non public type members allowed in public signatures

[scabug]

=== What steps will reproduce the problem? ===

Given this trait, with a private type member:

 scala> trait X { private[this] type A = Int; def a: A = 1 }
 defined trait X

The access restrictions correctly prevent me from accessing the type
member from outside of x.

 scala> type xA = x.A
 <console>:10: error: type A is not a member of java.lang.Object with X
        type xA = x.A
                  ^
 scala> val x = new X {}
 x: java.lang.Object with X = $$anon$$1@58bf7b3e

 scala> x.a
 res6: Int = 1

But I can see the type in the eta-expanded signature here:

 scala> x.a _
 res7: () => x.A = <function0>

Not sure if this is a problem, it just strikes me as a bit odd to
allow private type members to appear in public signatures, by contrast
with the way private template definitions are handled.

 scala> trait X { private[this] trait A; def a: A = null }
 <console>:8: error: private trait A escapes its defining scope as
part of type X.this.A
        trait X { private[this] trait A; def a: A = null }

Similar problems exists with other access modifiers.

=== What is the expected behavior? ===

Whatever Adriaan deems sound upon his return from the Antipodes.

=== What do you see instead? ===

=== Additional information ===

https://groups.google.com/d/topic/scala-language/_txHz6q3Rw4/discussion

=== What versions of the following are you using? ===

• Scala: 2.8.1 / 2.9.0.RC1

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-4400?orig=1
Reporter: @retronym

[scabug]

@SethTisue said:
this is changing in Dotty: scala/scala3#997

[som-snytt]

The example from 2011 is i1130.scala. I'll see how close I can get i997.scala.

[SethTisue]

in both Scala 2 and 3, it matters whether private is qualified or unqualified:

scala 2.13.15> trait Foo { private[Foo] trait A; def a: A = null }
trait Foo

scala 2.13.15> trait Foo { private trait A; def a: A = null }
                                                   ^
               error: private trait A escapes its defining scope as part of type Foo.this.A

Welcome to Scala 3.6.2 (17.0.13, Java OpenJDK 64-Bit Server VM).
                                                                                                    
scala> trait Foo { private[Foo] trait A; def a: A = null }
// defined trait Foo
                                                                                                    
scala> trait Foo { private trait A; def a: A = null }
-- Error: ----------------------------------------------------------------------
1 |trait Foo { private trait A; def a: A = null }
  |                                 ^
  |               non-private method a in trait Foo refers to private trait A
  |               in its type signature => Foo.this.A
1 error found

I don't know why that would be.

[SethTisue]

https://scala-lang.org/files/archive/spec/2.13/05-classes-and-objects.html#private is relevant:

Members marked private without a qualifier are called class-private, whereas members labeled with private[this] are called object-private. A member is private if it is either class-private or object-private, but not if it is marked private[C] where C is an identifier; in the latter case the member is called qualified private.

so in spec-ese, "qualified private" is in fact a separate concept, it seems. But I'm not currently finding spec language that gives further insight into the difference

https://www.scala-lang.org/old/node/10488 gives some insight and pointers — but it concludes with Heiko asking my exact question:

Why doesn't the compile-time check prevent a trait/class defined qualified private (private[foo]) to escape its defining scope? Is this intentional?

and Martin never answered

[som-snytt]

@SethTisue private means doesn't participate in overriding 5.1.4. I don't know about leakability.