feat(parser/ts): support optional variance annotations

[magic-akari]

Description:
Implement: Optional variance annotations

BREAKING CHANGE:

Related issue (if exists):

• Optional variance annotations microsoft/TypeScript#48240

[kdy1]

Is it included in rc? I didn't check the typescript blog recently...

[magic-akari]

No, I just made it "ready for review".
I was concerned that there might be some issues that needed to be revised.

do not merge until microsoft/TypeScript#48240 is merged.

[kdy1]

I see, thanks!

[kdy1]

LGTM. Thank you!
Not marking as approval as to prevent mistake

[magic-akari]

I'm testing the type variance annotations with JSX(tsx).

const validJSX = <in T>() => {}</in>;

const validTypeVarianceFN = <in T,>() => {};

const expectedError = <in T>() => {};

The first two are parsed expected.
However, the last one did not report an error.

Single type parameter T should have a trailing comma.

When I investigated, I found that this was a pre-existing issue.
What do you think about it?
Maybe we should open a new issue to track it.

const expectedError = <T>() => {};

[kdy1]

I didn't know that const expectedError = <T>() => {}; is an error. I think we need an issue, thank you!

[magic-akari]

Babel will revert this feature and defer it to Babel 8.
cc @kdy1

• Defer <T>() => {} TSX error to Babel 8 babel/babel#14367

[nicolo-ribaudo]

Note that it should be an error only in TSX files, not in TS files. Due to how Babel needs to be configured, many people enabled TSX mode also for .ts files: swc might not have the same problem and you might be able to ship it without introducing breakages.

[kdy1]

@magic-akari I found that typescript PR is merged, and you added some tests.
Is this PR ready to review?

[magic-akari]

Yes. It's ready for review.

[kdy1]

Thank you!

---

swc-bump:

• swc_atoms
• swc_ecma_ast --breaking