javac sees better types with new mixin forwarder scheme

Thanks to mixin forwarders now being emitted as bridges, we can resolve
some long-standing TODOs. I've also manually verified that this works
with ecj.

Author: smarter

test/files/run/t3452d/Test.java

@@ -1,11 +1,6 @@
-import scala.collection.immutable.Nil;
-import scala.collection.immutable.List;
-
 public class Test {
 	public static void main(String[] args) {
 		C<String> c = new C<String>();
-		// TODO add a bridge during mixin so we can expose
-		// sharper generic signature for `tail`.
-		/*Traversable<String>*/ Object ls = c.tail();
+		scala.collection.Iterable<String> ls = c.tail();
 	}
 }

test/files/run/t3452g/A.scala

@@ -4,6 +4,8 @@ trait TraversableLike[A, Repr] {
 
 abstract class AbstractTrav[A] extends TraversableLike[A, AbstractTrav[A]]
 
+class C1 extends AbstractTrav[String]
+
 object O extends AbstractTrav[String]
 
-class C[A] extends AbstractTrav[A]
+class C2[A] extends AbstractTrav[A]

test/files/run/t3452g/Test.java

@@ -1,14 +1,10 @@
-
 public class Test {
-	public static void main(String[] args) {
-		// To get better types here, we would need to
-		// add bridge during mixin so we can expose
-		// a generic return type of Traversable<A>, because the erasure
-		// of this (Traversable) differs from the erasure of the mixed
-		// method (erasure(Repr) = Object)
+  public static void main(String[] args) {
+    AbstractTrav<String> lsSharp1 = new C1().tail();
 
-		Object lsSharp = O.tail();
+    // Object is the result type for the static forwarder (might be because of #11305)
+    Object lsSharp2 = O.tail();
 
-		Object lsSharp2 = new C<String>().tail();
-	}
+    AbstractTrav<String> lsSharp3 = new C2<String>().tail();
+  }
 }