Better handling of merge errors

Instead of picking one at random, throw a MergeError
which might be caught later in mergeDenot. MergeDenot
has enough info to pick a simulate Scala2 linarization
if the prefix comes from Scala2, or it rethrows the
exception so that it becomes a type error.

Author: odersky

src/dotty/tools/dotc/core/Denotations.scala

@@ -308,7 +308,17 @@ object Denotations {
                   else if (!sym2.exists) sym1
                   else if (preferSym(sym2, sym1)) sym2
                   else sym1
-                new JointRefDenotation(sym, info1 & info2, denot1.validFor & denot2.validFor)
+                val jointInfo =
+                  try info1 & info2
+                  catch {
+                    case ex: MergeError =>
+                      if (pre.widen.classSymbol.is(Scala2x))
+                        info1 // follow Scala2 linearization -
+                              // compare with way merge is performed in SymDenotation#computeMembersNamed
+                      else
+                        throw new MergeError(s"${ex.getMessage} as members of type ${pre.show}")
+                  }
+                new JointRefDenotation(sym, jointInfo, denot1.validFor & denot2.validFor)
               }
             }
           } else NoDenotation

src/dotty/tools/dotc/core/TypeComparer.scala

@@ -1057,6 +1057,8 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
     println(TypeComparer.explained { implicit ctx => tp1 <:< tp2})
     assert(false, s"andConflict ${tp1.show} and ${tp2.show}")
     */
+    if (!ctx.isAfterTyper && !ctx.mode.is(Mode.Scala2Unpickling))
+      throw new MergeError(msg)
     ctx.warning(msg, ctx.tree.pos)
     winner
   }

src/dotty/tools/dotc/core/Types.scala

@@ -492,10 +492,8 @@ object Types {
 
       try go(this)
       catch {
-        case ex: MergeError =>
-          throw new MergeError(s"${ex.getMessage} as members of type ${pre.show}")
         case ex: Throwable =>
-          ctx.println(i"findMember exception for $this member $name")
+          core.println(i"findMember exception for $this member $name")
           throw ex // DEBUG
       }
       finally {

tests/neg/i827.scala

@@ -0,0 +1,7 @@
+trait A { trait Inner }
+trait B { self: A =>
+  trait Inner extends self.Inner
+}
+
+
+class C extends C