Limit bad advice given for erroneous pattern.

Don't suggest "_: <none>" as an alternative when the pattern
type doesn't conform to the expected type.

Author: paulp

src/compiler/scala/tools/nsc/typechecker/TypeDiagnostics.scala

@@ -285,8 +285,8 @@ trait TypeDiagnostics {
           case xs   => xs map (_ => "_") mkString (clazz.name + "[", ",", "]")
         })+ "`"
 
-    "\nNote: if you intended to match against the class, try "+ caseString
-
+    if (!clazz.exists) ""
+    else "\nNote: if you intended to match against the class, try "+ caseString
   }
 
   case class TypeDiag(tp: Type, sym: Symbol) extends Ordered[TypeDiag] {

test/files/neg/bad-advice.check

@@ -0,0 +1,6 @@
+bad-advice.scala:4: error: pattern type is incompatible with expected type;
+ found   : Bip.type
+ required: Int
+    case Bip => true
+         ^
+one error found

test/files/neg/bad-advice.flags

@@ -0,0 +1 @@
+-Xfatal-warnings

test/files/neg/bad-advice.scala

@@ -0,0 +1,6 @@
+object Bip
+object Test {
+  def f(x: Int) = x match {
+    case Bip => true
+  }
+}