Fix emit for optional chain with non-null assertion

[rbuckton]

Fixes emit for optional chaining when a non-null assertion is present in the chain:

// ts
obj?.a!.b

// js (old)
(obj === null || obj === void 0 ? void 0 : obj.a).b

// js (new)
obj === null || obj === void 0 ? void 0 : obj.a.b

Fixes #36031

[DanielRosenwasser]

Why did we decide not to change the types here? Seems like we really shouldn't have one without the other.

[rbuckton]

There are some odd quirks to consider if we change the precedence of !:

// note: `undefined*` indicates an `undefined` that was added to the type by `?.`
declare const a: undefined | { b: number | null };

// current behavior 
a!.b // number | null
a!.b! // number
a?.b // number | null | undefined*
a?.b! // number

// proposed behavior (differences) 
a?.b! // number | undefined*

If we give ! a lower precedence than ?. and OptionalChain, then it must always have a lower precedence than OptionalChain. With the proposed changes, to get the current behavior for a?.b! you would instead need to write (a?.b)!.

[rbuckton]

With the latest changes, a postfix-! inside of an optional chain will remove optionality only in the non-optional control flow branch, while a postfix-! at the end of an optional chain will remove optionality for the whole expression:

declare const a: undefined | { b: { c: number } | null };

a?.b.c; // error
a?.b!.c; // number | undefined
a?.b!; // { c: number }

This best matches user expectations.

[DanielRosenwasser]

@typescript-bot pack this

[typescript-bot]

Heya @DanielRosenwasser, I've started to run the tarball bundle task on this PR at 4214548. You can monitor the build here.

[DanielRosenwasser]

Do we have tests for

let x = a?.b!();

[elibarzilay]

(To be taken with plenty of salt.)

[typescript-bot]

Hey @DanielRosenwasser, I've packed this into an installable tgz. You can install it for testing by referencing it in your package.json like so:

{
    "devDependencies": {
        "typescript": "https://typescript.visualstudio.com/cf7ac146-d525-443c-b23c-0d58337efebc/_apis/build/builds/66655/artifacts?artifactName=tgz&fileId=96C836092C4432923665DCDF014700A31A0F45018B869320D4A73487C34B8B9E02&fileName=/typescript-3.9.0-insiders.20200228.tgz"
    }
}

and then running npm install.

---

There is also a playground for this build.

[sandersn]

@rbuckton is this ready to merge?

[DanielRosenwasser]

@rbuckton @RyanCavanaugh this should have landed in the beta

[DanielRosenwasser]

@typescript-bot user test this
@typescript-bot test this

[typescript-bot]

Heya @DanielRosenwasser, I've started to run the parallelized community code test suite on this PR at 4214548. You can monitor the build here.

[typescript-bot]

Heya @DanielRosenwasser, I've started to run the extended test suite on this PR at 4214548. You can monitor the build here.

[DanielRosenwasser]

@typescript-bot cherry-pick this to release-3.9

[typescript-bot]

Heya @DanielRosenwasser, I've started to run the task to cherry-pick this into release-3.9 on this PR at 4214548. You can monitor the build here.

[typescript-bot]

Hey @DanielRosenwasser, I've opened #37599 for you.

[typescript-bot]

The user suite test run you requested has finished and failed. I've opened a PR with the baseline diff from master.

[rbuckton]

@DanielRosenwasser I manually cherry-picked the last commit over, which just addresses @elibarzilay's comments about a dead-code branch and fixes a comment.