Do not unpack array patterns that update a referenced binding

[nicolo-ribaudo]

Q	A
Fixed Issues?	Fixes #8528
Patch: Bug Fix?	yes
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

Do not unpack array patterns that update a referenced binding

We used this.scope.traverse to check if the array initializer referenced
a binding which is present in the array pattern. It doesn't work because,
in order to check if an identifier is a reference, we need to check its
parent.
this.scope.binding can't set the correct parentPath, because the only
path that it knows is the root scope's (e.g. the function path).
Use t.traverse which gives the correct parent.

This regression has been brought to the surface by 29d249e because it made the isReferenced check stricter, but it is unrelated.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/8974/

[loganfsmyth]

I'm not a huge fan of return true in traversal-like APIs because it's not clear if the behavior is .skip()-like or .stop()-like.

My preference would probably be for it to either:

• Explicitly return a STOP constant
• Skip the return value and throw an exception instead, e.g.

const STOP = {};
// ...
throw STOP;

and do

try {
  t.traverseFast(arr, arrayUnpackVisitor, state);
} catch (err) {
  if (err !== STOP) throw err;
}

[JSteunou]

Any step missing for this PR to be accepted?