Readonly object can be used as parameter for function expecting mutable object

[xaviergonz]

TypeScript Version: 2.8.1

Search Terms: mutate readonly immutable mutable

Code

interface Mutable {
  x: number;
}

interface Immutable {
  readonly x: number;
}

function mutate(mut: Mutable) {
  mut.x = 5;
}

function immutate(imm: Immutable) {
  // imm.x = 5; // this does not compile as expected
  mutate(imm); // but this does
}

Expected behavior: There should be a compiler error inside immutate saying that 'imm' cannot be used as a parameter to mutate since 'imm' is readonly while 'mut' is not

Actual behavior: Compiles successfully

Playground Link: https://codesandbox.io/s/vnp9k2z32y

Related Issues:

[xaviergonz]

Another example

interface Mutable {
  x: number;
}

interface Immutable {
  readonly x: number;
}

const imm: Immutable = {x: 3};
// imm.x = 5; // not allowed, good :D
const mut: Mutable = imm; // allowed :/
mut.x = 5; // allowed :(

// vs

const roA: ReadonlyArray<number> = [5];
// const writeA: number[] = roA; // not allowed, good :D

[mhegazy]

Duplicate of #11180

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.