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, 27 insertions, 0 deletions

diff --git a/test/files/neg/newpat_unreachable.check b/test/files/neg/newpat_unreachable.check
new file mode 100644
index 0000000000..08453cac19
--- /dev/null
+++ b/test/files/neg/newpat_unreachable.check
@@ -0,0 +1,27 @@
+newpat_unreachable.scala:6: error: patterns after a variable pattern cannot match (SLS 8.1.1)
+If you intended to match against parameter b of method contrivedExample, you must use backticks, like: case `b` =>
+      case b => println("matched b")
+           ^
+newpat_unreachable.scala:7: error: unreachable code due to variable pattern 'b' on line 6
+If you intended to match against parameter c of method contrivedExample, you must use backticks, like: case `c` =>
+      case c => println("matched c")
+                       ^
+newpat_unreachable.scala:8: error: unreachable code due to variable pattern 'b' on line 6
+If you intended to match against value d in class A, you must use backticks, like: case `d` =>
+      case d => println("matched d")
+                       ^
+newpat_unreachable.scala:9: error: unreachable code due to variable pattern 'b' on line 6
+      case _ => println("matched neither")
+                       ^
+newpat_unreachable.scala:22: error: patterns after a variable pattern cannot match (SLS 8.1.1)
+If you intended to match against parameter b of method g, you must use backticks, like: case `b` =>
+          case b => 1
+               ^
+newpat_unreachable.scala:23: error: unreachable code due to variable pattern 'b' on line 22
+If you intended to match against parameter c of method h, you must use backticks, like: case `c` =>
+          case c => 2
+                    ^
+newpat_unreachable.scala:24: error: unreachable code due to variable pattern 'b' on line 22
+          case _ => 3
+                    ^
+7 errors found