useDefineForClassFields with recent Next.js

[hyperknot]

Has anybody set up useDefineForClassFields with recent Next.js, using SWC? (I believe 12+ uses SWC).

My problem is the following:

1. On a new create-next-app project, with Typescript, I add "useDefineForClassFields": true
2. If I add the test from the docs

if (!new class { x }().hasOwnProperty('x')) throw new Error('Transpiler is not configured correctly');

At the top of _app.tsx, it'll always throw the error, both in "dev" and in "build".

=> This would mean that useDefineForClassFields is broken in Next / SWC.

3. However there is a PR for Next which is supposed to fix this behaviour (made especially for MobX):
   feat:Add support for useDefineForClassFields typescript option vercel/next.js#36335

Now that PR has a test which in theory tests this behaviour. In practice I couldn't run the test with "build", only "dev", so I couldn't test it. But let's suppose it's just a small compiler warning, and the PR + test is good.

=> This would mean that useDefineForClassFields works well in Next / SWC and the if hasOwnProperty cannot be used in Next.

Now this is in conflict here. What is the case? Is anyone using it with recent Next?

[hyperknot]

OK, it turns out it's actually working correctly with Next, both the one liner .hasOwnProperty() and an actual MobX makeAutoObservable() example as well.

The problem is that the .next folder is caching the compiled files and needs to be manually deleted when changing tsconfig.json.

So basically after rm -rf .next it works correctly, including with MobX. I opened a Next bug for the cache problem:
vercel/next.js#44396