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).

この issue の TypeScript 2.1.4 の再現から始め、readonly プロパティと分離された getter/setter インターフェースの代入可能性を比較してください。意図的に選択された現在の動作についてのリンク先の議論を確認し、互換性または strict-mode への影響を判断してください。完了条件は、文書化されている readonly-array の動作を壊さずに、要求された代入が拒否されることです。