Linter rule to catch type information loss in generic state class definitions

[Tomburgs]

Use case

It would be beneficial to have a linter feature that catches errors related to the loss of type information in generic class definitions, especially in the context of Flutter's Stateful Widgets. This issue often arises when the state class does not properly specify the generic type on the State class, leading to potential type safety issues at run time.

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For example, the following code would cause a runtime error when using the onTap method. Something along the lines of type '(MyConcreteTypeImplementation) => void' is not a subtype of type '(dynamic) => void'.

class MyGenericWidget<T> extends StatefulWidget {
  final T item;
  final void Function(T) onTap;

  // Imagine a constructor here

  @override
  _MyGenericWidgetState createState() => _MyGenericWidgetState<T>();
}

class _MyGenericWidgetState<T> extends State<MyGenericWidget> {
  // Imagine there is a widget here that calls the onTap with a generic type T.
}

This is because when declaring the _MyGenericWidgetState the generic T is not passed to MyGenericWidget.
In the given example, it would have to be declared as:

class _MyGenericWidgetState<T> extends State<MyGenericWidget<T>>

In my opinion this is extremely easy to overlook and can cause a lot of confusion, especially to newer flutter developers who have not ran into this exact issue before.

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Related issues:

• _TypeError (type '(MyObject) => Null' is not a subtype of type '(dynamic) => void') #40825
• '(TypleClass) => String' is not a subtype of type '(dynamic) => String' #110718

Proposal

I think a linter rule would be a good solution. Even something as case-specific as the MyStatefulWidget class accepts a generic, therefore when extending State and passing the MyStatefulWidget (State<MyStatefulWidget>), the generic values for MyStatefulWidget must also be passed (State<MyStatefulWidget<T>>).

[bartekpacia]

I think this issue belongs to https://github.com/dart-lang/linter

[darshankawar]

@Tomburgs
This would belong to the repo mentioned above. flutter_lints is a plugin to enable existing lints but since you are looking to actually add a new linter rule, it need to be implemented in dart first.
Closing from here.

[github-actions]

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