Extension type should be able to have assert for the parameter of the constructor

[Cat-sushi]

AFAIK, currently extension types doesn't have a way to define assertion for their parameter of their constructors.
As a smart alternative of typedef, assertion seems important.

[Cat-sushi]

Ex.

extension type NaturalNumber(int value): assert(value > 0){}

[lrhn]

Not planned.

You can write a public constructor with an assert:

extension type NaturalNumber._(int value) {
  NaturalNumber(this.value) : assert(value > 0);
}

The declaration header is then only used to declare the representation type and representation object name. You can choose to ignore the private constructor it introduces, and nobody else can see it.

If you really don't want the private constructor to exist (which doesn't change anything, other people can still write -1 as NaturalNumber), that's issue #3343.