Use # prefix on private class field with eslint-plugin-flowtype cause "Cannot read property 'type' of undefined"

Bug Report

Clone of https://github.com/babel/babel-eslint/issues/688

As reported by @SebastienGllmt:

This exact issue still persists for me even if I crank all eslint & bable dependencies to their latest version. What’s more, this doesn’t always fail. For example in my case

This fails

  #publicDeriverId: number;
  #bip44Wallet: IBip44Wallet;
  #derivationId: number;
  #pathToPublic: Array<number>; // only this specific private field causes an error.

This passes

  #publicDeriverId: number;
  #bip44Wallet: IBip44Wallet;
  #derivationId: number;
  #pathToPublic: string; // note: this changed

Adding this totally unrelated type makes it work

type TotallyUnrelatedType = Array<number>;
export class Foo {
  #publicDeriverId: number;
  #bip44Wallet: IBip44Wallet;
  #derivationId: number;
  #pathToPublic: Array<number>;
}

_Originally posted by @SebastienGllmt in https://github.com/babel/babel-eslint/issues/688#issuecomment-531608020_

[kaicataldo]

Hey, sorry for the delay here. Do you mind sharing the versions of all @babel/core, eslint, @babel/eslint-parser, and eslint-plugin-flowtype you’re using as well as the actual output from ESLint for the various examples? It’ll make it easier to understand what is going on.

[JLHwung]

We can not enable the class features by default since it is a breaking change to ecosystem (but it is inevitable since Babel AST differs to ESTree). If you want to try on this feature

module.exports = {
  parser: "@babel/eslint-parser",
  babelOptions: {
    parserOpts: {
      plugins: [
        ["estree", { classFeatures: true }],
        "classPrivateProperties",
        "classProperties",
      ],
    },
  },
};