As pointed out to me by plocinic, the pattern m...

As pointed out to me by plocinic, the pattern matcher has been
indiscriminately clearing the MUTABLE flag on synthetic vals because it
is signalling itself with that bit and it didn't think anyone else would
ever notice. Someone did. Closes #3699, review by plocinic.

3 files changed, 28 insertions, 13 deletions

diff --git a/src/compiler/scala/tools/nsc/matching/Matrix.scala b/src/compiler/scala/tools/nsc/matching/Matrix.scala
index f7f7b2cc41..800200b897 100644
--- a/src/compiler/scala/tools/nsc/matching/Matrix.scala
+++ b/src/compiler/scala/tools/nsc/matching/Matrix.scala
@@ -8,6 +8,7 @@ package matching
 
 import transform.ExplicitOuter
 import symtab.Flags
+import scala.collection.mutable
 
 trait Matrix extends MatrixAdditions {
   self: ExplicitOuter with ParallelMatching =>
@@ -16,7 +17,7 @@ trait Matrix extends MatrixAdditions {
   import analyzer.Typer
   import CODE._
   import Debug._
-  import Flags.{ TRANS_FLAG, SYNTHETIC }
+  import Flags.{ TRANS_FLAG, SYNTHETIC, MUTABLE }
 
   /** Translation of match expressions.
    *
@@ -117,6 +118,19 @@ trait Matrix extends MatrixAdditions {
     // TRANS_FLAG communicates there should be no exhaustiveness checking
     private def flags(checked: Boolean) = if (checked) Nil else List(TRANS_FLAG)
 
+    // Recording the symbols of the synthetics we create so we don't go clearing
+    // anyone else's mutable flags.
+    private val _syntheticSyms = mutable.HashSet[Symbol]()
+    def clearSyntheticSyms() = {
+      _syntheticSyms foreach (_ resetFlag (TRANS_FLAG|MUTABLE))
+      log("Cleared TRANS_FLAG/MUTABLE on " + _syntheticSyms.size + " synthetic symbols.")
+      _syntheticSyms.clear()
+    }
+    def recordSyntheticSym(sym: Symbol): Symbol = {
+      _syntheticSyms += sym
+      sym
+    }
+
     case class MatrixInit(
       roots: List[PatternVar],
       cases: List[CaseDef],
@@ -235,7 +249,7 @@ trait Matrix extends MatrixAdditions {
     {
       val n: Name = if (name == null) newName("temp") else name
       // careful: pos has special meaning
-      owner.newVariable(pos, n) setInfo tpe setFlag (SYNTHETIC.toLong /: flags)(_|_)
+      recordSyntheticSym(owner.newVariable(pos, n) setInfo tpe setFlag (SYNTHETIC.toLong /: flags)(_|_))
     }
 
     def typedValDef(x: Symbol, rhs: Tree) =
diff --git a/src/compiler/scala/tools/nsc/matching/MatrixAdditions.scala b/src/compiler/scala/tools/nsc/matching/MatrixAdditions.scala
index 55613c5bd7..ee4eb64e89 100644
--- a/src/compiler/scala/tools/nsc/matching/MatrixAdditions.scala
+++ b/src/compiler/scala/tools/nsc/matching/MatrixAdditions.scala
@@ -127,18 +127,8 @@ trait MatrixAdditions extends ast.TreeDSL
           })
         }
       }
-      object resetTraverser extends Traverser {
-        import Flags._
-        def reset(vd: ValDef) =
-          if (vd.symbol hasFlag SYNTHETIC) vd.symbol resetFlag (TRANS_FLAG|MUTABLE)
-
-        override def traverse(x: Tree): Unit = x match {
-          case vd: ValDef => reset(vd)
-          case _          => super.traverse(x)
-        }
-      }
 
-      returning(lxtt transform tree)(resetTraverser traverse _)
+      returning(lxtt transform tree)(_ => clearSyntheticSyms())
     }
   }
 
diff --git a/test/files/run/bug3699.scala b/test/files/run/bug3699.scala
new file mode 100644
index 0000000000..0475353887
--- /dev/null
+++ b/test/files/run/bug3699.scala
@@ -0,0 +1,11 @@
+object Test {
+  def act: Int => Int = {
+    case _ =>
+      lazy val (a, b) = (3,9)
+      a
+      b
+  }
+  def main(args: Array[String]) = {
+    assert(act(1) == 9)
+  }
+}
\ No newline at end of file