First refactoring related to Error trees.

There are no more direct calls to context.error from Typers and Infer,
so more work needs to be done to finish it for Implicits and Namers.
I am pushing it to trunk so that all of you can share my pain (and
complain). Please do not add any more context.error randomly in that
code, instead deal with it appropriately (by creating specific error
tree).

I was trying to be as informative when it comes to error tree names
as possible, but if you feel like changing names to something more
appropriate then feel free to do so. When it comes to printing error
messages I tried to follow test suite as closily as possible but
obviously there were few changes to some tests (mostly positive, I
believe).

On my machine performance drawback was neglible but I am working on more
aggressive caching to reduce the penalty of containsError() calls even
more. Any suggestions welcome.

At the moment the code supports both styles i.e. throwing type errors
for the cases that are not yet handled and generating error trees. But
in the future we will drop the former completely (apart from cyclic
errors which can pop up almost everywhere).

Review by odersky, extempore and anyone who feels like it.

1 files changed, 7 insertions, 4 deletions

diff --git a/test/files/neg/t1878.check b/test/files/neg/t1878.check
index 4b9cfebde1..de59b023ab 100644
--- a/test/files/neg/t1878.check
+++ b/test/files/neg/t1878.check
@@ -1,15 +1,18 @@
-t1878.scala:3: error: _* may only come last
-  val err1 = "" match { case Seq(f @ _*, ',') => f }
-                                 ^
 t1878.scala:3: error: scrutinee is incompatible with pattern type;
  found   : Seq[A]
  required: java.lang.String
   val err1 = "" match { case Seq(f @ _*, ',') => f }
                                 ^
+t1878.scala:3: error: not found: value f
+  val err1 = "" match { case Seq(f @ _*, ',') => f }
+                                                 ^
+t1878.scala:3: error: _* may only come last
+  val err1 = "" match { case Seq(f @ _*, ',') => f }
+                                 ^
 t1878.scala:9: error: _* may only come last
   val List(List(_*, arg2), _) = List(List(1,2,3), List(4,5,6))
                  ^
 t1878.scala:13: error: _* may only come last
     case <p> { _* } </p> =>
                 ^
-four errors found
+5 errors found