vue-template-compiler fails when parsing a template with a comment as a root node.

[elevatebart]

Version

2.5.22

Reproduction link

https://codesandbox.io/s/52k2p9po54

Steps to reproduce

Go to the codesandbox.
Try to parse the given template with comments on.

<template>
 <!-- here is a nice component template -->
 <button class="buttonComponent">click</button>
</template>

What is expected?

The comment AST node should be attached to the root object.

What is actually happening?

node_modules/vue-template-compiler/build.js:2469:21)
TypeError: Cannot read property 'children' of undefined

(currentParent is undefined)

---

I am going to try my hand at a PR. Wish me luck ;)

[elevatebart]

Ideally we would be able to retrieve this comment the normal way.
But this would mean creating an extra parentNode that I guess we do not want.

How about using the same syntax as babel parser? Use the "leadingComment" property for the node if there is a next sibling. traiingComment the same way.
This way, no need to attach it to a root that does not exist.

The fix would then be 2 fold.

1. check if there is a currentParent and if there is not, drop the comment.
2. add a leadingComment and a trailingComment property to each node.

My PR will tke care of 1. I can create another issue for 2.