Contradiction: had an empty possible set indicating an uninitialized location

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When compiling scala-io compiler crashes with:

[info] java.lang.AssertionError: assertion failed: Contradiction: had an empty possible set indicating an uninitialized location
[info] 	at scala.tools.nsc.backend.opt.ConstantOptimization$ConstantOptimizer$Possible.<init>(ConstantOptimization.scala:166)
[info] 	at scala.tools.nsc.backend.opt.ConstantOptimization$ConstantOptimizer.interpretLast(ConstantOptimization.scala:500)
[info] 	at scala.tools.nsc.backend.opt.ConstantOptimization$ConstantOptimizer.interpretBlock(ConstantOptimization.scala:552)
[info] 	at scala.tools.nsc.backend.opt.ConstantOptimization$ConstantOptimizer.interpretMethod(ConstantOptimization.scala:610)
[info] 	at scala.tools.nsc.backend.opt.ConstantOptimization$ConstantOptimizer.optimizeMethod(ConstantOptimization.scala:65)
[info] 	at scala.tools.nsc.backend.opt.ConstantOptimization$ConstantOptimizer.optimizeClass(ConstantOptimization.scala:57)
[info] 	at scala.tools.nsc.backend.opt.ConstantOptimization$ConstantOptimizationPhase.apply(ConstantOptimization.scala:49)
[info] 	at scala.tools.nsc.backend.icode.ICodes$ICodePhase$$anonfun$apply$2.apply(ICodes.scala:123)
[info] 	at scala.tools.nsc.backend.icode.ICodes$ICodePhase$$anonfun$apply$2.apply(ICodes.scala:123)

It can be reproduced with dbuild using this branch: https://github.com/gkossakowski/community-builds/tree/play

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Imported From: https://issues.scala-lang.org/browse/SI-8306?orig=1
Reporter: @gkossakowski

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@gkossakowski said:
I'll be trying to isolate and minimize this.

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@retronym said:
Yay for dbuild!

You might also be able to workaround (to find other problems) with -Yconst-opt:false.

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@gkossakowski said:
Minimized (compile with -optimize):

class Si8306 {
	def foo: Int = 123
	lazy val extension: Int =
    	foo match {
      		case idx if idx != -1 => 15
      		case _ => 17
    	}
}

Lazy val is important here. If I change it to val or def then the crash is gone.

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@magarciaEPFL said:
Just checked, the new optimizer http://magarciaepfl.github.io/scala/ isn't prone to this bug.

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@gkossakowski said:
Thanks for checking!

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@retronym said:
patmat emits a switch with just a default case:

(x1: Int) match {
  case _ => if (x1.!=(-1)) 15 else17
}

That's enough to lead to an empty set of integer tags in the SWITCH in the icode tree, which trips the assertion in constant optimization.

We could change one or more of patmat, genicode, and constopt to improve the situation.

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@gkossakowski said:
I think patmat is fine here. Switch with just default case should be supported by the backend. I think the same goes for genicode.

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@adriaanm said:
scala/scala#3560