Breaking: validate parser options (fixes #384)

[aladdin-add]

it will throw an error if any of the conditions is true:

• ecmaVersion is invalid
• sourceType is invalid
• ecmaVersion < 6 & sourceType: "module"

is there some more conditions need to be covered?

[platinumazure]

Leaving some small suggestions. This will be useful to pull into ESLint 6.x-- thanks for doing this!

[platinumazure]

LGTM, thanks!

[mysticatea]

LGTM, thank you!

[not-an-aardvark]

Thanks! This mostly looks good, I just have a suggestion for a better error message.

[not-an-aardvark]

Does this also fix eslint/eslint#9687, or are more changes in ESLint needed to fix that?

[aladdin-add]

Does this also fix eslint/eslint#9687, or are more changes in ESLint needed to fix that?

we need to upgrade espree, since this will be in a major release. not sure some more, like adding a test?

[not-an-aardvark]

LGTM, thanks!

[simonbrunel]

So does that mean ESLint shouldn't be used in projects targeting ecmaVersion < 2015, while not using any transpiler but for which sources rely on ES6 import/export bundler (e.g. rollup). I don't think ESLint should enforce the ecmaVersion >= 2015 when sourceType: module since this should be an acceptable use case:

module.exports = {
    parserOptions: {
        ecmaVersion: 5,       // disable ES6 features (const, let, etc.)
        sourceType: module    // allow ESM import/export syntax
    }
}

[not-an-aardvark]

@simonbrunel To be clear, ESLint already does not support that case. The config you showed currently has the effect of setting ecmaVersion: 2015 despite the ecmaVersion: 5 value provided in the config. This change just makes the error more obvious to avoid setting any misleading expectations.

[simonbrunel]

@not-an-aardvark yes I know, but these kind of projects exist and I don't think ESLint should exclude them.

[simonbrunel]

What I mean is instead of setting ecmaVersion to 2015 automatically or trigger an error if it doesn't match when sourceType: module, it may be better to make a distinction between the scope of these 2 options and allow projects that use ESM import/export to disable other ES6 features.

[aladdin-add]

you can use eslint-plugin-es.

[simonbrunel]

@aladdin-add thanks, I will give it a try!