Question

This doesn’t work:

trait Trait
class Class extends Trait with Trait

Compiler complains:

<console>:8: error: trait Trait is inherited twice
       class Class extends Trait with Trait
                           ^
<console>:8: error: trait Trait is inherited twice
       class Class extends Trait with Trait
                                      ^

This does:

trait Trait
class Abstraction extends Trait
class Implementation extends Abstraction with Trait

Questions:

• Why does it work?
• How is the second snippet different? (concerning the double inheritance issue)
• Is the second snippet or pattern somehow useful?

Answer 1

Second snippet works because of trait linearization. The compiler will organize the traits into a linear list so that Trait only appears once. I think the linearization is

Implementation, Trait, Abstraction, ScalaObject, AnyRef, Any

See this chapter from Programming Scala for a great explanation.

This is primarily done to have a consistent approach to the diamond inheritance problem and is useful in that case.

Since Trait cannot appear twice after linearization, it does not make sense to write Trait with Trait and it makes sense to be disallowed.