Does the Java compiler hallucinate?

    private ButtonGroup groupButtons(AbstractButton... buttons) {
        return groupButtons(() -> new ButtonGroup(), buttons);
    }

    private <BG extends ButtonGroup> BG groupButtons(Supplier<BG> buttonGroupSupplier, AbstractButton... buttons) {
        return groupButtons(buttonGroupSupplier.get(), buttons);
    }

    private <BG extends ButtonGroup> BG groupButtons(BG buttonGroup, AbstractButton... buttons) {
        for (AbstractButton button : buttons) {
            buttonGroup.add(button);
        }
        return buttonGroup;
    }

It doesn’t like the first call

Ambiguous method call.

Both groupButtons (Supplier, AbstractButton…) in FormItemTableEditDlg and

groupButtons(ButtonGroup, AbstractButton…) in FormItemTableEditDlg match

Nope, they don’t!

It’s clearly a lambda, not a javax.swing.ButtonGroup (it’s not even an interface which is plain as day even at compile-time)

Yes, I can cast, but I don’t want to. It should be accepted by javac as is

Why is it ambiguous to Java?

Java 8