Weird errors and crashes with v-for in ES5 mode (ecmaVersion: 5)

[catrope]

I've been working on setting up Vue support for eslint in a repo where we use ES5, and the Vue plugin doesn't work well with eslint's ES5 mode.

Installing the Vue plugin silently overwrites certain settings: ecmaVersion is set to 2018, sourceType to module, and JSX is enabled. This caused us to accidentally disable enforcement of our "no ES6" rule for about a month until I found out.

I tried to remedy this by changing these settings back in our .eslintrc.json, but that causes strange errors and even crashes to happen when linting templates that use v-for.

Tell us about your environment

• ESLint version: 6.8.0
• eslint-plugin-vue version: 6.1.2
• Node version: 10.15.2

Please show your full configuration:

{
	"extends": [
		"plugin:vue/recommended"
	],
	"parserOptions": {
		"ecmaVersion": 6,
		"sourceType": "script",
		"ecmaFeatures": {
			"jsx": false
		}
	}
}

What did you do?
First file (simple v-for):

<template>
  <div class="foo">
    <p
      v-for="x in xs"
      :key="x"
    >
      {{ x }}
    </p>
  </div>
</template>

Second file (v-for using parens to get both the value and the index):

<template>
  <div class="foo">
    <p
      v-for="(x, index) in xs"
      :key="index"
    >
      {{ x }}
    </p>
  </div>
</template>

What did you expect to happen?
Both of these files should lint without errors, even in ES5 mode (ecmaVersion: 5 in the eslint config).

What actually happened?
For the first file, I get confusing errors about the v-for line:

  4:7   error  'v-for' directives require that attribute value                                                  vue/valid-v-for
  4:14  error  Parsing error: Unexpected token x                                                                vue/no-parsing-error
  5:7   error  Expected 'v-bind:key' directive to use the variables which are defined by the 'v-for' directive  vue/valid-v-for

For the second file, I get an error message that exposes a JS error / crash in the plugin code, without reference to a line number:

  0:0  error  Parsing error: Cannot read property 'block' of undefined

Both of these errors only happen if ecmaVersion: 5 is set. If I set ecmaVersion: 6, the errors go away.

[catrope]

As a workaround, I'm enabling the Vue plugin only for *.vue files, leaving the ecmaVersion setting unchanged, then using es/no-2015 to prohibit ES6 features (eslint config here). But it would be great if the Vue plugin didn't break with ecmaVersion: 5, so that I can use eslint's "real" ES5 mode instead.

[ota-meshi]

Thank you for posting this issue.

I have debugged and checked this. This is a problem with vue-eslint-parser.

v-for="(x, index) in xs" directive are parsed by converting them to for(let [x, index] in xs); code, which is almost invalid syntax in ES5.
Here, it analyzes the invalid AST and fails to analyze the variables.

Please post your issue to vue-eslint-parser to solve this issue.