Why can't I omit "apply" in this.apply(_) in Scala?

Question

Observe the following code

trait Example {
  type O
  def apply(o: O)
  def f(o: O) = this.apply(o)
}

which compiles fine in Scala. I would expect that I can leave out apply as usual, writing def f(o: O) = this(o). However, this results in the exciting error message

type mismatch;  found   : o.type (with underlying type Example.this.O)
                required: _31.O where val _31: Example
possible cause: missing arguments for method or constructor

Can anyone explain to me what's going on?

Answer 1

You can't because this() inside an constructor is a call to this object's constructor (this() anywhere else generates a compilation failure) and can not be made into an apply() call as it would hide the constructor and make it impossible to call another constructor in your object. this(args) is always a call to a constructor method (both in Java and Scala), so when inside your own object, you always have to explicitly call apply(args).

Answer 2

The accepted answer is incorrect. You can infer what the actual problem is from the fact that this compiles fine:

trait Example {
  def apply(o: String): String = o
  def f(o: String) = this(o)
}

this(...) only represents a call to a constructor when the call site is an auxiliary constructor. The remainder of the time it is a call to apply, just as you imagined.