meta limitation: useResult not available in factory constructors

[bwc-x]

Can't seem to find any existing open/closed issue regarding this.

Currently facing a situation, roughly akin to:

import 'package:meta/meta.dart';

extension Ext on Class {
  void doSomeWork(int i) {
    print('$this + $i = ...');
    return this;
  }
}

class Class {
  const Class();
  const factory Class.sub() = SubClass;
}

class SubClass extends Class {
  const SubClass();
}

void main() {
  Class.sub(); // .doSomeWork(5) meant to be called, yet skipped on accident
}

Given a new instance, built using a factory - sometimes containing a great deal of inner parameters, passed to it - it is impossible to check whether the result is actually used. The slightly more practical (actual) use case for this involves passing a rather complex instance, built through a redirecting const factory into a Flutter's BuildContext for subsequent state propagation / widget tree update. Assigning it to a temporary variable beforehand, though necessary at times, is completely redundant in the rest of cases.

The @useResult annotation would be nothing short of perfect for this. Yet it vehemently refuses to do its job, insisting that The annotation 'useResult' can only be used on fields, getters, methods, top-level functions, or top-level variables. While ignoring that message makes it ignore all the uses of the resulting instance completely.

Any way to patch this up?

A rather messy fix for this currently involves either prior redundant assignment, or an even more redundant static method declaration - reduplicating the whole factory constructor in whole merely for a chance to annotate it with @useResult.

class Class {
  const Class();
  @useResult // no errors
  static Class sub() => SubClass();
}

Wouldn't mind doing this either, if it wasn't for the freezed 's own reliance on factory constructors for its code generation.

[srawlins]

I don't see why you shouldn't be able to annotate any kind of constructor with @useResult. The docs don't include it currently. https://pub.dev/documentation/meta/latest/meta/useResult-constant.html

@bwilkerson @pq any opinions?

[pq]

I don't see why you shouldn't be able to annotate any kind of constructor with @useResult.

I agree. I expect it was just an oversight.

[bwilkerson]

I also agree. I would think that allowing it on constructors should be well defined and reasonable.

[srawlins]

Short memory. I did this two months ago. You just need the next Dart version. :D Maybe not released yet. eb7ba49