TS doesn't see when we add symbol properties to functions.

[anurbol]

TypeScript Version: 3.4.0-dev.20190207

Search Terms:
function, property, symbol

Code

const mySymbol = Symbol()

interface Foo {
    [mySymbol]: true
    (): any
}

const foo: Foo = () => {} // error: 
// Property '[mySymbol]' is missing in type '() => void' 
// but required in type 'Foo'.
foo[mySymbol] = true

Expected behavior:
Just like with regular (non-symbol) props, show no error if the symbol property actually has been added .

Actual behavior:
There is an error.

Playground Link: link

[anurbol]

What is the cleanest workaround BTW, anyone knows? I mean except adding as any after the function.

[dragomirtitian]

Object.assign can work

const foo: Foo = Object.assign(() => { }, {
  [mySymbol]: true as true // must cast, literal types will be widened here to boolean causing an error on assignment, but it will work with other things.
});

[anurbol]

@dragomirtitian Woah! Indeed! Cool! Thanks! 👍 👍 👍

[Andarist]

This one was already fixed by #54726 , the given playground still doesn't work though and I prepared a fix for it here: #55357 . I think that this issue can already be closed since what I noticed is orthogonal, cc @jakebailey