[explicit-function-return-type] allowExpressions false positives and incorrect documentation

[gilbsgilbs]

Repro

{
  "rules": {
    "typescript/explicit-function-return-type": ["error", {allowExpressions: true}]
  }
}

node.addEventListener('click', () => {});

Expected Result

No error

Actual Result

Reported as missing return type

Additional Info

I can see at least two issues:

• As indicated here the default value for allowExpressions should be false but it's not
• As the option is all borked, even when enabled (which is the actual default), the rule reports a ton of false positives which lets the user think that the option is disabled even when it's enabled (explicitly or implicitly). This is probably why we didn't had this reported before.

I investigated a bit and was able to figure out why tests are not failing: filename key is either not set or set to 'something.js' in tests with allowExpressions. This tells eslint to skip all typescript rules somehow. I am easily able to reproduce in tests by setting the filename to 'test.ts'.

Versions

package	version
@typescript-eslint/eslint-plugin	cebcfe64
@typescript-eslint/parser	cebcfe64
TypeScript	3.3.3
ESLint	5.14.1
node	11.12.0
npm	6.9.0