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

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.