Fix mergeDenot one more time.

It's easy to get this wrong. What happened was that when
computing deferred members of a class a deferred member
was preferred over a concrete one because the types did not match.
Thsi should not happen. We now change the scheme to always
prefer concrete over abstract, and subclass-owned over superclass-owned.
But we pick a denotation only if the overrides relationship on types
coincides with the preference on symbols.

1 files changed, 19 insertions, 0 deletions

diff --git a/tests/pos/t2503.scala b/tests/pos/t2503.scala
new file mode 100755
index 000000000..d0983f2ca
--- /dev/null
+++ b/tests/pos/t2503.scala
@@ -0,0 +1,19 @@
+import scala.collection.mutable._
+
+trait SB[A] extends Buffer[A] {
+
+  import collection.Traversable
+
+  abstract override def insertAll(n: Int, iter: Traversable[A]): Unit = synchronized {
+     super.insertAll(n, iter)
+  }
+
+  abstract override def update(n: Int, newelem: A): Unit = synchronized {
+    super.update(n, newelem)
+  }
+}
+
+object Test extends dotty.runtime.LegacyApp {
+  new ArrayBuffer[Int] with SB[Int]
+}
+