This failing test case demonstrates a regression between 7.0.0-beta.38 and
7.0.0-beta.39 in the @babel/plugin-proposal-object-rest-spread package.
I distilled this test case from a larger configuration of plugins in my
application, one of which calls api.traverse.cache.clearScope(). Although
calling clearScope() is an uncommon thing for a plugin to do, it was a
reliable way to reproduce the problem. If I can find other reliable
reproductions, I'll push some additional failing tests to this PR.
Regardless of how common it is, clearing the scope cache should be a safe
operation that only slows down the transform (because scopes have to be
recreated and re-crawled). Crashing due to a spurious duplicate
declaration seems like a bug worth fixing.
My hunch is that [these two lines](eb38ea2b10/packages/babel-plugin-proposal-object-rest-spread/src/index.js (L75-L76))
(which were changed in `7.0.0-beta.39`) are not actually removing the
original rest element as a binding from the enclosing `Scope`, in certain
circumstances, so the new variable declaration ends up colliding with the
old (removed) binding.
Possibly related: #7304 (reported by @julien-f)
