intrusive_ptr<Expression> parseIdExpression(const intrusive_ptr<ExpressionContext>& expCtx,
                                            BSONElement groupField,
                                            const VariablesParseState& vps) {
    if (groupField.type() == Object) {
        // {_id: {}} is treated as grouping on a constant, not an expression
        if (groupField.Obj().isEmpty()) {
            return ExpressionConstant::create(expCtx.get(), Value(groupField));
        }

        const BSONObj idKeyObj = groupField.Obj();
        if (idKeyObj.firstElementFieldName()[0] == '$') {
            // grouping on a $op expression
            return Expression::parseObject(expCtx.get(), idKeyObj, vps);
        } else {
            for (auto&& field : idKeyObj) {
                uassert(17390,
                        "$group does not support inclusion-style expressions",
                        !field.isNumber() && field.type() != Bool);
            }
            return ExpressionObject::parse(expCtx.get(), idKeyObj, vps);
        }
    } else {
        return Expression::parseOperand(expCtx.get(), groupField, vps);
    }
}
Unknown