-Ydebug has been sabotaged

[scabug]

Driving me bonkers, keep failing to open. Does nobody use -Ydebug?

% scalac ./src/library/scala/collection/parallel/ParMapLike.scala  -Ydebug
...
[running phase icode on ParMapLike.scala]
The last instruction is not a control flow instruction: PJUMP method matchEnd5
	at scala.reflect.internal.SymbolTable.abort(SymbolTable.scala:51)
	at scala.tools.nsc.Global.abort(Global.scala:251)
	at scala.tools.nsc.backend.icode.ICodes.dumpClassesAndAbort(ICodes.scala:61)
	at scala.tools.nsc.backend.icode.BasicBlocks$BasicBlock.directSuccessors(BasicBlocks.scala:488)
	at scala.tools.nsc.backend.icode.BasicBlocks$BasicBlock$SuccessorList.updateConserve(BasicBlocks.scala:50)
	at scala.tools.nsc.backend.icode.BasicBlocks$BasicBlock$SuccessorList.updateSuccs(BasicBlocks.scala:104)
	at scala.tools.nsc.backend.icode.BasicBlocks$BasicBlock.successors(BasicBlocks.scala:472)
	at scala.tools.nsc.backend.icode.BasicBlocks$BasicBlock$$anonfun$predecessors$1.apply(BasicBlocks.scala:496)
	at scala.tools.nsc.backend.icode.BasicBlocks$BasicBlock$$anonfun$predecessors$1.apply(BasicBlocks.scala:496)

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-7427?orig=1
Reporter: @paulp

[scabug]

@paulp said:
Regressed in b50a0d811f, hello J. Iry.

 commit b50a0d811f
 Author: James Iry <jamesiry@gmail.com>
 Date:   8 weeks ago
 
     SI-7006 Prevent unreachable blocks in GenICode

     This commit makes GenICode prevent the generation of
     most unreachable blocks. The new unreachable block prevention code can
     be disabled with a compiler flag. ...

[scabug]

@paulp said:
If it isn't easy to fix, just not crashing -Ydebug would be great.

[scabug]

@paulp said:
I guess I will be fixing this myself.

[scabug]

@paulp said:
In case there was any question, here's a repro. It doesn't take much.

trait Bippy { 3 match { case 3 => } }

It crashes with -Ydebug by itself; compiles with no options, or with -Ydebug -Ydisable-unreachable-prevention.

[scabug]

@paulp said:
16e92f48e7b07

(Whatever the underlying issue is remains, but the crash is now a warning.)