@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.

@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?

@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)

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

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.