Dart does not recognise nullable value in extension at runtime

[javiermrz]

The problem is pretty straightforward. I have the following extension:

extension NullableListExtension on List<dynamic>? {
  String getOrEmptyString(int index) =>
      this == null || index >= this!.length ? "" : this![index]!.toString();
}

When editing, compiler shows no error. However, the following warning appears at runtime:

Warning: Operand of null-aware operation '!' has type 'List<dynamic>' which excludes null.
 - 'List' is from 'dart:core'.
      this == null || index >= this!.length ? "" : this![index];

(With ^ symbol pointing to the exclamation in this![index]).
But if I actually were to remove the ! from the this![index], it would ask me to introduce the symbol again (which I think is the correct thing to do).

[lrhn]

I never remember whether we can promote this or not, but in either case we should be consistent.

(Also, the code won't work because the empty string is not assignable to T).

[javiermrz]

@irhn yep, that was an error of my own, thanks for pointing it out, instead of T there should be a String there, I will update that. However, the error still persists.

I don't know if this can be promoted either, but when adding the ! the compiler seems to be happy (not at runtime though). It works like a charm though, this is only a warning, but quite annoying.

[javiermrz]

You are right, the error is gone, thanks. Shouldn't the compiler show an error to warn the developer about this?

[leafpetersen]

I believe that there is a known issue where the analyzer incorrectly promotes this only in extension methods (it is specified never to promote). I can't find the issue we discussed this in at the moment. We had considered fixing this forward by making this promotable everywhere, but this looked to be too long of an arc of work in the CFE, so for the time being this is a known errata. cc @scheglov @johnniwinther

Related:

#39959
#32120

[javiermrz]

Great then. Thanks all for your fast answers, I'll close this issue as everything was solved. Happy Holidays :)

[scheglov]

@leafpetersen actually, it worked in the CFE, and required some additional work in the analyzer. IIRC, I prototyped a solution for the analyzer, but at the end we decided that we don't want such promotion. It seems that in the CFE promotion still works though.

[leafpetersen]

@scheglov I'm probably misremembering details. I'm pretty sure that we do want the promotion, but decided that the implementation cost was too high somewhere to do it now (perhaps the CFE only promotes this in extensions, and making it do so in classes was going to be hard?).

[scheglov]

Ah, yes, it is quite possible that this promotion in CFE works only in extensions because it has an artificial variable for this in extensions.