FULL PRODUCT VERSION :
java version "9-ea"
Java(TM) SE Runtime Environment (build 9-ea+161)
Java HotSpot(TM) 64-Bit Server VM (build 9-ea+161, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
not OS specific (currently reproducing on Darwin syrah 16.4.0 Darwin Kernel Version 16.4.0: Thu Dec 22 22:53:21 PST 2016; root:xnu-3789.41.3~3/RELEASE_X86_64 x86_64)

A DESCRIPTION OF THE PROBLEM :
See the code, compiles with JDK8 alright, fails with latest JDK9 161.

/Users/rhusar/wildfly/clustering/jgroups/extension/src/main/java/org/jboss/as/clustering/jgroups/JChannelFactory.java:[68,13] error: no suitable method found for addProtocols(List<Object>)
    method ProtocolStack.addProtocols(Protocol...) is not applicable
      (varargs mismatch; inference variable R has incompatible bounds
          equality constraints: List<T#2>
          upper bounds: Protocol,Object)
    method ProtocolStack.addProtocols(List<Protocol>) is not applicable
      (argument mismatch; inference variable T#2 has incompatible bounds
          equality constraints: Protocol
          lower bounds: Object)
  where R,A,T#1,T#2 are type-variables:
    R extends Object declared in method <R,A>collect(Collector<? super T#1,A,R>)
    A extends Object declared in method <R,A>collect(Collector<? super T#1,A,R>)
    T#1 extends Object declared in interface Stream
    T#2 extends Object declared in method <T#2>toList()
[ERROR] /Users/rhusar/wildfly/clustering/jgroups/extension/src/main/java/org/jboss/as/clustering/jgroups/JChannelFactory.java:[72,76] error: cannot find symbol
  symbol:   method createProtocol(ProtocolStackConfiguration)
  location: variable pc of type Object

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
see code

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
the test attached compiles OK like jdk9
ACTUAL -
[ERROR] COMPILATION ERROR :
[INFO] -------------------------------------------------------------
[ERROR] /Users/rhusar/git/jdk9typebug/src/main/java/JChannelFactory.java:[14,14] no suitable method found for addProtocols(java.util.List<java.lang.Object>)
    method JChannelFactory.ProtocolStack.addProtocols(JChannelFactory.Protocol...) is not applicable
      (varargs mismatch; inference variable R has incompatible bounds
          equality constraints: java.util.List<T>
          upper bounds: JChannelFactory.Protocol,java.lang.Object)
    method JChannelFactory.ProtocolStack.addProtocols(java.util.List<JChannelFactory.Protocol>) is not applicable
      (argument mismatch; inference variable T has incompatible bounds
          equality constraints: JChannelFactory.Protocol
          lower bounds: java.lang.Object)
[ERROR] /Users/rhusar/git/jdk9typebug/src/main/java/JChannelFactory.java:[17,77] cannot find symbol
  symbol:   method createProtocol(JChannelFactory.ProtocolStackConfiguration)
  location: variable pc of type java.lang.Object
[INFO] 2 errors

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.util.List;
import java.util.Objects;
import java.util.function.Function;
import java.util.stream.Collectors;
import java.util.stream.Stream;

/**
 * @author Radoslav Husar
 */
public class JChannelFactory {
    void createChannel() {
        ProtocolStackConfiguration configuration = new ProtocolStackConfiguration();
        ProtocolStack stack = new ProtocolStack();
        stack.addProtocols(Stream.of(
                Stream.of(configuration.getTransport()),
                configuration.getProtocols().stream()
        ).flatMap(Function.identity()).filter(Objects::nonNull).map(pc -> pc.createProtocol(configuration)).collect(Collectors.toList()));
    }

    public abstract static class Protocol {
    }

    public interface ProtocolConfiguration<P extends Protocol> {
        P createProtocol(ProtocolStackConfiguration stackConfiguration);
    }

    static class ProtocolStackConfiguration {

        ProtocolConfiguration<? extends TP> getTransport() {
            return null;
        }

        List<ProtocolConfiguration<? extends Protocol>> getProtocols() {
            return null;
        }
    }

    static class TP extends Protocol {
    }

    public static class ProtocolStack {

        public ProtocolStack addProtocols(Protocol... prots) {
            return this;
        }

        public ProtocolStack addProtocols(List<Protocol> prots) {
            return this;
        }

    }
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
explicitly cast:

.map(pc -> ((ProtocolConfiguration)pc).createProtocol(configuration))