Preserve modifiers in homomorphic mapped types

[ahejlsberg]

WIth this PR we preserve property modifiers in homomorphic (structure preserving) mapped types. A mapped type of the form { [P in keyof T]: X } is homomorphic with T (because it has the same set of properties as T) and now preserves the optional and readonly modifiers as they exist on the properties in T.

Since the predefined mapped types Partial<T> and Readonly<T> are homomorphic they now preserve already existing property modifiers.

EDIT: With #12826 the predefined Pick<T, K> type now also preserves property modifiers. Before that PR, Pick<T, K> could be used to strip modifiers, but that is no longer possible.

type T1 = { a?: number, b: string };
type T2 = Partial<T1>;  // { a?: number, b?: string }
type T3 = Readonly<T1>;  // { readonly a?: number, readonly b: string }
type T4 = Partial<Readonly<T1>>;  // { readonly a?: number, readonly b?: string }
type T5 = Readonly<Partial<T1>>;  // { readonly a?: number, readonly b?: string }
type T6 = Pick<T1, "a">;  // { a?: number }

Fixes #12542.

[mhegazy]

@DanielRosenwasser and @sandersn, we will need to clarify in the handbook that there is a subset of mapped types (isomorphic mapped types) that we treat them specially in inference, and in trafficking the optional and readonly modifiers through the transformation.

[weswigham]

@ahejlsberg How can I create a Writable<T>, now? I had been using the stripping of modifiers to allow for temporarily writable versions of immutable types during their piecemeal construction.

Edit: Nevermind, read what you said about Pick. Seems like a bit of a hack at a glance; would it be useful to add a Writable<T> (and Required<T>) alias(es) to the lib?

[mhegazy]

@DanielRosenwasser brings a good point about terminology. these are actually "homomorphic" mapped types and not "isomorphic" since the transformation is not bidirectional (i.e T => ReadOnly<T>, but not the opposite)

[ahejlsberg]

@DanielRosenwasser Changing to "homomorphic" makes everything so much clearer! 😛

[guncha]

In --strictNullChecks mode, the type T6 above is actually { a: number | undefined, b: number | undefined } because the undefined that was added as a result of making the properties optional persists even when the modifiers are stripped.

Is there any way currently to get rid of that undefined? I'm trying to write a get function, similar to Lodash's get (https://lodash.com/docs/4.17.2#get) and it's almost possible except for the fact that (T | undefined)[keyof T] is never and I can't make the get function work more than one level deep because of that.