private void assertConvertible(JCTree tree, Type actual, Type formal, Warner warn) {
        if (types.isConvertible(actual, formal, warn))
            return;

        if (formal.isCompound()
            && types.isSubtype(actual, types.supertype(formal))
            && types.isSubtypeUnchecked(actual, types.interfaces(formal), warn))
            return;

        if (false) {
            // TODO: make assertConvertible work
            chk.typeError(tree.pos(), diags.fragment("incompatible.types"), actual, formal);
            throw new AssertionError("Tree: " + tree
                                     + " actual:" + actual
                                     + " formal: " + formal);
        }
    }