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cycle with default arguments creates null field #5366

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scabug opened this issue Jan 11, 2012 · 2 comments
Open

cycle with default arguments creates null field #5366

scabug opened this issue Jan 11, 2012 · 2 comments
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named/default args
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@scabug
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scabug commented Jan 11, 2012 

class IdAndMsg(val id: Int,  val msg: String = "")

case object ObjA extends IdAndMsg(1)
case object ObjB extends IdAndMsg(2)

object IdAndMsg {
  val values = List(ObjA , ObjB)
}

object Test {
  def main(args: Array[String]): Unit = {
    ObjA
    println(IdAndMsg.values)
  }
}

/*
Output:
List(null, ObjB)
*/

The cycle is that if one references ObjA first, its superclass has a default argument which is retrieved from object IdAndMsg, which has a val which refers to ObjA, which is null.

It doesn't seem necessary - IdAndMsg.init$default$2 is a static method, but it creates IdAndMsg$ and populates the MODULE$ field so it can call IdAndMsg$.init$default$2. At least in the case of constants, the static method could return it directly and avoid loading the object.
@scabug
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scabug commented Jan 11, 2012

Imported From: https://issues.scala-lang.org/browse/SI-5366?orig=1
Reporter: @paulp
Affected Versions: 2.8.1, 2.9.3, 2.10.6, 2.11.8, 2.12.0-RC1

@scabug
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scabug commented May 14, 2012

@lrytz said:
I tried once implementing constructor defaults using static methods but it was not straightforward. I don't remember the details, in principle it could most likely be done.

@scabug scabug added this to the Backlog milestone Apr 7, 2017
This was referenced Apr 7, 2017
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@liufengyun liufengyun mentioned this issue Jun 7, 2021
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liufengyun added a commit to scala/scala3 that referenced this issue Jun 16, 2023
@liufengyun
Check safe initialization of static objects (#16970)
5caa6aa 
The problem is illustrated by the example below:

``` Scala
class Foo(val opposite: Foo)
case object A extends Foo(B)     // A -> B
case object B extends Foo(A)     // B -> A
```
The check aims to be simple for programmers to understand, expressive,
fast, and sound.

The check is centered around two design ideas: (1) initialization-time
irrelevance; (2) partial ordering.

The check enforces the principle of _initialization-time irrelevance_,
which means that the time when a static object is initialized should not
change program semantics. For that purpose, it enforces the following
rule:

> **The initialization of a static object should not directly or
indirectly read or write mutable state owned by another static object**.

This principle not only puts the initialization of static objects on a
solid foundation but also avoids whole-program analysis.

Partial ordering means that the initialization dependencies of static
objects form a directed-acyclic graph (DAG). No cycles with length
bigger than 1 allowed --- which might lead to deadlocks in the presence
of concurrency and strong coupling & subtle contracts between objects.

Related Issues:

#16152
#9176
#11262
scala/bug#9312
scala/bug#9115
scala/bug#9261
scala/bug#5366
scala/bug#9360
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Check of cyclic object initialization v3 dotty-staging/dotty
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