Upstream is gone!

A: branch checked out from branch dev

B: branch checked out from branch A

Situtation: We finish coding in branch A and then merge A to dev. When we are in B, we get some notification

On branch source
Your branch is based on 'origin/dev', but the upstream is gone.
  (use "git branch --unset-upstream" to fixup)

Solution:

git branch --unset-upstream
git branch --set-upstream-to origin/dev