The javadoc specification for the MethodHandleProxies.asInterfaceInstance method states:
  "The interface must be public. No additional access checks are performed."

However, this method still accepts private/default-access/protected *nested* interfaces.

Minimized test:
===============
--- Test.java ---
import java.lang.invoke.*;
import static java.lang.invoke.MethodType.*;
import static java.lang.invoke.MethodHandles.*;
import static java.lang.invoke.MethodHandleProxies.*;

public class Test {
   /*protected*/ /*private*/ interface PrivateIntf {
       void m();
   }

   public static class C {
       public static void m() {}
   }

   public static void main(String argv[]) throws Throwable {
       MethodHandle mh = lookup().findStatic(
               C.class, "m", methodType(void.class));

       PrivateIntf intf = asInterfaceInstance(PrivateIntf.class, mh);
   }
}
--- Test.java ---

Minimized test output:
======================
$ java -showversion -verify Test
java version "1.7.0-ea"
Java(TM) SE Runtime Environment (build 1.7.0-ea-b145)
Java HotSpot(TM) Server VM (build 21.0-b15, mixed mode)