How to debug core-js polyfills?

[ChristophP]

From our error tracking I can see that some users with older browsers aren't getting the polyfills that they need to.
I wanted to look into whether core-js correctly expands imports how it should.

Here's what I checked so far.

• npm ls core-js -> version 3.34.0
• npx browserslist -> Safari 15.3 is listed as supported by our .browserlistrc
• index.tsx imports -> we import import "core-js" according to the core-js docs this should expand to all the core-js features which need to be polyfilled in the given browsers
• looks like parcel uses mode: entry for core-js (code)
• checked the compiled output but it's hard to find the relevant lines

Is there any way to check this well? Specifically I am trying to find out why structuredClone was not polyfilled for Safari 15.3 even though I believe it should have been polyfilled given the configuration supplied above.
It would be helpful to see how for example

// this
import 'core-js/actual';

// turns into this
import 'core-js/actual/promise';
import 'core-js/actual/set';
import 'core-js/actual/iterator';
import 'core-js/actual/array/from';
import 'core-js/actual/array/flat-map';
import 'core-js/actual/structured-clone';

So maybe like the output of the JS transformer without additional bundling or so

(Using parcel 2.12, with Node 20 on a Manjaro Linux Machine)

EDIT:
I believe it is related to how the coreJs option is or isn't passed to swc. More info below

[ChristophP]

PS: I tried reproducing it in this parcel REPL but I am not sure how to specify browsersupport there granularly

[ChristophP]

I think I have a repro.

# .browserslistrc
safari 15.3 # to make sure to use an old browser

// index.ts
import "core-js/stable/structured-clone";

const p = structuredClone({});
console.log(p)

Then build and look at the output

npx parcel build index.ts --no-optimize
cat dist/index.js

The output then looks like this

(() => {
   const $bdbb458e2a7a1037$var$p = structuredClone({});
   console.log($bdbb458e2a7a1037$var$p);
})();

No structuredClone polyfill visible even though it would be needed for Safari 15.3 according to can-i-use

[ChristophP]

For comparison: this SWC playground seems to include the structuredClone polyfill correctly.

But only if the coreJs: "3.37" version is set correctly. When set to coreJs: "3" (which is equivalent to 3.0 according to the docs) or leaving this option off altogether the polyfill for structuredClone is not included. For 3.0 this would make sense since the structuredClone polyfill was only added in core-js version 3.24-ish.
I have a feeling that its related to the coreJs option/lack of the coreJs option to swc.

Could that be the reason?
How does parcel set the coreJs version for swc? Or in case it's not set at all, what does it default to?
If so, would this be considered a bug?