Still giddy with the thrill of fixing #266, I vanquish another

pattern matcher bug from the dawn of time.  If you've always wanted
to write code like this:

  class Bob[K[_]] {
    def foo(other: Any) = other match {
      case x: (Bob[X] forSome { type X[_] })  =>
    }
  }

Now is your chance.  Closes #1427, review by moors.  (Is there
a better way to "shake off" the pattern existential?)

git-svn-id: http://lampsvn.epfl.ch/svn-repos/scala/scala/trunk@23159 5e8d7ff9-d8ef-0310-90f0-a4852d11357a

src/compiler/scala/tools/nsc/matching/Matrix.scala

@@ -155,8 +155,12 @@ trait Matrix extends MatrixAdditions {
       // XXX how will valsym.tpe differ from sym.tpe ?
       def tpe = valsym.tpe
 
-      lazy val ident  = ID(lhs)
-      lazy val valDef = tracing("typedVal", typer typedValDef (VAL(lhs) === rhs) setPos lhs.pos)
+      // See #1427 for an example of a crash which occurs unless we retype:
+      // in that instance there is an existential in the pattern.
+      lazy val ident  = typer typed { ID(lhs) setType null }
+      lazy val valDef = typer typedValDef {
+        (VAL(lhs) withType ident.tpe) === rhs
+      }
 
       override def toString() = "%s: %s = %s".format(lhs, lhs.info, rhs)
     }

test/files/run/bug1427.scala

@@ -0,0 +1,15 @@
+class Bob[K[_]] {
+  def foo(other: Any) = other match {
+    case x: (Bob[X] forSome { type X[_] })  => true
+    case _                                  => false
+  }
+}
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    val x = new Bob[List]
+    val results = List(x, new Bob[Set], 55) map (x foo _)
+    
+    assert(results == List(true, true, false))
+  }
+}