This plantuml specification doesn't parse:
@startuml
actor Alice
database Bob
autonumber
Alice -> Bob : Hello
@enduml
By removing the first two lines that define the shape of participants, the autonumber works properly. The autonumber feature should be independent of the shape, hence this appears to be a bug.
If not, how could I use autonumber in this case? Do I have to use a different syntax?