main = ()
  where
    when ?x <- ?y
    then True
--
3:10-3:12: Existential variable ?x is referred only once. Replace by _ if this is a wildcard.
3:10-3:18: Cannot solve the query.
3:16-3:18: Existential variable ?y is referred only once. Replace by _ if this is a wildcard.
--
import "Prelude"

main = getRef r
  where
    r = ref 1
  
    when True
    then Foo ()
    
    when Foo _
    then r := 2
--
2