Covariant return types for nullable defaults

[giladwo]

Functions like E firstWhere(bool test(E element), {E orElse()?}) have an invariant E return type.

Considering the fact that E extends E?, and orElse can usefully return null, I suspect a covariant contravariant T super E may be a more useful return type. (I'm not sure how to express this in Dart though).

What do you think?

[mraleph]

You can't express this in Dart.

See related discussions at #42947

[giladwo]

Sorry, I got mixed up and thought about this kind of variance:
https://github.com/Team1690/Scouting-Frontend/blob/master/lib/views/mobile/selector.dart#L24-L34
Could you please explain why wouldn't a T extends E work here?

[mraleph]

Because T extends E allows for a more specific type and not for a less specific type.

You, on the other hand, would like to allow T to be E? if E is non-nullable - which is less specific. (E extends E? is satisfied, but E? extends E is not).

Unfortunately Dart does not support lower type bounds on type parameters, only upper type bounds, while languages like Java and Scala support both lower and upper type bounds.

[giladwo]

So how come the above validate works as if E? extends E?

[mraleph]

I am not sure I understand the question. Could you clarify which part of the code works as if E? extends E?

[giladwo]

I've understood the mistake while trying to explain it here.
Thanks for the helpful clarification 😄

[eernstg]

Note the overlap with dart-lang/language#1674.