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What is the difference between scala self-types and trait subclasses?
I understand a self-annotation as a promise to the compiler, where the programmer manifests that a trait will be mixin with the annotated one. For example:
scala> trait X
defined trait X
scala> trait Y { this: X => }
defined trait Y
scala> new Y {}
<console>:10: error: illegal inheritance;
self-type Y does not conform to Y's selftype Y with X
new Y {}
^
scala> new Y with X {}
res1: Y with X = $anon$1@1125a40
In the previous example, the third expression failed because we did not set a valid X to the new instance. Obviously, the last one works nice. So far, so good. And now, let's see another example which involves an object.
scala> object Z { this: X => }
defined module Z
I understand the object is being instantiated failing with the X promise (we are creating an instance now with a future promise!), as represented in the next lines, where the traits have been slightly modified:
scala> trait X { class X1 }
defined trait X
scala> trait Y { this: X => new X1 }
defined trait Y
scala> object Z { this: X => new X1 }
<console>:8: error: not found: type X1
object Z { this: X => new X1 }
^
So, what does the object self-annotation imply?
