Simplify Trees.flatten

Also: fix error annotations in test

Author: odersky

compiler/src/dotty/tools/dotc/ast/Trees.scala

@@ -779,9 +779,8 @@ object Trees {
               buf += scanned.head
               scanned = scanned.tail
             }
-            recur(buf, remaining)
           }
-          else recur(recur(buf, elems), remaining1)
+          recur(recur(buf, elems), remaining1)
         case tree :: remaining1 =>
           if (buf != null) buf += tree
           recur(buf, remaining1)

tests/neg/parser-stability-17.scala

@@ -1,2 +1,2 @@
-trait x0[] {      x0: x0 => }
-        class x0[x1] extends x0[x0 x0] x2 x0
+trait x0[] {      x0: x0 => }      // error // error
+        class x0[x1] extends x0[x0 x0] x2 x0  // error // error