Using symbol as object-key type in TypeScript

I'm trying to define an object with a symbol as key-type since MDN says:

A symbol value may be used as an identifier for object properties [...]

But using it as type for the key-property:

type obj = {
    [key: symbol | string]: string
}

results in the following error:

TS1023: An index signature parameter type must be either 'string' or 'number'.

even it can be used as index-type. I'm using the latest typescript version (v3.7.2), related questions I've found:

Typescript: destructuring an object with symbols as keys (He's using an actual instance of a Symbol, I want the type symbol)
TypeScript: An index signature parameter must be a 'string' or 'number' when trying to use string | number
ES6: destructuring an object with symbols as keys (That can't be a solution - it seems kinda wrong to use an actual instance as type since every Symbol instance is unique...)

I've also took a look at the typescript symbol docs but they only show how it's used as value, not as type.

Example:

const obj = {} as {
    [key: number | symbol]: string // Won't work
};

const sym = Symbol('My symbol');
obj[sym] = 'Hi';

Issue on Microsoft/TypeScript

Open feature request

Unfortunately this is not possible at the moment in TypeScript. If you have to interoperate with some APIs that expect this or really want to use symbols as keys, you can do this awkward version:

// Ensure we can not pass regular map to our custom functions
type SymbolMapTag = { readonly symbol: unique symbol }

type SymbolMap = SymbolMapTag & {
    [Key in string | number | symbol]: string;
}

function set_symbol<T extends SymbolMap, TSym extends symbol>
(target: T, sym: TSym, value: T[TSym]) {
    target[sym] = value;
}

function get_symbol<T extends SymbolMap, TSym extends symbol>
(target: T, sym: TSym): T[TSym] {
    return target[sym];
}

const symbol_map = {} as SymbolMap;

const sym = Symbol('My symbol');
set_symbol(symbol_map, sym, "hi");
get_symbol(symbol_map, sym); // string


type NonSymbolMap = {
    [Key in string | number]: string;
}

const non_symbol_map = {} as NonSymbolMap;
set_symbol(non_symbol_map, sym, "hi"); // error
get_symbol(non_symbol_map, sym); // error