Quite the opposite, any developer that so much as glances at the leaked source code is banned from the project. The legality of ReactOS rests upon its developers reverse engineering Windows without having access to the source code.
As far as I'm aware you can however look at the leaked source code and use those insights to fix or write documentation about the behavior of the APIs and general behavior. You just can't write any ReactOS code yourself because you are tained once you've seen the leak. But I think the overlap between people who can extract insights from the leaked code and people who enjoy writing documentation in their free time (more than reverse engineering and programming) is pretty small.
> As far as I'm aware you can however look at the leaked source code and use those insights to fix
You can, but if your solution is at all like theirs the onus will be on you (what-ever legal team you can afford against their 800lb gorilla) to prove that you didn't copy it.
It is far safer not to look at all, and IIRC ReactOS's developer guidelines prohibit their devs from doing so for the avoidance of doubt.
> or write documentation about the behavior of the APIs and general behavior.
What Compaq did to implement the behaviour of IBM's BIOS (the only part of the original PC that wasn't off-the-shelf) was to double-clean-room. Two completely separate teams were on the job: in one "clean room" they analysed the chip's behaviour in detail and documented everything significant, and in the other a team implemented a design that mimicked said behaviour.
I doubt the ReactOS team have the resources to properly pull this off for something as large as the Windows source code though, and even if they did that still wouldn't get close to their target of Win2003 & later compatibility (there have been some significant changes to parts of the driver model since the XP days).
