Emit all bridge methods non-final

src/compiler/scala/tools/nsc/transform/Erasure.scala

@@ -565,7 +565,7 @@ abstract class Erasure extends InfoTransform
       if (!bridgeNeeded)
         return
 
-      var newFlags = (member.flags | BRIDGE | ARTIFACT) & ~(ACCESSOR | DEFERRED | LAZY)
+      var newFlags = (member.flags | BRIDGE | ARTIFACT) & ~(ACCESSOR | DEFERRED | LAZY | FINAL)
       // If `member` is a ModuleSymbol, the bridge should not also be a ModuleSymbol. Otherwise we
       // end up with two module symbols with the same name in the same scope, which is surprising
       // when implementing later phases.

test/files/run/t12532.scala

@@ -0,0 +1,27 @@
+class Sync
+
+class Async extends Sync { def x = 1 }
+
+trait Base {
+  def foo: Sync
+}
+
+trait BaseSync extends Base {
+  override def foo: Sync
+}
+
+trait BaseAsync extends Base  {
+  override def foo: Async
+}
+
+abstract class ImplAsync extends BaseAsync {
+  final override def foo: Async = new Async
+}
+
+final class StrangeClass extends ImplAsync with BaseSync
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    assert((new StrangeClass).foo.x == 1)
+  }
+}