interface Film {
    name : string;
    durationInSeconds : number;
const myFunction = function(
    film: Film,
    filmPropertyName: keyof Film,
    newValue: string | number
    let value = film[filmPropertyName];
    console.log('Existing value:', value); //value is of type `string | number`, inferred in prior line
    //That means that on read, film[filmPropertyName] is of type `string | number` as expected.
    //However, on write, it's of type `never.`
    film[filmPropertyName] = newValue; //not assignable to type 'never'.ts(2322)
    value = newValue; //works just fine with both sides as `string | number`
    //Checking that types match seems like a sensible requirement but doesn't help:
    if(typeof film[filmPropertyName] === typeof newValue) {
        value = newValue; //works fine
        film[filmPropertyName] = newValue; //same error as above
    //even checking that both are a specific standard type doesn't help:
    if(typeof film[filmPropertyName] === 'string' && typeof newValue === 'string') {
        value = newValue; //works fine
        film[filmPropertyName] = newValue; //same error as above