Respect variant declaration ordering

[OpenByteDev]

internal sealed record ProgressHidden;
internal sealed record ProgressCompleted;
internal sealed record ProgressRunning(
    WorkItem? Work = null,
    string? Message = null,
    double? Progress = null
);
[UnionType<ProgressHidden, ProgressRunning, ProgressCompleted>]
internal readonly partial struct ProgressState;

I would expect state.Switch(hidden, running, completed) but its state.Switch(completed, hidden, running)

[SleepWellPupper]

Yes, the ordering is generator-defined and an implementation detail. I have (currently) no plans to change this.
Can you share a little about your specific usecase so I can understand better why this would be required?

[OpenByteDev]

For me there is an intuitive ordering for some unions and i would expect this to be ordering for Switch.
One example is Result<TOk, TErr> where, at least for me, Ok should come for Err.
Also many state unions occur in a particular (partial) order like Started -> Running -> Done.

[SleepWellPupper]

Yeah, having an order there would make sense. Personally I always add the parameter name to the Switch invocation argument, so I can order them whichever way I like at the callsite.
For now, I will leave the ordering unchanged for this reason.