Case-classes and its default constructors

Suppose, I defined the case class:

case class A(id: Int)

Then I suddenly typed this code:

val a = A

The question is - why compiler allows such writings? In Java, defining constructor with any parameters, makes the default constructor not available.

EDIT: Ok, as Ende Neu pointed, the a now contains A.type. How can I avoid scenario, when suddenly typing a = A may break my code in:

a match {
   case A(id) => do something
   case _ => I didn't expect fall into here
}

I'm trying to avoid to fall into the second case in my Akka message processing code. Is it possible?

UPDATE: After some time spent on working with Akka + Scala I figured out, the answer to the second part of this question. Actor's receive method has signature PartialFunction[Any, Unit], that means there's no typecheck for the input parameter. So any type is accepted, including A.type. Best advice to the user in this context: be careful when sending the messages to actors.

Solution

You are assigning A.type to a:

scala> case class A(id: Int)
defined class A

scala> val a = A
a: A.type = A

scala> a()
<console>:11: error: not enough arguments for method apply: (id: Int)A in object A.
Unspecified value parameter id.
            a()
             ^

scala> a(1)
res1: A = A(1)

After the assignment a is of type A.type, then you can use a to construct new instances of the case class A, but you will still need to specify the id parameter.

Regarding your edit:

a match {
  case A(id) => do something
  case _ => I didn't expect fall into here
}

I think this correctly throws an exception in this case since you wanted a variable of type A and instead you ended up with a variable of type A.type, which is most probably a mistake, if you really want to differentiate between A and A.type in your match something like this should work:

scala>  val a = A
a: A.type = A

scala>  a match {
     |    case a: A.type => "A.type"
     |    case someA: A => "case class"
     |    case _ => "default"
     |  }
<console>:13: warning: fruitless type test: a value of type A.type cannot also be a A
                    case someA: A => "case class"
                            ^
<console>:13: warning: unreachable code
                    case someA: A => "case class"
                                 ^
res8: String = A.type

scala> val someA = A(1)
someA: A = A(1)

scala>  someA match {
     |    case a: A.type => "A.type"
     |    case someA: A => "case class"
     |    case _ => "default"
     |  }
<console>:12: warning: fruitless type test: a value of type A cannot also be a A.type
                    case a: A.type => "A.type"
                         ^
res9: String = case class

But as I said, this feels weird, probably there's a better solution to this kind of problems that I don't know.