Regression: Cannot parse arrow fn exprs in conditional expr cons and alt in object literals in TypeScript

[dsherret]

Describe the bug

Regression of #2174

#3685 (comment)

This syntax no longer parses, yet it is valid JavaScript. Maybe the TS compiler can't figure out how to parse it right now(Edit: it can figure this out now), but it works in V8.

https://astexplorer.net/#/gist/d8417911fb6ea22a87b63d441b2a0e87/20b56c10cf6e34b3e7ddc60e9328702e09c930fd

cc @williamtetlow

Input code

const x = {
    prop: true
        ? fn => ({ })
            : fn => true,
};

Config

No response

Playground link

https://play.swc.rs/?version=1.2.151&code=H4sIAAAAAAAAA0vOzysuUahQsFWo5uVSAIKCovwCK4WSotJUCB8E7BXS8hRs7RQ0qhVqNRHCIGAFlQJp0OHlqrUGAGjLrV1RAAAA&config=H4sIAAAAAAAAA0WOQQ6DMAwE%2F%2BIz1%2FaQP%2FCIKBiUipDIa6RGiL%2FXoaG9edfeWR%2F0QiB3UPECljahburf5EhrYQSJRWkghVmzX8GnCS8Lq50wHrZbcwaTU9l5oBS3ONcGCjkVYaDnbOW3ZeUbY5yUp70Zx9V18Z7Gg0oM%2Bot95Zgn%2Fn9wt%2FSTiLGj2hPnBwXfhU7WAAAA

Expected behavior

Should parse.

Actual behavior

error: Expected ':', got ','
 --> input.ts:4:25
  |
4 |             : fn => true,
  |                         ^



Caused by:
    0: failed to process js file
    1: failed to process js file
    2: Syntax Error

Version

1.2.151

Additional context

No response

[kdy1]

I think the patch for #2174 was removed for some reasons, but I don't remember the PR removed it.

[dsherret]

It was in #3685. I think it's reasonable that this doesn't parse in TS given that the TS compiler can't figure it out, but it should still at least parse in JS.

[williamtetlow]

@dsherret this does parse as JS. The playground you shared has language set to Typescript

see

https://play.swc.rs/?version=1.2.151&code=H4sIAAAAAAAAA0vOzysuUahQsFWo5uVSAIKCovwCK4WSotJUCB8E7BXS8hRs7RQ0qhVqNRHCIGAFlQJp0OHlqrUGAGjLrV1RAAAA&config=H4sIAAAAAAAAA0WOSw7DMAhE78La23bhO%2BQQyCGRK%2F8ERKpl5e61G6fdATPzmAYvcWAbFGQhHpPUpPgGC%2BQiimNfFEy39dOGQeg0oMg76bDIo2shZyGwygcZiD75rQ6Qy7EwicxclzDtgW5M58S8HuPQQGuhL%2B%2FZeaLsnf5i17rklf4N7i%2FT4mWZqFHi%2FACR4MgV1gAAAA%3D%3D

[dsherret]

@williamtetlow yup, I missed that and my bad! I think though that we should still try to support this in TS because it might be fixed in tsc in the future.

That said, given that it parses in JS and the TS compiler itself has this bug then I think it's very low priority. Edit: It is fixed in the latest TS nightly.

[dsherret]

I just went to report this to TypeScript's repo and at the last second I discovered it is fixed in the latest TS nightly:

https://ts-ast-viewer.com/#code/MYewdgzgLgBAHjAvDA3gKBpmAHATibALhilwFcBTDLGgfhgDMwkA+GAChRgF8BKamoJjEmrEuQoAaNNwDcQA (select @next in Options > Version)

https://www.typescriptlang.org/play?ts=4.7.0-dev.20220302#code/MYewdgzgLgBAHjAvDA3gKBpmAHATibALhilwFcBTDLGgfhgDMwkA+GAChRgF8BKamoJjEmrEuQoAaNNwDcQA

[williamtetlow]

No probs 🙂

For context, I can see why TSC doesn't support it at the moment as it's quite a difficult issue to fix.

The following will parse as we can tell that what comes after the : is the falsy expression.

true ? fn => { } : fn => 1

but when parsing

true ? fn => ({ }) : fn => true

This bit({ }) : fn => true could be the beginning of a type definition or the falsy expression and we assume the former the same way TSC does.

[williamtetlow]

I just went to report this to TypeScript's repo and at the last second I discovered it is fixed in the latest TS nightly:

https://ts-ast-viewer.com/#code/MYewdgzgLgBAHjAvDA3gKBpmAHATibALhilwFcBTDLGgfhgDMwkA+GAChRgF8BKamoJjEmrEuQoAaNNwDcQA (select @next in Options > Version)

https://www.typescriptlang.org/play?ts=4.7.0-dev.20220302#code/MYewdgzgLgBAHjAvDA3gKBpmAHATibALhilwFcBTDLGgfhgDMwkA+GAChRgF8BKamoJjEmrEuQoAaNNwDcQA

Oh ok let me take a look at how they fixed it

[williamtetlow]

It looks like pretty simple fix they just do the opposite now 😄 I can take a look

microsoft/TypeScript#47550

[dsherret]

@williamtetlow oh nice! Thanks so much for looking into this 🙂

[swc-bot]

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