We should may add a test for the concrete issue, too. I just created one for the root issue.
(the test-project seems to have no ref assembly to Net9.0)

Oh, then it becomes a breaking change to existing code. But this is wanted???

Oh, then it becomes a breaking change to existing code. But this is wanted???

Please see https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-06-17.md#params-span-breaks for more information.

I see - then we would need to fix the lowering (and use an array).

I see - then we would need to fix the lowering (and use an array).

Please see #74163 (comment). First, we need to understand the root cause of the problem. Where and what is going wrong. Only then we can decide what the fix should look like. I suggest you to start with that if you would like to continue working on the issue.

then we would need to fix the lowering (and use an array).

We probably don't want to change the lowering in this fashion either.

We probably don't want to change the lowering in this fashion either.

I wouldn't like to change the lowering code for interpolated string, too.
We could simply dissallow such code (breaking change) - but this is also not very well.

I wouldn't like to change the lowering code for interpolated string, too. We could simply dissallow such code (breaking change) - but this is also not very well.

Let's discuss possible fix strategies in the issue. While proposing a strategy, please make the proposal very specific and detailed. For example, the quote above looks neither specific, nor detailed to me.