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The national security law that China has imposed on Hong Kong is already curtailing speech in the territory - but it is having a far wider impact.
It applies to everyone in the world, everywhere in the world. People who break the law can be prosecuted if they go to Hong Kong.
That has brought an unexpected headache for foreign universities, which are scrambling to work out how to protect their students from saying and writing things that might later be used against them.
Foreign institutions renowned as bastions of free speech are having to deal with Chinese censorship.
Anyone who criticises China and travels to Hong Kong is potentially at risk of arrest under the new law.
But Hong Kong students studying abroad face a particular threat because they will at some point return to the former British territory. They cannot avoid going to Hong Kong in the same way foreigners can.
One big problem for students is that it is difficult to know what is allowed and what is illegal, because the law was drafted so broadly.
The legislation outlaws behaviour that undermines China's national security, but that even includes acts that provoke "hatred" of the Chinese government.
Does that include criticism of the government?
"A lot of key concepts are very ambiguous, so we don't know what speech has the potential to provoke," said Prof Tang.
Many think the ambiguity is deliberate, to spread fear and uncertainty.