strictPropertyInitialization with subclasses that refine the types of properties

[rictic]

TypeScript Version: 2.7.0-dev.20171226

Code

class A {
  x: {y: string} = {y: 'y'};
}

class B extends A {
  x: {y: string, z?: number};
}

Expected behavior:
I expected this not to warn because A#x is assignable to B#x.

Actual behavior:

index.ts(6,3): error TS2564: Property 'x' has no initializer and is not definitely assigned in the constructor.

[DanielRosenwasser]

Seems undesirable. CC @ahejlsberg.

[Jessidhia]

Note: this is not the case in TypeScript's implementation, but in the current stage-3 class properties, x in B is a completely different own prop that has the same name.

This does mean that, in a stage-3 compliant implementation, (new B).x will be a new own prop with the value undefined since it lacks an initializer. It'd have to be initialized with = this.x in order to copy the value that comes from the superclass constructor before it is overridden with the new property definition.

[rictic]

@Kovensky Huh! Reading through: https://tc39.github.io/proposal-class-fields/#initialize-public-instance-fields it seems like [[Fields]] will have two entries, first A's x then B's x, and CreateDataPropertyOrThrow will assign undefined when evaluating B's x.

What the code is trying to do is declare the type of the existing field rather than declaring the field itself. This works today and expresses that intent pretty well. Typescript even gives an error if the declared type of B#x is not assignable to A#x:

class A {
  x: {y: string} = {y: 'y'};
}

class B extends A {
}

interface B {
  x: {y: string, z?: number};
}

I'd like someone else to double check my reasoning here, but if all of the above is correct then I think this issue can be closed as working correctly.

[thw0rted]

Ryan posted a very helpful suggestion over in #35831. If you're using at least TS 3.7, there's a declare modifier for class properties that lets you narrow the type of a property in a subclass without emitting any runtime initializer code, and it also makes strictPropertyInitialization happy.