Object type not assignable to equivalent value of mapped type

[ethanroday]

Code

// Pack<{ a: number }> is { a: { value: number } }
type Pack<T> = { [K in keyof T]: { value: T[K] } };

function patch<T, K extends keyof T>(obj: Partial<Pack<T>>, key: K, value: T[K]) {
    obj[key] = { value }; // error: Type '{ value: T[K]; }' is not assignable to type 'Pack<T>[K]'.
}

Example usage of patch():

type SomeType = {
  a: number, b: string
};

const obj: Partial<Pack<SomeType>> = {
    a: { value: 1 }
}

patch(obj, 'b', 'foo'); // desired outcome is for obj.b to equal 'foo'

Expected behavior:

Definition of patch() function should be fine, and calling should produce the desired outcome.

Actual behavior:

patch() function fails to compile with error Type '{ value: T[K]; }' is not assignable to type 'Pack<T>[K]' even though { value: T[K] } does appear to be assignable to Pack<T>[K]:

function getPack<T, K extends keyof T>(obj: T, key: K): Pack<T>[K]  {
    const value = obj[key]; // T[K]
    return { value }; // { value: T[K]; }
}

Playground Link:
Additional examples here: https://bit.ly/2le5qZq

[DanielRosenwasser]

Technically, but perhaps infuriatingly, this is correct because you really don't know if an instance of Pack<T>[K] resolves to a more derived type than { value: T[K] }. All you're guaranteed is that it's an upper bound constraint on whatever obj[key] is, so you can read values out assuming they have type { value: T[K] }, but you can't write into it.

[ethanroday]

@DanielRosenwasser I'm not sure I totally understand. So then why does Pack<T> (rather than Partial<Pack<T>>) work as expected? What's an example of a situation where the expected behavior I outlined above would be incorrect?

[DanielRosenwasser]

Okay, a couple of things:

First, this works the way you want under strictNullChecks which is odd. I don't know why that is off the bat.

Second, really neither should work. Seems like actually that's the bug.

Third,

What's an example of a situation where the expected behavior I outlined above would be incorrect?

interface Box<T> { value: T }
type Pack<T> = { [K in keyof T]: Box<T[K]> }

function patch<T, K extends keyof T>(obj: Partial<Pack<T>>, key: K, value: T[K]) {
    obj[key] = { value };
}


let boxCollection = {
    foo: {
        value: 100,
        shapeType: "cube",
    },
    bar: {
        value: 200,
        shapeType: "sphere",
    }
};

patch(boxCollection, "foo", 100);

// Crash!
boxCollection.foo.shapeType.toLowerCase();

[ethanroday]

Thanks for the example - very helpful. If I understand correctly, the issue is that you can't just blindly assign something to obj[key] because the type of obj[key] may be more derived than Box<T>. Not to muddy the waters, then, but shouldn't the following work just fine? I would expect the type of obj[key].value to be T[K].

function patch<T, K extends keyof T>(obj: Partial<Pack<T>>, key: K, value: T[K]) {
    obj[key].value = value; // Error: Property 'value' does not exist on type 'Partial<Pack<T>>[K]'.
}

[ethanroday]

@DanielRosenwasser any ideas here?