Say I have a branch A, and from that I branch B. I make a bunch of changes on A, then checkout B and do a git pull. Now I make a change on B but realize that it should've been in A. If I now try to git checkout A, I get "Your local changes to the following files would be overwritten by checkout" to the file I touched.
Why would my change be overwritten if I just did a git pull in B and haven't touched that file in A since?
The reason you get that message is that the underlying file (before your uncommitted modifications) is different between branch A and branch B. If the files were the same, Git would switch branches and keep the same uncommitted modifications after switching to branch A.
One way to bring these changes across is to stash them:
(on branch B)$ git stash
git checkout A
git stash pop
If there are conflicting changes, you may have to resolve the conflict at this point. If there are changes but they don't conflict, then this will succeed.
来源：https://stackoverflow.com/questions/12044716/why-would-my-local-changes-in-git-be-overwritten-by-checkout-in-this-circumstanc