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

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)!.

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.

@typescript-bot pack this

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

Do we have tests for

let x = a?.b!();

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.

@rbuckton is this ready to merge?

@rbuckton @RyanCavanaugh this should have landed in the beta

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

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

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

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

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.

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

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

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