Why can a method returning Unit be overridden with method returning String when return types are not explicitly given?

Question

I was working through the code examples from the chapter on Traits in Programming in Scala Edition1 https://www.artima.com/pins1ed/traits.html

and came across a weird behavior because of my typo. While overriding method of a trait below code snippet doesn't give any compile error although the return types of the overridden method is different Unit vs String. But upon calling the method on an object it returns Unit but doesn't print anything.

trait Philosophical {
    def philosophize = println("I consume memory, therefore I am!")
}

class Frog extends Philosophical {
  override def toString = "green"
  override def philosophize = "It aint easy to be " + toString + "!"
}

val frog = new Frog
//frog: Frog = green

frog.philosophize
// no message printed on console

val f = frog.philosophize
//f: Unit = ()

But when I give the explicit return type in the overridden method , it gives a compile error:

class Frog extends Philosophical {
  override def toString = "green"
  override def philosophize: String = "It aint easy to be " + toString + "!"
}
         override def philosophize: String = "It aint easy to be " + toString +
                      ^
On line 3: error: incompatible type in overriding
       def philosophize: Unit (defined in trait Philosophical);
        found   : => String
        required: => Unit

Can anyone help explain why no compile error in the first case.

Answer 1

When the expected type is Unit, any value can be accepted:

Value Discarding

If e has some value type and the expected type is Unit, e is converted to the expected type by embedding it in the term { e; () }.

Answer 2

my question is why it got through the compiler in the 1st case

When you did not specify the return type explicitly it was inferred by the type it needs to have for the override to work.

That turned out to be Unit.

Since String values (the value of the expression making up the function body) can be assigned to Unit, the compiler is happy.