40 to 60% [of patients in a sample who hadn't yet been exposed to the novel coronavirus] had CD4+ cells that already respond to the new coronavirus. This doesn’t mean that people have already been exposed to it per se, of course – immune crossreactivity is very much a thing, and it would appear that many people have already raised a response to other antigens that could be partially protective against this new virus.

https://blogs.sciencemag.org/pipeline/archives/2020/05/15/good-news-on-the-human-immune-response-to-the-coronavirus

https://www.cell.com/action/showPdf?pii=S0092-8674%2820%2930610-3

My understanding is that a lot of coronaviri just cause "the common cold".

I'd happily take a few weeks of cold symptoms if there was a 10% chance it might confer some protection. Probably even a 1% chance.

A little bit of previous discussion here mentions the possibility of immune enhanced infections..