Overload issue - more-specific-ness

Thu Mar 13 23:48:12 UTC 2014

Consider this program:
 // like Runnable, but throws
 interface RunnableX extends Callable<Void>
 {
 void run() throws Exception;
 default Void call() throws Exception
 {
 run();
 return null;
 }
 }
 static void foo(RunnableX r){}
 static void foo(Callable<List<?>> c){}
The overload should be fine, because both functional interfaces are
0-arg. And foo#2 should be more specific than foo#1.
However the following program fails to compile
 public void test()
 {
 foo( ()->new ArrayList<Void>() );
 // javac: reference to foo is ambiguous
 // both method foo(RunnableX) and method foo(Callable<List<?>>) match
 }
Is it the correct behavior or a bug? It seems to me that foo#2 should
be the most-specific method here.
( If RunnableX does NOT extend Callable<Void>, the program compiles.)
Thanks,
Zhong Yu