Better error when abstract methods are missing.

When many methods are missing, print a list of signatures the way they
need to be implemented, and throw in ??? stub implementations so it
should be compilable code. If anyone would like this logic exposed more
generally (for the IDE or whatever) just let me know. No review.

1 files changed, 18 insertions, 8 deletions

diff --git a/test/files/neg/t2213.check b/test/files/neg/t2213.check
index f59503ee2a..9fb3bb2eb7 100644
--- a/test/files/neg/t2213.check
+++ b/test/files/neg/t2213.check
@@ -1,15 +1,25 @@
 t2213.scala:9: error: class C needs to be abstract, since:
-value y in class A of type Int is not defined
-value x in class A of type Int is not defined
-method g in class A of type => Int is not defined
-method f in class A of type => Int is not defined
+it has 4 unimplemented members.
+/** As seen from class C, the missing signatures are as follows.
+ *  For convenience, these are usable as stub implementations.
+ */
+  def f: Int = ???
+  def g: Int = ???
+  val x: Int = ???
+  val y: Int = ???
+
 class C extends A {}
       ^
 t2213.scala:11: error: object creation impossible, since:
-value y in class A of type Int is not defined
-value x in class A of type Int is not defined
-method g in class A of type => Int is not defined
-method f in class A of type => Int is not defined
+it has 4 unimplemented members.
+/** As seen from object Q, the missing signatures are as follows.
+ *  For convenience, these are usable as stub implementations.
+ */
+  def f: Int = ???
+  def g: Int = ???
+  val x: Int = ???
+  val y: Int = ???
+
 object Q extends A { }
        ^
 two errors found