List<int>.fold not compiling to javascript because of operator not found

[ravish-tech]

This can be replicated on dartpad.dev.
When trying to run the below code, I get error. I was expecting this to run and give me 2 as answer.

void main() {
  List<int> a = List<int>();
  a.add(2);
  print(a.fold(0, (prev, v) => prev + v));
}

Error compiling to JavaScript:
main.dart:4:37:
Error: The operator '+' isn't defined for the class 'Object'.
 - 'Object' is from 'dart:core'.
  print(a.fold(0, (prev, v) => prev + v));
                                    ^
Error: Compilation failed.

Based on Dart SDK 2.8.2

[eernstg]

You need to give type inference some extra information:

void main() {
  List<int> a = List<int>();
  a.add(2);
  print(a.fold<int>(0, (prev, v) => prev + v));
}

With the original version, the inferred type argument to fold will be Object, which makes prev a parameter of type Object.

This issue arises because the Dart type inference does not transfer information between parameters in this situation, cf. dart-lang/language#731. It rarely comes up except for fold.

[sigmundch]

(I believe Erik's comment should address your question, but feel free to reopen if we misunderstood)