Scala generic method - No ClassTag available for T

To instantiate an array in a generic context (instantiating an array of T where T is a type parameter), Scala needs to have information at runtime about T, in the form of an implicit value of type ClassTag[T]. Concretely, you need the caller of your method to (implicitly) pass this ClassTag value, which can conveniently be done using a context bound:

def foo[T:ClassTag](count: Int, value: T): Array[T] = Array.fill[T](count)(value)

For a (thorough) description of this situation, see this document:

https://docs.scala-lang.org/sips/scala-2-8-arrays.html

(To put it shortly, ClassTags are the reworked implementation of ClassManifests, so the rationale remains)