Interface with readonly property is assignable to interface with mutable property

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).

Start with the TypeScript 2.1.4 reproduction in this issue, comparing assignability for readonly properties with the separate getter/setter interfaces. Review the linked discussion about the deliberate current behavior and determine the compatibility or strict-mode implications; done means the requested assignment is rejected without breaking the documented readonly-array behavior.