Better pattern matcher error message.

For the common case when someone hasn't quite grokked
the significance of lower case in a pattern match.  I'd
like to make all the unreachables errors, not warnings,
but there may be a bug or two to clear out first.

class A {
  def badEquals(x: Any, y: Any) = x match {
    case y => true
    case _ => false
  }
}

a.scala:3: warning: patterns after a variable pattern cannot match (SLS 8.1.1)
If you intended to match against parameter y of method badEquals, you must use backticks, like: case `y` =>
    case y => true
         ^
a.scala:4: warning: unreachable code due to variable pattern 'y' on line 3
    case _ => false
              ^
two warnings found

1 files changed, 29 insertions, 0 deletions

diff --git a/test/files/neg/newpat_unreachable.scala b/test/files/neg/newpat_unreachable.scala
new file mode 100644
index 0000000000..c9cc85cec6
--- /dev/null
+++ b/test/files/neg/newpat_unreachable.scala
@@ -0,0 +1,29 @@
+object Test {
+  class A {
+    val d = 55
+
+    def contrivedExample[A, B, C](a: A, b: B, c: C): Unit = a match {
+      case b => println("matched b")
+      case c => println("matched c")
+      case d => println("matched d")
+      case _ => println("matched neither")
+    }
+
+    def correctExample[A, B, C](a: A, b: B, c: C): Unit = a match {
+      case `b` => println("matched b")
+      case `c` => println("matched c")
+      case `d` => println("matched d")
+      case _ => println("matched neither")
+    }
+
+    def f[A](a: A) = {
+      def g[B](b: B) = {
+        def h[C](c: C) = a match {
+          case b => 1
+          case c => 2
+          case _ => 3
+        }
+      }
+    }
+  }
+}