Why does scalac only emit dispersion errors with specific access modifiers?

Question

This code does not compile:

trait Invariant[T] trait Covariant[+T] { protected val example: Invariant[T] } 

 error: covariant type T occurs in invariant position in type => Invariant[T] of value example protected val example: Invariant[T] ^ one error found 

However, this code compiles just fine:

 trait Invariant[T] trait Covariant[+T] { protected[this] val example: Invariant[T] } 

After some experimentation, it seems that the variance error only occurs when the access modifier is not tied to this . Both of them do not work:

 val example: Invariant[T] private val example: Invariant[T] = ??? 

While private[this] working.

I see similar behavior when a type is contravariant rather than invariant.

I feel like I'm missing something about how a system like Scala works. Why does the compiler behave this way?