fix for SI-5672

3 files changed, 25 insertions, 5 deletions

diff --git a/src/compiler/scala/tools/nsc/backend/icode/Members.scala b/src/compiler/scala/tools/nsc/backend/icode/Members.scala
index 71795a02aa..019e887c4e 100644
--- a/src/compiler/scala/tools/nsc/backend/icode/Members.scala
+++ b/src/compiler/scala/tools/nsc/backend/icode/Members.scala
@@ -257,11 +257,23 @@ trait Members {
           var succ = bb
           do {
             succ = nextBlock(succ);
-            bb.removeLastInstruction
-            succ.toList foreach { i => bb.emit(i, i.pos) }
-            code.removeBlock(succ)
+            val lastInstr = bb.lastInstruction
+            /* Ticket SI-5672
+             * Besides removing the control-flow instruction at the end of `bb` (usually a JUMP), we have to pop any values it pushes.
+             * Examples:
+             *   `SWITCH` consisting of just the default case, or
+             *   `CJUMP(targetBlock, targetBlock, _, _)` ie where success and failure targets coincide (this one consumes two stack values).
+             */
+            val oldTKs = lastInstr.consumedTypes
+            assert(lastInstr.consumed == oldTKs.size, "Someone forgot to override consumedTypes() in " +  lastInstr)
+
+              bb.removeLastInstruction
+              for(tk <- oldTKs.reverse) { bb.emit(DROP(tk), lastInstr.pos) }
+              succ.toList foreach { i => bb.emit(i, i.pos) }
+              code.removeBlock(succ)
+              exh foreach { e => e.covered = e.covered - succ }
+
             nextBlock -= bb
-            exh foreach { e => e.covered = e.covered - succ }
           } while (nextBlock.isDefinedAt(succ))
           bb.close
         } else
diff --git a/src/compiler/scala/tools/nsc/backend/icode/Opcodes.scala b/src/compiler/scala/tools/nsc/backend/icode/Opcodes.scala
index 576cc72f82..3179fc5c56 100644
--- a/src/compiler/scala/tools/nsc/backend/icode/Opcodes.scala
+++ b/src/compiler/scala/tools/nsc/backend/icode/Opcodes.scala
@@ -436,6 +436,8 @@ trait Opcodes { self: ICodes =>
       override def consumed = 1
       override def produced = 0
 
+      override val consumedTypes = List(INT)
+
       def flatTagsCount: Int = { var acc = 0; var rest = tags; while(rest.nonEmpty) { acc += rest.head.length; rest = rest.tail }; acc } // a one-liner
     }
 
@@ -470,6 +472,8 @@ trait Opcodes { self: ICodes =>
 
       override def consumed = 2
       override def produced = 0
+
+      override val consumedTypes = List(kind, kind)
     }
 
     /** This class represents a CZJUMP instruction
@@ -489,6 +493,8 @@ trait Opcodes { self: ICodes =>
 
       override def consumed = 1
       override def produced = 0
+
+      override val consumedTypes = List(kind)
     }
 
 
@@ -499,6 +505,8 @@ trait Opcodes { self: ICodes =>
     case class RETURN(kind: TypeKind) extends Instruction {
       override def consumed = if (kind == UNIT) 0 else 1
       override def produced = 0
+
+      // TODO override val consumedTypes = List(kind)
     }
 
     /** This class represents a THROW instruction
diff --git a/src/compiler/scala/tools/nsc/typechecker/PatternMatching.scala b/src/compiler/scala/tools/nsc/typechecker/PatternMatching.scala
index 61e02edaff..c48c81ea7a 100644
--- a/src/compiler/scala/tools/nsc/typechecker/PatternMatching.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/PatternMatching.scala
@@ -1646,7 +1646,7 @@ trait PatternMatching extends Transform with TypingTransformers with ast.TreeDSL
       // TODO: if patterns allow switch but the type of the scrutinee doesn't, cast (type-test) the scrutinee to the corresponding switchable type and switch on the result
       if (regularSwitchMaker.switchableTpe(scrutSym.tpe)) {
         val caseDefsWithDefault = regularSwitchMaker(cases map {c => (scrutSym, c)}, pt)
-        if (caseDefsWithDefault.length <= 2) None // not worth emitting a switch... also, the optimizer has trouble digesting tiny switches, apparently, so let's be nice and not generate them
+        if (caseDefsWithDefault isEmpty) None // not worth emitting a switch.
         else {
           // match on scrutSym -- converted to an int if necessary -- not on scrut directly (to avoid duplicating scrut)
           val scrutToInt: Tree =