Generic type mismatch with Readonly<T>

[dpogue]

This feels like #16985 in a slightly different form.
/cc Anders Hejlsberg (@ahejlsberg)

TypeScript Version: 2.5.1, but probably broken since 2.4.1

Code

interface MyInterface {
  something: number;
}

class MyClass<T extends MyInterface> {
  doIt(data : Readonly<T>) {}
}

interface II2 extends MyInterface {
  foo: string;
}

class MySubClass extends MyClass<II2> {}

function fn(arg: typeof MyClass) {};

fn(MySubClass);

Expected behavior:
No compilation error.

Actual behavior:

error TS2345: Argument of type 'typeof MySubClass' is not assignable to parameter of type 'typeof MyClass'.
  Type 'MySubClass' is not assignable to type 'MyClass<T>'.
    Types of property 'doIt' are incompatible.
      Type '(data: Readonly<II2>) => void' is not assignable to type '(data: Readonly<T>) => void'.
        Types of parameters 'data' and 'data' are incompatible.
          Type 'Readonly<T>' is not assignable to type 'Readonly<II2>'.
            Property 'foo' is missing in type 'Readonly<T>'.

[ahejlsberg]

This is working as intended and is an effect of #16368. The type checker is pointing out that it is not safe to treat the doIt(data: Readonly<II2>) method in MySubClass as the more general doIt(data: Readonly<T>) in MyClass<T>. Effectively, you can't treat a method that expects the specific type II2 as a method that accepts any type that derives from MyInterface.

[dpogue]

This feels like it limits generics to the point where they're hardly useful anymore...

What we're trying to model is a "Model" class that wraps some data handled by a JSON API:

interface IModelData {
  href : string; // All models have an href
}

class ModelBase<DataType extends IModelData> {
  persist(data : Readonly<DataType>) {
    return callToServer(data['href'], data);
  }
}


interface IUserData extends IModelData {
  username : string;
}

class User extends ModelBase<IUserData> {}

What is the correct way to model this in TypeScript that allows proper typechecking?

[ahejlsberg]

One way to model it is to have a non-generic base class:

interface IModelData {
  href : string; // All models have an href
}

abstract class ModelBase {
  abstract persist(data: IModelData): void;
}

class GenericBase<DataType extends IModelData> extends ModelBase {
  persist(data : Readonly<DataType>) {
    return callToServer(data['href'], data);
  }
}

interface IUserData extends IModelData {
  username : string;
}

class User extends GenericBase<IUserData> {}

function fn(arg: typeof ModelBase) {};

fn(User);

Another way is to not use typeof ModelBase but instead write out the actual factory type:

interface IModelData {
  href : string; // All models have an href
}

class ModelBase<DataType extends IModelData> {
  persist(data : Readonly<DataType>) {
    return callToServer(data['href'], data);
  }
}

interface IUserData extends IModelData {
  username : string;
}

class User extends ModelBase<IUserData> {}

function fn(arg: new() => ModelBase<IModelData>) {}

fn(User);

With either approach you're ensuring that the arg parameter of fn is a contract that is satisfied by every subclass constructor function.

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.