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Compiler crash using "implicit alternative constructor": assertion failed: type error: can't convert from UNIT to REF #4882

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scabug opened this issue Aug 6, 2011 · 4 comments
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@scabug scabug commented Aug 6, 2011

Experimenting with a snippet posted in a SO question I got a compiler crash. Here are the steps to reproduce it:

Welcome to Scala version 2.10.0.r25450-b20110806020344 (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_26).
Type in expressions to have them evaluated.
Type :help for more information.

scala> class A(i: Int) { implicit def this(s: String) = this(s.toInt) }
defined class A

scala> val a: A = new A(1)
a: A = A@7579f56e

scala> val a1: A = "1"
<console>:8: error: type mismatch;
 found   : java.lang.String("1")
 required: A
       val a1: A = "1"
                   ^

scala> import a._
import a._

scala> val a1: A = "1"
java.lang.AssertionError: assertion failed: type error: can't convert from UNIT to REF(class $read$$iw$$iw$A) in unit <console> at source-<console>,line-12,offset=178
	at scala.tools.nsc.backend.icode.GenICode$ICodePhase.adapt(GenICode.scala:1109)
	at scala.tools.nsc.backend.icode.GenICode$ICodePhase.scala$tools$nsc$backend$icode$GenICode$ICodePhase$$genLoad(GenICode.scala:1095)
	at scala.tools.nsc.backend.icode.GenICode$ICodePhase.scala$tools$nsc$backend$icode$GenICode$ICodePhase$$genStat(GenICode.scala:164)
	at scala.tools.nsc.backend.icode.GenICode$ICodePhase$$anonfun$genStat$1.apply(GenICode.scala:144)
	at scala.tools.nsc.backend.icode.GenICode$ICodePhase$$anonfun$genStat$1.apply(GenICode.scala:144)
	at scala.collection.LinearSeqOptimized$class.foldLeft(LinearSeqOptimized.scala:111)
	at scala.collection.immutable.List.foldLeft(List.scala:45)
	at scala.tools.nsc.backend.icode.GenICode$ICodePhase.scala$tools$nsc$backend$icode$GenICode$ICodePhase$$genLoad(GenICode.scala:1017)
	at scala.tools.nsc.backend.icode.GenICode$ICodePhase.gen(GenICode.scala:115)
	at scala.tools.nsc.backend.icode.GenICode$ICodePhase$$anonfun$gen$1.apply(GenICode.scala:71)
	at scala.tools.nsc.backend.icode.GenICode$ICodePhase$$anonfun$gen$1.apply(GenICode.scala:71)
	at scala.collection.LinearSeqOptimized$class.foreach(LinearSeqOptimized.scala:59)
	at scala.collection.immutable.List.foreach(List.scala:45)
	at scala.tools.nsc.backend.icode.GenICode$ICodePhase.gen(GenICode.scala:71)
	at scala.tools.nsc.backend.icode.GenICode$ICodePhase.gen(GenICode.scala:137)
	at scala.tools.nsc.backend.icode.GenICode$ICodePhase.gen(GenICode.scala:90)
	at scala.tools.nsc.backend.icode.GenICode$ICodePhase$$anonfun$gen$1.apply(GenICode.scala:71)
	at scala.tools.nsc.backend.icode.GenICode$ICodePhase$$anonfun$gen$1.apply(GenICode.scala:71)
	at scala.collection.LinearSeqOptimized$class.foreach(LinearSeqOptimized.scala:59)
	at scala.collection.immutable.List.foreach(List.scala:45)
	at scala.tools.nsc.backend.icode.GenICode$ICodePhase.gen(GenICode.scala:71)
	at scala.tools.nsc.backend.icode.GenICode$ICodePhase.gen(GenICode.scala:81)
	at scala.tools.nsc.backend.icode.GenICode$ICodePhase.gen(GenICode.scala:67)
	at scala.tools.nsc.backend.icode.GenICode$ICodePhase.apply(GenICode.scala:63)
	at scala.tools.nsc.Global$GlobalPhase.applyPhase(Global.scala:357)
	at scala.tools.nsc.Global$GlobalPhase$$anonfun$run$1.apply(Global.scala:325)
	at scala.tools.nsc.Global$GlobalPhase$$anonfun$run$1.apply(Global.scala:325)
	at scala.collection.Iterator$class.foreach(Iterator.scala:663)
	at scala.collection.mutable.ListBuffer$$anon$1.foreach(ListBuffer.scala:316)
	at scala.tools.nsc.Global$GlobalPhase.run(Global.scala:325)
	at scala.tools.nsc.backend.icode.GenICode$ICodePhase.run(GenICode.scala:56)
	at scala.tools.nsc.Global$Run.compileSources(Global.scala:1003)
	at scala.tools.nsc.interpreter.IMain$ReadEvalPrint.compileAndSaveRun(IMain.scala:758)
	at scala.tools.nsc.interpreter.IMain$ReadEvalPrint.compile(IMain.scala:733)
	at scala.tools.nsc.interpreter.IMain$Request.compile(IMain.scala:875)
	at scala.tools.nsc.interpreter.IMain.interpret(IMain.scala:578)
	at scala.tools.nsc.interpreter.IMain.interpret(IMain.scala:545)
	at scala.tools.nsc.interpreter.ILoop.reallyInterpret$1(ILoop.scala:693)
	at scala.tools.nsc.interpreter.ILoop.interpretStartingWith(ILoop.scala:744)
	at scala.tools.nsc.interpreter.ILoop.command(ILoop.scala:650)
	at scala.tools.nsc.interpreter.ILoop.processLine$1(ILoop.scala:541)
	at scala.tools.nsc.interpreter.ILoop.loop(ILoop.scala:549)
	at scala.tools.nsc.interpreter.ILoop.process(ILoop.scala:821)
	at scala.tools.nsc.interpreter.ILoop.main(ILoop.scala:850)
	at xsbt.ConsoleInterface.run(ConsoleInterface.scala:51)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at sbt.compiler.AnalyzingCompiler.call(AnalyzingCompiler.scala:57)
	at sbt.compiler.AnalyzingCompiler.console(AnalyzingCompiler.scala:48)
	at sbt.Console.console0$1(Console.scala:23)
	at sbt.Console$$anonfun$apply$2$$anonfun$apply$1.apply$mcV$sp(Console.scala:24)
	at sbt.TrapExit$.executeMain$1(TrapExit.scala:33)
	at sbt.TrapExit$$anon$1.run(TrapExit.scala:42)

