A smartphone application that gives each user an identity (like an email address, or one specific to the app) and shares their identity with people in close proximity. Distance is estimated through signal strength or something.

The ad-hoc communications would be similar to the AirDrop system that can transfer files between nearby Apple devices when enabled.

History of contacts would be tracked. If someone is infected with coronavirus, they can tell the app this, and it would send a message to everyone with recent contact with the person, based on an estimate of time of infection and how long they have been contagious.

Is there an app that can do this already? Is anyone working on one? Should it be developed, if it hasn't already?

Of course it wouldn't help for people who are too poor to have a smartphone, like me, but most people do have one. It also wouldn't help much to p̶r̶e̶v̶e̶n̶t̶ predict (prevention is from early self-isolation or treatment) infection from someone who has already left the vicinity, unless there was a (much more involved and centralized) process of tracking physical locations, possibly including moving public transport.