t4098.scala:3: error: forward reference not allowed from self constructor invocation
    this(b)
         ^
t4098.scala:8: error: forward reference not allowed from self constructor invocation
    this(b)
         ^
t4098.scala:13: error: forward reference not allowed from self constructor invocation
    this(b)
         ^
t4098.scala:18: error: forward reference not allowed from self constructor invocation
    this(b)
         ^
four errors found