Readonly<T> should not be assignable to T without a type assertion

[octogonz]

TypeScript Version: 3.3.0-dev.20181207

Search Terms: Readonly interface assignable

Code

interface IThing {
    x: number;
}

let readonlyThing: Readonly<IThing> = { x: 123 };

// This is an error, as expected
readonlyThing.x = 321; 

// Why is this not an error?
let thing: IThing = readonlyThing;
thing.x = 321;

Expected behavior: Attempting to assign Readonly<IThing> to IThing should result in an error.

Actual behavior: The code compiles without any error, allowing "x" to be assigned even though it was intended to be readonly.

Playground Link: http://www.typescriptlang.org/play/#src=%0D%0Ainterface%20IThing%20%7B%0D%0A%20%20%20%20x%3A%20number%3B%0D%0A%7D%0D%0A%0D%0Alet%20readonlyThing%3A%20Readonly%3CIThing%3E%20%3D%20%7B%20x%3A%20123%20%7D%3B%0D%0A%0D%0A%2F%2F%20This%20is%20an%20error%2C%20as%20expected%0D%0AreadonlyThing.x%20%3D%20321%3B%20%0D%0A%0D%0A%2F%2F%20Why%20is%20this%20not%20an%20error%3F%0D%0Alet%20thing%3A%20IThing%20%3D%20readonlyThing%3B%0D%0Athing.x%20%3D%20321%3B%0D%0A

Related Issues:
#26864
#13002
#12542

[octogonz]

This apparently isn't specific to Readonly<T>. It works with a regular interface:

interface IThing {
    x: number;
}

interface IReadonlyThing {
    readonly x: number;
}

const readonlyThing: IReadonlyThing = { x: 123 };

// This is an error, as expected
readonlyThing.x = 321; 

// Why is this allowed without a type assertion?
let thing: IThing = readonlyThing;
thing.x = 321;

[jack-williams]

I think this is related: #13347

[weswigham]

Aye duplicate of #13347 - we're considering a kind of --strictReadonly in the future to opt in to strict behavior here.

[octogonz]

Thanks! I'll close this issue then.

[typescript-bot]

This issue has been marked as a duplicate and has seen no activity in the last day. It has been closed automatic house-keeping purposes.