Readonly-ness is lost in function call

[thecotne]

TypeScript Version: 3.6.3

Search Terms:

Code

type EditablePerson = {
    name: string
}
type ReadonlyPerson = {
    readonly name: string
}

const editablecotne: EditablePerson = {
    name: 'cotne'
}
const readonlycotne: ReadonlyPerson = {
    name: 'cotne'
}

function updateName (person: EditablePerson, newName: string) {
    person.name = newName
}

//ok
editablecotne.name = 'not so cotne'
updateName(editablecotne, 'not so cotne')

//err
readonlycotne.name = 'not so cotne'// ok
updateName(readonlycotne, 'not so cotne')//fuck!

Expected behavior:
you can't modify readonly property of object

Actual behavior:
you can pass object to function and edit it there

Playground Link: try

Related Issues:

[jack-williams]

Duplicate of #13347, I think.

[thecotne]

@jack-williams looks similar but i am not sure if it's same problem (description looks cryptic to me)

[jack-williams]

The issue description exactly matches your scenario.

Interface with readonly property (ReadonlyPerson) is assignable to interface with mutable property (EditablePerson)

Assignable in TypeScript applies both to assignment expressions (like in #13347) and passing arguments to functions (like in this issue).

[thecotne]

@jack-williams you are right...