That entry seems to have slain the compiler.  Shall I replay
your session? I can re-run each line except the last one.
[y/n]
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@scabug scabug commented Aug 6, 2011

Imported From: https://issues.scala-lang.org/browse/SI-4882?orig=1
Reporter: gerferra

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@scabug scabug commented Aug 6, 2011

Commit Message Bot (anonymous) said:
(extempore in r25454) Disallowed implicit modifier on auxiliary constructors, since it either
silently accomplishes nothing or crashes the compiler. If it should do
something useful let me know. Closes #4882, review by odersky.

@scabug scabug closed this Aug 6, 2011
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@scabug scabug commented Aug 6, 2011

gerferra said:
Wow! that was fast. Amazing.

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@scabug scabug commented Aug 6, 2011

@soc said (edited on Aug 6, 2011 3:21:17 PM UTC):
Thanks for the fast fix.

I think the debate about "implicit classes" which comes up once in a while is closely related to that.

One use-case for implicit constructors would be to replace the current "implicit def + wrapper class"-pattern:

class Foo(value: Int)

implicit def int2Foo(i: Int) = new Foo(i)

by an "equivalent":

class Foo implicit (value: Int)

So basically reducing the need for these simple "trampoline" methods calling constructors.

I'm not sure which role secondary secondary constructors would play here ...

In the end I would prefer it if primary/secondary constructors would behave as much as possible as "normal" methods in this question.

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