t6146b.scala:15: warning: match may not be exhaustive.
It would fail on the following inputs: S2(), S3()
  def foo(f: F[Int]) = f match { case X.S1 => }
                       ^

scala> :power
Power mode enabled. :phase is at typer.
import scala.tools.nsc._, intp.global._, definitions._
Try :help or completions for vals._ and power._

scala> val u = rootMirror.universe; import u._, language._
u: $r.intp.global.type = <global>
import u._
import language._

scala> val S1 = typeOf[c.X.S1.type forSome { val c: C[_] }].typeSymbol.tpeHK
S1: u.Type = C.X.S1.type

scala> val S2 = typeOf[O.S2].typeSymbol.tpeHK
S2: u.Type = C.this.S2

scala> val S3 = typeOf[O.S3].typeSymbol.tpeHK
S3: u.Type = O.S3

scala> val S4 = typeOf[S4].typeSymbol.tpeHK
S4: u.Type = S4

scala> val F = typeOf[c.F[_] forSome { val c: C[_] }].typeSymbol.tpeHK
F: u.Type = C.this.F

scala> val fTpe = typeOf[O.type].decl(newTermName("foo")).paramss.head.head.tpe
fTpe: u.Type = O.F[Int]

scala> def memType(sub: Type, scrut: Type): Type =
  nestedMemberType(sub.typeSymbol, scrut.prefix, scrut.typeSymbol.owner)
memType: (sub: u.Type, scrut: u.Type)u.Type

scala> 

scala> val mt1 = memType(S1, fTpe)
mt1: u.Type = O.X.S1.type

scala> global.typeDeconstruct.show(mt1)
res0: String =
TypeRef(
  pre = SingleType(pre = ThisType(object O), object X)
  TypeSymbol(class S1 extends C.this.F[T])
)

scala> memType(S2, fTpe)
res1: u.Type = O.S2

scala> memType(S3, fTpe)
res2: u.Type = O.S3

scala> memType(S4, fTpe)
res3: u.Type = S4

scala> :quit