private-ness of trait constructor ignored

[pweisenburger]

Compiler version

3.3.0-RC3

Minimized code

object o:
  trait T private[o]()

def test = new o.T { }

Output

The code compiles. I would expect it not to. My expectation was that trait T private[o]() scopes the constructor of T private in o (but maybe this is not the case?). I assume that the anonymous class created in new o.T { } calls the T's constructor. If that is the case, then I think creating instances of T should only be possible within o (which was the intention here).

[som-snytt]

object o:
  trait T private[o]()
  trait U private[o] (i: Int)

def test = new o.T { }
def ouest = new o.U(42) { }

class C extends o.T
class D extends o.U(42) // constructor U cannot be accessed as a member of o.U from class D.

Error as expected with parameters.

[odersky]

@som-snytt Yes, that's the point here. We don't call the constructor of a trait unless we have to pass parameters. So declaring it as private[o] does not help.

[pweisenburger]

Makes sense that the constructor is not called if there are no parameters to pass. But this sounds more like an optimization that should not affect the surface language. I find it a bit concerning that trait T private[o](...) behaves differently depending on whether the argument list is empty or not, in particular that the private modifier is sometimes silently ignored by the compiler.

If the code is legal, private should not be a no-op IMO. Thus, I believe the compiler should issue a warning or even reject the program, or – as a much more useful semantics – private should prevent instances of the trait from being created/mixed-in out of scope (to this end, the compiler could check that the constructor is accessible, regardless of whether it is actually called or not).

Also, the statements in the trait's body are executed during construction (either as part of the constructor or of some special mixin initialization method). I think a reasonable expectation is that private[o] makes construction private.

[odersky]

I agree this warrants further investigation.