Missing open-source contributor presents a dilemma when accepting their PR

Unfortunately, this would be intentional copyright infringement (assuming the code is copyrightable, blah blah blah), since you are doing it on purpose with knowledge that it is copyrighted.

In a number of countries, copyright infringement is also strict liability - it doesn't matter if you had any intent to commit it, but if you did, the damages often start much much higher. So the former case you'd probably have some nominal statutory damages, assuming you can't prove any actual loss. But in the later case, those damages get quite high.

In the US, for example, statutory damages for intentional copyright infringement (IE you don't have to prove any actual damage) are 150k per infringement.

I make no claims any of this makes sense, or someone will actually sue you, or that you should do anything different than "nothing".

My only claim is that "and I find it hard to see how damages could be levied in this situation." is totally the wrong view in a lot of countries - you should expect, if it did get to that point, you would have plenty of damages levied against you.

The author appears to be in the UK, where statutory damages for infringement were historically not available. but post-brexit, they were actually doing consultation/blah blah blah on making them available. I have no idea what happened.

But even if they have no statutory damages, it won't prevent you from being sued wherever the contributor is, and having that law apply rather than your home law :)

It just makes it harder to collect.

A confirmation is simply unnecessary. Can't it work like, writing this somewhere that says, by creating a pull request, you agree all your code and the discussions around the pull request is now copyright free? Saves everybody time and avoid hassles like this.

The other side of this is I get very annoyed by CLAs -- there have been a number of times I want to contribute to Google and Microsoft's open source projects, but they all require CLAs which require me to get explicit permissions from my employer to contribute to those projects. It is possible, but is a slow and complicated process that nobody wants to go through at my company. So instead of creating a pull request to address the problem, I open an issue and mention how it can be addressed. Which may or may not be picked up by someone else who wants to work on this. This is just frustrating.