Should [] on Map not be of type K -> V?

[Guyren Howe]

Since Map is K -> V [] surely should also be K -> V . It is currently Object? -> V?

Why is it ? and why is not from K?

This makes making a map-like thing more challenging, as `operator [](K key} => _store[key]` fail with:

''MemoryModel.[]' ('V? Function(K)') isn't a valid override of 'MapMixin.[]' ('V? Function(Object?)'). (Documentation)'

[Nate Bosch]

The map will return a `null` from operator [] in the case of missing keys so the signature must have a nullable return type.

Throwing an exception on missing keys is a design we have discussed - there are tradeoffs between the designs and some authors prefer one over the other. Switching to exceptions would be a significant breaking change, and we don't have any plans to make that change.

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[James D. Lin]

I think the OP is asking more about why `Map.operator []` accepts `Object?` as an argument instead of `K`.  This has been brought up many times before:

https://github.com/dart-lang/sdk/issues/41427

https://github.com/dart-lang/sdk/issues/41544

https://github.com/dart-lang/sdk/issues/43517

https://github.com/dart-lang/sdk/issues/48030

(I stopped searching, but there almost certainly are many more older issues discussing it too.)

Also see https://github.com/dart-lang/linter/issues/1307 which is where something about it is most likely to be done.

- James

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