JCExpression makeString(JCExpression tree) {
        if (tree.type.tag >= CLASS) {
            return tree;
        } else {
            Symbol valueOfSym = lookupMethod(tree.pos(),
                                             names.valueOf,
                                             syms.stringType,
                                             List.of(tree.type));
            return make.App(make.QualIdent(valueOfSym), List.of(tree));
        }
    }