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scabug opened this issue Apr 4, 2015 · 1 comment
Open

Circular object dependency results in null field #9261

scabug opened this issue Apr 4, 2015 · 1 comment
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@scabug
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scabug commented Apr 4, 2015

The following code compiles without errors but results in a field unexpectedly being null:

$ scala
Welcome to Scala version 2.11.6 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_40).
scala> :paste
// Entering paste mode (ctrl-D to finish)

sealed abstract class OrderType(val reverse: OrderType)
case object Buy extends OrderType(Sell)
case object Sell extends OrderType(Buy)

// Exiting paste mode, now interpreting.

defined class OrderType
defined object Buy
defined object Sell

scala> Buy.reverse
res0: OrderType = Sell

scala> Sell.reverse
res1: OrderType = null

(May or may not be related to #9115 and/or #5366.)
@scabug
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scabug commented Apr 4, 2015

Imported From: https://issues.scala-lang.org/browse/SI-9261?orig=1
Reporter: Mirko Nasato (mnasato)
Affected Versions: 2.11.6
See #5366, #9115

@scabug scabug added the backend label Apr 7, 2017
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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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