remarks

import { Debug } from 'ts-typedefs';
declare const val: string | number;

if (typeof val === 'string') {
     return null;
} else if (typeof val === 'number') {
     throw new Debug.UnreachableCodeError(val); // compiler error: val is not of type `never`
     return;
} else {
     throw new Debug.UnreachableCodeError(val); // this is ok val has `never` type here
}