New of T

[loic-sharma]

Introduction

It would be nice if Dart could instantiate a generic type. For example:

T myFactory<T>() {
  return T();
}

This would likely need some sort of generic constraint to ensure T can be instantiated.

Use case

Today, a Flutter StatefulWidget must implement the createState method:

class Foo extends StatefulWidget {
  const Foo();

  @override
  State<Foo> createState() => _FooState();
}

This language feature would be a step towards removing the createState method: StatefulWidget would accept a generic parameter and use that to provide a default createState implementation:

- class Foo extends StatefulWidget {
+ class Foo extends StatefulWidget<_FooState> {
    const Foo();
-
-   @override
-   State<Foo> createState() => _FooState();
  }

Previous discussions

The following issues appear relevant:

1. could support T : new() like C#? #1787
2. More capable Type objects #4200

Feel free to close this if there is a better tracking issue already :)

[lrhn]

I think the current place-holder issue for this is #356.

The issue is the same in every case: There needs to be some way to ensure that the type bound to T has an unnamed constructor with that signature. That means some restriction on which types can be passed as arguments, which has to be expressed in some way (what #1787 was one syntax for).
Having specific static members is also a property of a type that cannot be tested at runtime, so there is no way to go from a type parameter <T> to a type argument to <T : new()>. That's what #4200 tries to solve by making the type objects implement interfaces that can be checked at runtime.

Closing this deferring to #356.