TypeScript Version: nightly

Code

interface Test {
  optional?: string;
}

function getProperty<K extends keyof Test>(t: Test, key: K): Test[K]|never {
  if (t[key]) {
    return t[key];
  }
  throw new Error('Value does not exist at key');
}

const a: string = getProperty({}, 'optional');

Target behavior:
The given function is coded such that it will always return a non-empty value, so it would be great if there were a way to make something like this compile. Is there a way to communicate the opposite of Partial<T>, e.g. that we know Test[K] will be non-empty?

Actual behavior:
error TS2322: Type 'string | undefined' is not assignable to type 'string'. Type 'undefined' is not assignable to type 'string'.