Improved union/intersection type inference

TypeScript 1.8 improves type inference involving source and target sides that are both union or intersection types.For example, when inferring from string | string[] to string | T, we reduce the types to string[] and T, thus inferring string[] for T.

Example
  1. type Maybe<T> = T | void;
  3. function isDefined<T>(x: Maybe<T>): x is T {
  4.     return x !== undefined && x !== null;
  5. }
  7. function isUndefined<T>(x: Maybe<T>): x is void {
  8.     return x === undefined || x === null;
  9. }
  11. function getOrElse<T>(x: Maybe<T>, defaultValue: T): T {
  12.     return isDefined(x) ? x : defaultValue;
  13. }
  15. function test1(x: Maybe<string>) {
  16.     let x1 = getOrElse(x, "Undefined");         // string
  17.     let x2 = isDefined(x) ? x : "Undefined";    // string
  18.     let x3 = isUndefined(x) ? "Undefined" : x;  // string
  19. }
  21. function test2(x: Maybe<number>) {
  22.     let x1 = getOrElse(x, -1);         // number
  23.     let x2 = isDefined(x) ? x : -1;    // number
  24.     let x3 = isUndefined(x) ? -1 : x;  // number
  25. }