NeedsFBound.scala

 object Test {              
    trait BlahConfig[A <: BlahConfig[A]] {
      val server: Int        
      def updateCache(l: List[Int]): A
    }
                             
    def doBlahStuff[A <: BlahConfig[A]](i: Int) = State[A, Int]( c => (c.updateCache(List(i)), c.server) )
  
    trait FooConfig[A <: FooConfig[A]] {
      def password: String   
    }
  
    def doFooStuff[A <: FooConfig[A]](s:String) = State[A, String](c =>(c, c.password))
  
    //CONCRETE CONFIG       
    case class RConfig(s: Int, p: String, cache: List[Int]) extends BlahConfig[RConfig] with FooConfig[RConfig] {
      val password = p       
      val server = s         
      def updateCache(l: List[Int]) = this.copy(cache = l ::: cache)
    }
   
    val res = for {
      str <- doSomeStuff[RConfig]("blah") 
      i   <- doStuff[RConfig](2)
    } yield i
  
   res(RConfig(1, "asdf", List[Int]()))
  }

thanks to @emrys and @dibblego who informed me lenses could solve the issue by zooming, I was able to redo this in a much nicer way:

https://gist.github.com/vmarquez/bac38b6605359124bb83