Following !if -1 == -1 produces an internal server error, if it is started on the plantuml web server http://www.plantuml.com (version 1.2022.13.beta5)
( !if "-1" == "-1" is working)
@startuml
[A]
!if -1 == -1
[Bob] -> [Alice]
!endif
@enduml
BR Helmut