Recursive references in objects

[KuceraMartin]

Compiler version

3.2.0

Minimized code

The first example works (I believe) correctly. I would expect the latter two to have the same output, yet the second one results in stack overflow and the third one loops infinitely.

Example 1:

case object A:
  val b = B

case object B:
  val a = A

println(A)
println(A.b)
println(A.b.a)

https://scastie.scala-lang.org/TWhOPNLMTYyAV18EqK6hLA

Output

A
B
A

(this is correct)

Example 2:

{
  case object A:
    val b = B

  case object B:
    val a = A

  println(A)
  println(A.b)
  println(A.b.a)
}

https://scastie.scala-lang.org/Wki4Z9XJR3Gx13RFmCzpXQ

Output

Exception in thread "main" java.lang.StackOverflowError

Expectation

A
B
A

Example 3:

class MyClass:
  case object A:
    val b = B

  case object B:
    val a = A

val myClass = new MyClass()
println(myClass.A)
println(myClass.A.b)
println(myClass.A.b.a)

https://scastie.scala-lang.org/347k71gqSXuZhtZLOK69yQ

Output

(no output, loops infinitely)

Expectation

A
B
A

[som-snytt]

Compare #16152

The class initializer for the top-level object leaks its MODULE reference before it is initialized.

Local and member objects are lazy vals.

[olhotak]

@liufengyun -Ysafe-init-global warns about this issue now.

It warns about example 2 but not about example 3.

[liufengyun]

It warns about example 2 but not about example 3.

You mean the 1st example gets a warning? For the 2nd and 3rd, the objects are not global objects.

[olhotak]

The 1st example gets a warning. We all agree on that.

The 2nd example doesn't even compile: { is Illegal start of toplevel definition. Scastie must wrap it somehow. I wrapped it in an outer object and then Ysafe-init-global gives a warning. How else is it intended to be wrapped?

For the 3rd example neither checker (neither -Ysafe-init nor -Ysafe-init-global) gives a warning, yet the code infinitely loops at run time.

[som-snytt]

I took example 2 to mean local (not a member of a class or package).

➜  snips ll i*456*
-rw-r--r-- 1 amarki amarki 136 Dec  4  2022 i16456a.scala
-rw-r--r-- 1 amarki amarki 150 Dec  4  2022 i16456.scala
➜  snips cat i16456.scala

def f: Unit = {
  case object A:
    val b = B

  case object B:
    val a = A

  println(A)
  println(A.b)
  println(A.b.a)
}

@main def test() = f

Is example 3 a loop or rather deadlock on class initialization.

[liufengyun]

Is example 3 a loop or rather deadlock on class initialization.

As it's given, there are no threads, so it's non-termination. If we access myClass.A and myClass.B from two different threads at the same time, then it might lead to a deadlock.