MediaConstraintsImpl.cpp:

    WebMediaConstraints standardForm = convertConstraintsToWeb(constraintsIn);
    if (constraintsIn.hasOptional() || constraintsIn.hasMandatory()) {
        if (!standardForm.isEmpty()) {
            errorState.throwTypeError("Malformed constraint: Cannot use both optional/mandatory and specific or advanced constraints.");
            return WebMediaConstraints();
        }

There should be an use counter. We have had one bug (#614716) where the user mixed those constraint forms and was surprised.