Interface with readonly property is assignable to interface with mutable property

[danielearwicker]

TypeScript Version: 2.1.4

Code

interface MutableValue<T> {
    value: T;
}

interface ImmutableValue<T> {
    readonly value: T;
}

let i: ImmutableValue<string> = { value: "hi" };
i.value = "Excellent, I can't change it"; // compile-time error

let m: MutableValue<string> = i;
m.value = "Oh dear, I can change it";

Expected behavior:
The assignment of i to m would fail, to stop us accidentally allowing value to be modified.

Actual behavior:
The assignment is allowed.

The current behaviour was a deliberate choice so this is a breaking change (or strict flag) feature request rather than a bug report!

The Handbook has this snippet:

let a: number[] = [1, 2, 3, 4];
let ro: ReadonlyArray<number> = a;
ro[0] = 12; // error!
ro.push(5); // error!
ro.length = 100; // error!
a = ro; // error!

It notes that a = ro being an error is helpful. But this happens because ReadonlyArray has no push method, making it incompatible with Array.

My example above seems "morally equivalent" to modelling the input/output flow of values with separate methods:

interface MutableValue<T> {
    getValue(): T;
    setValue(v: T): void;
}

interface ImmutableValue<T> {
    getValue(): T;
}

declare let i: ImmutableValue<string>;
i.setValue("Excellent, I can't change it"); // compile-time error

let m: MutableValue<string> = i;
m.setValue("Oh dear, I can change it");

And sure enough, this stops the assignment of i to m.

Would be great if mutable and readonly properties had the same relationship as if they were modelled by separate get/set methods (which of course they might actually be, via property getter/setters).

[danielearwicker]

Judging from #6614 there was an idea to add a mutable modifier. If I'm correct in assuming that this would mean:

interface MutableValue<T> {
    mutable value: T;
}

interface ImmutableValue<T> {
    readonly value: T;
}

and that would stop i being assigned to m because I've explicitly said that value must be mutable, then this whole issue can be boiled down to "Please add the mutable modifier"! 👍

[aluanhaddad]

@danielearwicker I think you are channeling @Aleksey-Bykov and his issue readonly modifiers are a joke

[danielearwicker]

Sometimes I think I'm way too polite!

[DavidKDeutsch]

I don't see an obvious "vote for this" button, so count this as my +1. Just ran into a nasty bug of mine where I was incorrectly modifying a member of a class instance, so I changed the type using ReadOnly, hoping I would get a compile error to quickly point my lazy self to there I was assigning it to a "normal" T, but no luck. As far as I am concerned, there is no difference between allowing an assignment of a type ReadOnly to a T and allowing an assignment of an Object to, say, an Array.

[Pauan]

@DavidKDeutsch In the upper-right corner of every comment, there is a +:grinning: button. If you click that, you can choose the :+1: reaction. This counts as a plus vote. You can do this on any comment, but often it is done on the first comment.