Leading comment is lost when using named import just before

Bug Report

• I would like to work on a fix!

Current Behavior Parsing the code in the section below using @babel/parser should return a leadingComments on the export declaration

Input Code

import { test } from 'vue-test'

/**
 * lost comment
 */
export default {
   name: 'name-123',
}

Expected behavior/code For the leadingComment to be attached to the exportDeclaration node

Babel Configuration (babel.config.js, .babelrc, package.json#babel, cli command, .eslintrc)

const babelParserOptions: ParserOptions = {
	sourceType: 'module',
	strictMode: false,
	tokens: true,
	plugins: [
		'decorators-legacy',
		'doExpressions',
		'objectRestSpread',
		'classProperties',
		'classPrivateProperties',
		'classPrivateMethods',
		'exportDefaultFrom',
		'exportNamespaceFrom',
		'asyncGenerators',
		'functionBind',
		'functionSent',
		'dynamicImport',
		'numericSeparator',
		'optionalChaining',
		'importMeta',
		'bigInt',
		'optionalCatchBinding',
		'throwExpressions',
		'nullishCoalescingOperator'
	]
}

Environment

 Node 10.17

• Babel version(s): v7.6.0
• Node/npm version: Node 10
• OS: windows 10/linux in CI/MacOS Mojave
• Monorepo: M?A
• How you are using Babel: API

Possible Solution I am guessing a starting point would be in one of those 2 commits https://github.com/babel/babel/pull/10380 https://github.com/babel/babel/pull/10292 https://github.com/babel/babel/pull/10369

Additional context/Screenshots Here is a PR where the issue is visible. https://github.com/vue-styleguidist/vue-styleguidist/pull/681

[JLHwung]

@elevatebart Under the hood Babel uses comment attachment algorithm to consume the comments and assign to different AST nodes. It lives in packages/babel-parser/src/parser/comments.js, you can take a close look at https://github.com/babel/babel/blob/80e95d0c833d639157579c765b6624cd4c2696d6/packages/babel-parser/src/parser/comments.js#L171-L181

There is a detailed comment on the third argument takeAllComments of adjustCommentsAfterTrailingComments thanks to @nicolo-ribaudo

https://github.com/babel/babel/blob/80e95d0c833d639157579c765b6624cd4c2696d6/packages/babel-parser/src/parser/comments.js#L44-L52

When it’s running on an ImportDeclaration (import { test } from 'vue-test'), the commentPreviousNode is an ImportSpecifier (test) and it will take all leadingComments to be the trailingComment of ImportDeclaration.

In your case, I don’t think we can use takeAllComments: true here because apparently it can attach to the next node.

[JLHwung]

@elevatebart Correct. It should stay in the state.leadingCommets stack and attach it to the export declaration.