gistfile1.scala

class !<:<[A, B]
implicit def any[A, B]: A !<:< B = new !<:<[A, B]
implicit def sub1[A, B >: A]: A !<:< B = new !<:<[A, B]
implicit def sub2[A, B >: A]: A !<:< B = new !<:<[A, B]

sealed trait ShardKeyNotSpecified
sealed trait ShardAware

trait Sharded

class ShardingOk[M, S](a: String) { override def toString = a }

implicit def sharded[M <: Sharded, State <: ShardAware]: ShardingOk[M, State] = new ShardingOk("a")
implicit def unsharded[M, State](implicit ev: M !<:< Sharded): ShardingOk[M, State] = new ShardingOk("b")

class A extends Sharded
class B

implicitly[ShardingOk[A, ShardKeyNotSpecified]] // Should fail
implicitly[ShardingOk[A, ShardAware]]           // Should succeed
implicitly[ShardingOk[B, ShardKeyNotSpecified]] // Should succeed

/*
Sharded => ShardAware
~Sharded v ShardAware
~(Sharded & ~ShardAware)
~ShardAware => ~Sharded
*/