My project A (hosted on GitHub) somehow (not sure how) forgot that it was originally forked from another open-source project B. By 'forgot', I mean, when creating a pull request, I cannot choose B as a target for sending the pull request. Is there some way I tell GitHub that A is a fork of B?
(I can create a PR by forking B into A', then merging A into A' and sending the PR from A' to B which naturally works but, naturally, I don't like it)
No, there is no way to turn an existing repo into a fork. You can permanently switch to A' and abandon A, it's a one-time procedure. And there is no need to merge A into A'; you can simply push the whole A into A' instead.
来源：https://stackoverflow.com/questions/28672714/manually-set-forked-from-to-github-project