FooTest.java

Foo foo = new AutoValue_Foo.Builder().list(new ArrayList<String>()).build();
List<String> listNoWildcard = (List<String>) foo.list();
List<? extends String> listWithWildcard = foo.list();
Stream<? extends string> s = listWithWildcard.stream();

// alright, let's reduce this
s.reduce("", new BinaryOperator<String>() {
    // error: The method reduce(capture#3-of ? extends String, BinaryOperator<capture#3-of ? extends String>)
    // in the type Stream<capture#3-of ? extends String> is not applicable for the arguments
    // (String, new BinaryOperator<String>(){})

    @Override
    public String apply(String t, String u) {
        return t + u;
    }

});

// ok, let's satisfy that
s.reduce("", new BinaryOperator<? extends String>() {
    // error: The type new BinaryOperator(){} cannot extend or implement BinaryOperator<? extends String>.
    // A supertype may not specify any wildcard

    @Override
    public String apply(String t, String u) {
        return t + u;
    }

});
// And that is impossible to fix, so I need to cast
((Stream<String>) s).reduce("", new BinaryOperator<String>() {

    @Override
    public String apply(String t, String u) {
        return t + u;
    }

});
// unchecked casts are not good...but that's the only option