The following code compiles fine in Java-8 while fails with Java-9ea111:

import java.util.function.*;
import java.util.stream.*;
import java.util.*;

public class Test {
    interface MyStream<T> extends Stream<T> {
        public <U> List<U> toFlatList(Function<? super T, ? extends Collection<U>> mapper);
    }

    static class MyStreamSupplier<T> {
        public MyStream<T> get() {return null;}
    }

    public static <T> void myStream(Supplier<Stream<T>> base, Consumer<MyStreamSupplier<T>> consumer) {
    }

    public static void assertEquals(Object expected, Object actual) { }

    public void test() {
        List<List<String>> strings = Arrays.asList();
        List<String> expectedList = Arrays.asList();
        myStream(strings::stream, supplier -> {
            assertEquals(expectedList, supplier.get().toFlatList(Function.identity()));
        });
    }
}

Compiling with 9-ea+111 I see the following error message:

Test.java:23: error: method toFlatList in interface MyStream<T#1> cannot be applied to given types;
            assertEquals(expectedList, supplier.get().toFlatList(Function.identity()));
                                                     ^
  required: Function<? super List<String>,? extends Collection<U>>
  found: Function<Object,Object>
  reason: inference variable T#2 has incompatible bounds
    upper bounds: Collection<Object>,Object
    lower bounds: List<String>
  where U,T#1,T#2 are type-variables:
    U extends Object declared in method <U>toFlatList(Function<? super T#1,? extends Collection<U>>)
    T#1 extends Object declared in interface MyStream
    T#2 extends Object declared in method <T#2>identity()
1 error

Regression:
Compiles fine with javac 8u25, 8u40, 8u60, 8u71, 9ea57, 9ea80, 9ea82, 9ea83, 9ea91. Fails with 9ea105, 9ea111 (sorry, I have no 9ea versions installed between 9ea91 and 9ea105).

Work-around:
Replace Function.identity() with x -> x.