WIP: type-level assertion operator (!)

[kiara (KiaraGrouwstra)]

Fixes #14366.

I'm just extending the existing JSDocNullableType so it's usable outside of JSDoc and gets the functionality of the expression-level non-nullable / assertion operator (!).
This still evaluates too eagerly to deal with generics though.

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[kiara (KiaraGrouwstra)]

Anders Hejlsberg (@ahejlsberg): I feel a bit stuck on evaluation eagerness here -- my type Assert<T> = T!; just eagerly scrubs off the T before the type gets applied to some T value that might have | null.
Could you give a quick hint as to how to get it to evaluate when T is available? That'd help me a lot for #17884 as well.

[Marin Marinov (gcnew)]

@tycho01 While writing type class dictionaries by hand, I got stuck on #14366 as well. So I gave it a shot and got some mileage. Basic functionality is working, but higher order relations, comparability and type inference are missing. I also broke API compatibility, but it's an easy fix.

Basically, to make an operator lazy you have to create a new node for it and encode all relations.

https://github.com/gcnew/TypeScript/tree/nonNullType

[kiara (KiaraGrouwstra)]

Marin Marinov (@gcnew): whoa, pretty impressive progress there! I'd initially tried gone the way of adding a new node as well, but after realizing it overlapped with the JSDoc one figured that'd do to give a minimum repro of my issue with generics.

I was sorta struggling with elegantly tackling the parser part too -- looks like you handled that fairly well.
I guess my main reason to open PRs before finishing was to be able to communicate on a way forward anyway -- that part worked at least. Once you do have a PR up I guess I'll close this one.

I should try and incorporate the insights from your branch for the spread in tuple types then. Currently also still struggling with type-level calls though...

[Marin Marinov (gcnew)]

Sorry, I used bad terminology there. My addition of NonNullTypeNode was not strictly required, but I thought it'd be cleaner.

To make something lazy, you have to build an expression, instead of eagerly evaluating the said thing. For example, for keyofs that's the IndexType type; for type level indexing - IndexedAccessType. So I added a new NonNullType type. Values of these types are created by functions like getNonNullType which check whether their argument is still generic (isGenericNonNullType). If it is, they return a "to be evaluated expression", e.g. a NonNullType or else they carry the operation out and return the evaluated result.

[kiara (KiaraGrouwstra)]

Marin Marinov (@gcnew): yeah, I found the code in your branch pretty useful there. Much of it already looked familiar from just searching / plagiarizing existing nodes/types, but I found especially your tests to be a terrific checklist of the different scenarios. :)

[Mohamed Hegazy (mhegazy)]

We have discussed this in a few design meetings now, and do not feel that adding a new type operator is the right way to go.

Type operators come with a maintainability cost, as well as learning cost by adding a new concept to the language.

An alternative here is to always remove null|undefined in type positions. this allows the use case in #14366 to follow as expected.

[kiara (KiaraGrouwstra)]

Marin Marinov (@gcnew): just pushed a few examples of type subtraction including this at 4d14c9f.

[Marin Marinov (gcnew)]

@tycho01 Nice! I'm very glad you are making steady progress!

[kiara (KiaraGrouwstra)]

Marin Marinov (@gcnew): Thanks! You definitely helped me there! 😅