Prior to this change, we'd conduct an open-ended traversal on the 'id'
of any VariableDeclarator to find a RestElement. The 'id' of
a VariableDeclarator can contain an AssignmentPattern (to supply
a default value), and if the right-hand side of the AssignmentPattern
contained a RestElement, we'd transform it.
The problem here is that the right-hand side of an AssignmentPattern can
be *any* Expression. If the right-hand side is a function body, we'd
traverse the entire function body, and if a RestElement occurred
anywhere in that function body, we'd transform it and emit the
transformations wherever we began the traversal (at least one scope
outside its usage).
The fix is to stop the inner traversal if we encounter an
AssignmentPattern. The outer traversal will still visit the
AssignmentPattern, so RestElements within the right-hand side of an
AssignmentPattern will be properly transformed at that time.
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)
