How can I assign a val with the implementation of a trait? - scala

Question

I have the following trait and classes:
trait A{
def func1: String
def func2: Int
}
class F extends A{
def func1: String = ???
def func2: Int = ???
}
class G extends A{
def func1: String = ???
def func2: Int = ???
}
And I want to create the following trait and class:
trait X{
val a: ***
}
class Z extends X{
override val a = new G
}
What do I need to write on '***' in trait X so that I can assign val a with a class F or G ?
Thank you.
EDIT
Sorry, I forgot to mention that trait A is A[Any] and traits F and G can be 'F extends A[String]' and 'G extends A[Int]'. In other words, the classes can extends the trait A with different types of parameters.

Answer 1

Re. your edit, your code fails because trait A only takes type parameter of a fixed type. If you declare base class having a A[Any], you cannot override it with a A[Int] or A[String]. There are two ways to make your code works.
One is to make trait A type argument covariant, so that you can override with subclass of the type argument you declared in base class (note the + in trait A definition):
trait A[+T]
class F extends A[String]
class G extends A[Int]
trait X {
val a: A[Any]
}
class Z extends X {
val a = new G
}
See here: Sample
Another way to work is to declare a as a A[_] (_ means it takes any type as a parameter, which is different from Any):
trait A[T]
class F extends A[String]
class G extends A[Int]
trait X {
val a: A[_]
}
class Z extends X {
val a = new G
}
See here: Sample