Inconsistent tsc behavior on TS 5.4

[olegafx]

🔎 Search Terms

"tsc inconsistent behavior", "tsc 0 errors", "ts 5.4 bug", "5.4.0-dev.20240219"

🕗 Version & Regression Information

I wanted to update TS in our project from v5.3.3 to v5.4.3. And I noticed several weird things:

• When I was switching TS version in VS Code, it wasn't showing any errors. But tsc --noEmit was showing 2 errors. I thought that VS Code was just not ready to support v5.4.3 yet. But the new version comes with v5.4.3 and it still doesn't work correctly.
• I run tsc --noEmit. It shows 2 errors. If I open a problematic file and put // @ts-expect-error, it shows only 1 error. But if I remove that line, it instantly shows 0 errors.

This changed between versions 5.4.0-dev.20240218 and 5.4.0-dev.20240219.

5.4.0-dev.20240219 is the first version where I see this problem. I checked @next version which is 5.5.0-dev.20240412 and the error is still there.

Unfortunately, I wasn't able to make a reproducible example due complexity of the code. But both errors are about unknown type where it should be detected just fine.

⏯ Playground Link

No response

💻 Code

No response

🙁 Actual behavior

• It doesn't show errors in VS Code
• It shows errors in a console. But after applying // @ts-expect-error and removing that line, it shows 0 errors.

🙂 Expected behavior

• Errors in VS Code are detected
• It shows errors in a console. After applying // @ts-expect-error and removing that line, it shows all previous errors.

Additional information about the issue

No response

[RyanCavanaugh]

There's really not much we can do without a way to reproduce the problem, unfortunately

[olegafx]

Ok, I was able to reproduce it. Here is a link to StackBlitz. There are 2 main cases:

1. If you'll open fail.tsx, you won't see any errors in the editor. But if you will run yarn tsc --noEmit in the terminal, you'll see src/fail/fail.tsx:18:22 - error TS2322: Type 'unknown[]' is not assignable to type 'FailedItem[]'.
2. Start tsc in watch mode: yarn tsc --noEmit --watch. Open fail.tsx and add // @ts-expect-error in any place of the code. Remove this comment. It will show 0 errors in the console.

Also you could notice that there is import '@total-typescript/ts-reset/is-array'; in reset.d.ts. The only thing it's doing is changing any to unknown for Array.isArray. Source code.

React version doesn't really matter. It works the same on v16 and v18.

[DanielRosenwasser]

For what it's worth, the only thing that was merged in between those dates was #57122