Type parameters as constraints

With TypeScript 1.8 it becomes possible for a type parameter constraint to reference type parameters from the same type parameter list. Previously this was an error. This capability is usually referred to as F-Bounded Polymorphism.

Example
  1. function assign<T extends U, U>(target: T, source: U): T {
  2.     for (let id in source) {
  3.         target[id] = source[id];
  4.     }
  5.     return target;
  6. }
  8. let x = { a: 1, b: 2, c: 3, d: 4 };
  9. assign(x, { b: 10, d: 20 });
  10. assign(x, { e: 0 });  // Error