Upgrade: acorn to 5.0.1

[not-an-aardvark]

Bug fixes

Raise an error for duplicated lexical bindings.
Fix spurious error when an assignement expression occurred after a spread expression.
Accept regular expressions after of (in for/of), yield (in a generator), and braced arrow functions.
Allow labels in front or var declarations, even in strict mode.

Breaking changes

Parse declarations following export default as declaration nodes, not expressions. This means that class and function declarations nodes can now have null as their id.

[vitorbal]

LGTM. Can the breaking change affect us in any case?

[not-an-aardvark]

Can the breaking change affect us in any case?

Yes, our FunctionDeclaration ASTs will also sometimes have id: null as a result of this change.

I'm not sure whether this would be a bugfix for us under our semver policy.

If we wanted to avoid any possibility of breaking changes, it would probably be relatively simple to transform the AST back to the old behavior, because ExportDefaultDeclaration nodes can only appear at the top level.

[mysticatea]

I think the breaking change is not breaking our use case.

We have been following to ESTree spec about ExportDefaultDeclaration since the first time. I think we can just remove the convert espree.js#L198-L202.

[not-an-aardvark]

@mysticatea You're right, I hadn't realized we already convert the nodes. So this is not a breaking change for us.

However, when I tried removing the conversion, one of our tests failed due to acornjs/acorn#528.

[mysticatea]

Hmm? I think export default (async function() {}) should be a FunctionExpression because of parentheses. If so, our test is wrong.

[not-an-aardvark]

Well, in Acorn, export default (function() {}) is a FunctionDeclaration. So I think it should be consistent either way.

[mysticatea]

In the spec: https://tc39.github.io/ecma262/#prod-ExportDeclaration

If parentheses exist, it's the last case. So it's an AssignmentExpression.
If no parentheses, it's export default HoistableDeclaration, so it should be FunctionDeclaration.

This effects to the hoisting behavior.
The foo of export default function foo() {} is hoisting, but export default (function foo() {}); is not hoisting.

[not-an-aardvark]

@mysticatea I think you're right. But in that case, we have another bug: export default (function() {}) is currently parsed as a FunctionDeclaration when it should be a FunctionExpression.

[mysticatea]

I hadn't realized it. I'll open an issue about it.

EDIT: #302 already exists.

[not-an-aardvark]

I noticed this introduces a new parsing error:

(_ => 0) || (_ => 0)

This should parse successfully, but now espree fails to parse it.

edit: This is fixed in Acorn 5.0.1.