TRANSIT GUANGZHOU > HONGKONG > GUANGZHOU : VISA OR NOT VISA ?

Good morning. I am unable to obtain a clear answer to my question which consists of knowing whether a visa is necessary for a French citizen who has an 18-hour stopover in Guangzhou before going to Bali, and who would like to go to Hong Kong for a quick day trip? Thank you for your response and for all the contributions that make this site great. Yours truly. Cristian