JDK-8203436 introduced a more strict mode of execution for javac so that, as soon as an attemp to write an illegal signature (containing a non-denotable type) was detected, the compiler will terminate abruptly with an error.

There are two issues with this strategy:

1) There are cases where the client code doesn't catch the exception and doesn't report to log gracefully (e.g. uses in LambdaToMethod)

2) There are use cases where it's ok to have illegal signatures, because they won't end up in classfile.

Test case:

import java.awt.*;
import java.awt.event.*;
import java.util.List;
import java.util.function.*;

class JDK11CompilerBug {

    interface IFilter {
        Component getComponent();
    }

    static class Filter implements IFilter {

        @Override
        public Component getComponent() { return null; }

    }

    public Component buildFilter(List<? extends Filter> l, Dialog dialog) {
        Panel c = new Panel();
        l.stream()
            .map(f -> {
                Button btn = (Button) f.getComponent();
                btn.addActionListener((java.io.Serializable & ActionListener)evt -> {
                    applyFilter(f);
                    dialog.setVisible(false);
                });
                return btn;
            })
            .forEach(c::add);
        return c;
    }

    private void applyFilter(IFilter f) {}

}

bug report and discussion: http://mail.openjdk.java.net/pipermail/compiler-dev/2018-September/012372.html