Add a ghost property to `InjectionToken` for generic type parameter type safety

Which @angular/* package(s) are relevant/related to the feature request?

core

Description

Currently InjectionToken doesn’t use the type parameter and thus typescript does not type check it (it is marked as unused).

// Simplified version
class InjectionToken<T> {
  constructor(protected name: string) {}
}

interface Person {
  name: string;
}

interface Car {
  numOfWheels: number;
}

const a = new InjectionToken<Person>('test');
const b = new InjectionToken<Car>('test');

// No error :(
const c: InjectionToken<Person> = b;

Proposed solution

As the generic type is not used anyway - an unused property can be added to InjectionToken just for this type check ability with the type T

// Simplified version
class InjectionToken<T> {
  typeCheck!: T;
  // #typeCheck!: T; // Option 2
  // protected typeCheck!: T; // Option 3

  constructor(protected name: string) {}
}

interface Person {
  name: string;
}

interface Car {
  numOfWheels: number;
}

const a = new InjectionToken<Person>('test');
const b = new InjectionToken<Car>('test');

// Error :)
const c: InjectionToken<Person> = b;

Alternatives considered

1. Wrap InjectionToken with a custom class that does it (possible? recommended?)
2. Ignore the missing type check 😦

[crisbeto]

It’s not just about there being a major release, we also have to fix all the apps that would’ve been broken by the change. There were higher priority changes that we focused on for v15 instead.

[Harpush]

@crisbeto ok… Wasn’t 15 a major release? 🤔