Compile error: Nested loops with index broke after 3.9.1

[Xerios]

Describe the bug
You can see the exact same weird parsing behavior in the example below

Logs
Generated code is wrong, for instance when using {@debug paneIndex} it generates this:

(paneIndex is the index from {#each ... } loop)

To Reproduce
https://svelte.dev/repl/af7ece0c29f240ad97bf795bd3be1347?version=3.9.1

• Remove debug, it compiles properly
• Remove the inner loop, and it compiles properly.
• Debug is not at fault here, because in my project, the index is no longer passed onto nested loops which breaks my code.

Expected behavior

• Proper compilation without errors

Stacktraces

• No errors from the compiler, only compiled code gives an error

Information about your Svelte project:

• Svelte version 3.9.1 compared to every single version above

Severity

• Medium, project is live with over 100+ daily CCU. Having smooth animations is very important in my project. I would've loved to have latest transition/animations fixes which currently look a bit janky in 3.9.1, this compiler issue prevents me from upgrading.

@Xerios, the code from the REPL builds and works under 3.12.1 when building for production via npm run build and npm run start so I think it really is debugging/dev codegen that is causing the issue. The if () is still the culprit, as you said, though.

[Xerios]

Turns out my issue was some sort of race condition.

After diving into the generated code I noticed this:

function create_each_block$5(ctx) {
  // ctx.data has 0 elements, thus if_block is not created
  var if_block = ctx.data[ctx.paneIndex] && create_if_block_1$4(ctx);
  // ...
  const block = {
    p: function update(changed, new_ctx) {
      // ctx.data has one element, but since if_block wasn't created, it gives an error
      if (ctx.data[ctx.paneIndex]) if_block.p(changed, ctx);
    }
  }
}

[Conduitry]

This bug is still present with the changes in the code-red branch, but manifests as a compile-time exception rather than outputting invalid code.