Checking Unions with Index Signatures

In previous versions of TypeScript, the checker would not verify that undeclared fields in a union conform to any indexed types in the union. You can learn about indexed types here: example:indexed-types For example, the IdentifierCache below indicates that any key on the object will be a number: 

type IdentifierCache = { [key: string]: number };
// Meaning this will fail, because 'file_a' has a string value 
const cacheWithString: IdentifierCache = { file_a: "12343" };
// However, when you put that into a union, then the validation check would not happen: 
let userCache: IdentifierCache | { index: number };
userCache = { file_one: 5, file_two: "abc" };
// This is fixed, and there would be an error about 'file_two' from the compiler. This also takes into account when the key is a different type, for example: ([key: string] and [key: number]) 
type IdentifierResponseCache = { [key: number]: number };

let resultCache: IdentifierCache | IdentifierResponseCache;
resultCache = { file_one: "abc" };