Version 3.8.0 comma-dangle breaks Flowtype annotations when a object rest parameter is used

Tell us about your environment

• ESLint Version: v3.8.0
• Node Version: v6.7.0
• npm Version: 3.10.3

What parser (default, Babel-ESLint, etc.) are you using? Babel-ESLint

Please show your full configuration:

.eslintrc.js

module.exports = {
    "parser": "babel-eslint",
    "parserOptions": {
        "ecmaVersion": 6,
        "sourceType": "module"
    },
    "env": {
        "es6": true,
        "node": true
    },
    "plugins": [
        "flowtype",
        "flow-vars"
    ],
    "extends": "eslint:recommended",
    "rules": {
        "comma-dangle": [
            "error",
            "always-multiline"
        ],
    }
};

.babelrc

{
  "plugins": [
    "transform-flow-comments",
    "transform-function-bind",
    "transform-async-to-generator",
    "transform-exponentiation-operator",
    "transform-class-properties",
    "transform-es2015-destructuring",
    "transform-es2015-spread",
    "transform-es2015-parameters",
    "transform-object-rest-spread",
    "transform-es2015-classes",
    "transform-es2015-modules-commonjs"
  ]
}

What did you do? Please include the actual source code causing the issue. I created a “minimal” demo to reproduce the problem. My impression is that since 3.8.0 (it works on 3.7.1) comma-dangle (set to always-multiline) reports a comma in a flow annotation when a object rest parameter is used before.

Complete demo code: https://gist.github.com/mormahr/e6af53438a9bae9b9fe8905cc938b77d

export default class AntragsEvent {
    id: ?string
    rest: any

    constructor({
        id,
        ...rest, // <- this (the object rest spread) is causing
    }: {
        id?: string, // <- this comma to be reported by comma-dangle
    }) {
        this.id = id
        this.rest = rest
    }
}

What did you expect to happen? No report of an unexpected comma

What actually happened? Please include the actual, raw output from ESLint.

[PRIVATE]/CommaDangleBug/test.js 9:14 error Unexpected trailing comma comma-dangle

✖ 1 problem (1 error, 0 warnings)

[not-an-aardvark]

I understand that you’re using babel, but a trailing comma after a rest property is not valid syntax even according to the current proposal spec, because there is nothing that could follow a rest property.

The same thing applies with the already-standardized rest elements:

const [
  foo,
  ...bar, // <-- This is a syntax error in ES6
] = baz;

In order to be more compliant with the spec, babel’s parser temporarily reported this as a syntax error, but it was later reverted due to breakage.

The fix in this case is to simply remove the comma, so that it becomes valid syntax according to the Stage 3 proposal.

[not-an-aardvark]

For what it’s worth, I think your syntax is invalid based on the current version of the rest/spread spec; trailing commas aren’t allowed after rest properties, since no property could actually be added after it. (See https://github.com/eslint/eslint/issues/7297 for more info.)

class Foo {
  constructor ({
    bar,
    ...baz, // <-- trailing comma is not allowed here according to rest/spread spec
  }) {}
}

Does the problem go away if you remove that comma?