How to do it with reflection in Scala?

Suppose I have a non-sealed trait A and need a function convert[T <: A]: A => Option[T]:

def convert[T <: A](a: A): Option[T] = // don't compile
  if (a is instance of T) then Some(a) else None 

How would you suggest implement that convert ?

P.S I don't like reflection but need to deal with legacy, which uses it.