"implicit Manifest" is a Scala compiler trick and it does not make generics in Scala reified. The Scala compiler, when it sees a function with "implicit m: Manifest[A]" parameter and it knows the generic type of A at the call site, it will wrap the class of A and its generic type parameters into a Manifest and make it available inside the function. However, if it could not figure out the true type of A, then it has no way of creating a Manifest. In other words, Manifest has to be passed along the function calling chain if the inner function needs it.
scala> def typeName[A](a: A)(implicit m: reflect.Manifest[A]) = m.toString
typeName: [A](a: A)(implicit m: scala.reflect.Manifest[A])java.lang.String
scala> typeName(List(1))
res6: java.lang.String = scala.collection.immutable.List[int]
scala> def foo[A](a: A) = typeName(a)
<console>:5: error: could not find implicit value for parameter m:scala.reflect.Manifest[A].
def foo[A](a: A) = typeName(a)
^
scala> def foo[A](a: A)(implicit m: reflect.Manifest[A]) = typeName(a)
foo: [A](a: A)(implicit m: scala.reflect.Manifest[A])java.lang.String
scala> foo(Set("hello"))
res8: java.lang.String = scala.collection.immutable.Set[java.lang.String]