Isomorphic mapped types and unions

[ahejlsberg]

A mapped type of the form { [P in keyof T]: X }, where T is some type parameter, is said to be an isomorphic mapped type because it produces a type with the same shape as T. With this PR we make instantiations of isomorphic mapped types distributive over union types. For example, when { [P in keyof T]: X } is instantiated with A | B for T, we produce { [P in keyof A]: X } | { [P in keyof B]: X }. Furthermore, when a primitive type is substituted for T in an isomorphic mapped type, we simply produce that primitive type. For example, when { [P in keyof T]: X } is instantiated with A | undefined for T, we produce { [P in keyof A]: X } | undefined.

type A = { a: string };
type B = { b: string };
type C = { c: string };

type T1 = Partial<A | B | C>;  // Partial<A> | Partial<B> | Partial<C>
type T2 = Readonly<A | B | null | undefined>;  // Readonly<A> | Readonly<B> | null | undefined
type T3 = Readonly<A | B[] | string>;  // Readonly<A> | Readonly<B[]> | string
type T4 = Readonly<string>;  // string
type T5 = Readonly<string | number | boolean>;  // string | number | boolean

This PR also includes a minor fix that makes any isomorphic mapped type assignable to an empty object type.

[ahejlsberg]

@mhegazy Ping!

[mhegazy]

//cc: @DanielRosenwasser and @RyanCavanaugh