Not sure if it is useful to post here, but I think...

# compiler

[David Brunow]

Not sure if it is useful to post here, but I think I found a bug: https://youtrack.jetbrains.com/issue/KT-79078/Unexpected-NoWhenBranchMatchedException-when-using-sealed-class

[bnorm]

I can't speak definitively to the issue, as the JVM backend team should take a look, but it looks like a duplicate of this issue: https://youtrack.jetbrains.com/issue/KT-67222/Kotlin-object-is-null-incorrect-static-initialization. It looks like a cyclic initialization issue. To access the MySealedClass.One singleton, the outer class needs to be loaded as well, which triggers the initialization of TEST_LIST, which causes a cycle. Based on the information in the other ticket, I suspect your list actually contains a null value as the first element, which doesn't match any of the when branches.

[David Brunow]

Thanks for the link to that issue, it does look like the same thing.

[David Brunow]

And the first item in the array is null, as you suspected.