Make never rest type top-like

[jack-williams]

Not an actual fix as the issue is still under discussion. Figured I’d put up and implementation though.

Fixes #33495

[jack-williams]

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

[typescript-bot]

Heya @jack-williams, I've started to run the extended test suite on this PR at 76bc82b. You can monitor the build here. It should now contribute to this PR's status checks.

[typescript-bot]

Heya @jack-williams, I've started to run the parallelized community code test suite on this PR at 76bc82b. You can monitor the build here. It should now contribute to this PR's status checks.

[typescript-bot]

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

[weswigham]

Makes a lot of sense to me - making never behave as any already did here seems to work. Can we also see a test for the nested case, eg

function assignmentWithComplexRest2<T extends any[]>() {
    const fn1: (cb: (x: string, ...rest: T) => void) => void = (cb) => {};
    const fn2: (cb: (...args: never) => void) => void = fn1;
}

which should error, I believe?

[jack-williams]

which should error, I believe?

Yep, added!

[jack-williams]

Thanks for powering through all these PR's @sandersn and @weswigham!

[andrewbranch]

Was it known that this makes (...args: never) => unknown callable? #48840

[jack-williams]

Short answer: no. That function type should not be callable.

Longer answer: After looking through this again last night and trying to fix it, I do vaguely remember looking at that behaviour before. I think the code to trigger the error was move involved and touched a couple of the signature checking functions. I guess I never got around to resolving that and when the PR was revived, I had forgotten.

I started working on a fix yesterday. Haven't run it through perf and regression test. I do not think it is a big change. Also happy for someone to pick it up.

[andrewbranch]

Thanks @jack-williams, wasn’t expecting you to pick up a fix, just searching for info, but appreciate it!