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