I'd like to see that backed up, really. There are certainly criticisms to be made of how Apple has handled their open source projects (slow to update and push out new releases, not great at cooperating with upstream projects when Apple has forked them, typically cryptic Apple communication practices). But if Apple has taken open source projects, created closed-source forks of them, and hired original developers of and/or major contributors to the original open source project to maintain and develop the closed source fork without contributing back to the open source original, none of them immediately come to mind.

I got direct quotes from Apple employees about how their employment has banned them from doing anything related to GPLv3 code in their free time, specifically GCC.

I don't want to mention them in a public website in case that gets them in trouble.

That doesn't surprise me. The GPL, especially v3 and AGPL, are specific weird cases at a lot of companies -- sometimes because they want the freedom to close of forks, but often because they're convinced the GPL/AGPL could "poison" their work and force them to open previously closed source. (This turned out to be a problem for us at RethinkDB, which was AGPL-licensed.)

Well, depending on the employee, it would make sense. Apple funded a large part of clang/llvm, which many assume was specifically to avoid a GPL compiler. You'd hate to accidentally pollute a BSD(-ish) project with GPL code, if your primary purpose for funding the project is to avoid GPL code. Not that anyone would purposefully do such a thing, but it would be important for Apple to even avoid the appearance of anything like that.

Apple certainly hires people from their open source projects and keeps closed-source forks. Most of the changes make their way back to their original projects, though, but the caveat is that this only seems to happen if the project is used heavily outside of Apple and after the feature has "shipped" (which may be a while after the code was actually written).

I remember finding an article on HN about a developer for FreeBSD going to work on Mac OS[1], before returning to work in open source. Isn't the Mac OS user space an example of what you describe?

[1] https://www.wired.com/2013/08/jordan-hubbard/

I don't think I'd say it is, no, because Hubbard wasn't hired to work on a closed-source fork of FreeBSD at Apple but rather as an engineering manager. AFAIK, Apple did not make any closed forks of major BSD-related work.

They've certainly hired people to work on open-source projects and keep them open-source (e.g., LLVM, CUPS). What closed-source forks have they made?

Apple keeps internal forks of projects where they can do so legally; it helps them prevent leaks and lets them add new features quicker. These changes often do end up getting merged back in, but this is usually after they have shipped.