toJson() => {
        'index': A.values.indexOf(this),
      };
}

main() {
  print(jsonEncode(A.values[0]));
}

[julemand101]

Extensions are dependent on static types and does therefore not work on dynamic:
https://dart.dev/guides/language/extension-methods#static-types-and-dynamic

If using the debugger we can see the next part of the stacktrace:

#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:53:5)
#1      _defaultToEncodable (dart:convert/json.dart:521:55)
#2      _JsonStringifier.writeObject (dart:convert/json.dart:653:36)
#3      _JsonStringStringifier.printOn (dart:convert/json.dart:846:17)
#4      _JsonStringStringifier.stringify (dart:convert/json.dart:831:5)
#5      JsonEncoder.convert (dart:convert/json.dart:260:30)
#6      JsonCodec.encode (dart:convert/json.dart:168:45)
#7      jsonEncode (dart:convert/json.dart:82:10)
#8      main (file:///C:/Users/jacob/Documents/GitHub/AdventOfCode2019/bin/stackoverflow.dart:22:9)
#9      _startIsolate.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:301:19)
#10     _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:168:12)

The _defaultToEncodable method is defined as:

dynamic _defaultToEncodable(dynamic object) => object.toJson();

We can reproduce this problem fairly simple by:

dynamic _defaultToEncodable(dynamic object) => object.toJson();

class A { }

extension JsonA on A {
  void toJson() => print('Hey');
}

void main() {
  print(_defaultToEncodable(A()));
}

Which returns:

Unhandled exception:
NoSuchMethodError: Class 'A' has no instance method 'toJson'.
Receiver: Instance of 'A'
Tried calling: toJson()
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:53:5)
#1      _defaultToEncodable (file:///C:/Users/jacob/Documents/GitHub/AdventOfCode2019/bin/stackoverflow.dart:1:55)
#2      main (file:///C:/Users/jacob/Documents/GitHub/AdventOfCode2019/bin/stackoverflow.dart:10:9)
#3      _startIsolate.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:301:19)
#4      _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:168:12)

[simphotonics]

@julemand101 Thank you for taking the time to write such a detailed explanation. It might be useful for others who come across the same issue.

The use of the data-type dynamic was not obvious to me since jsonEncode and the first few functions in the calling sequence use Object. But I guess they had to use dynamic at some point in order to call the method toJson() on an object of unknown type.

I'll close this issue now.

[chonghorizons]

Issued a pull request (#47875) to update the docs, because the docs are not clear on this issue.

My use case was extending an enum: see https://stackoverflow.com/questions/70272959/jsonencode-doesnt-work-with-enum-extensions-is-there-a-workaround