Flow doesn't respect interface definition when implementation exists

[skevy]

I was trying to type check some code that uses Immutable.JS and this interface definition: https://gist.github.com/kastermester/ad79da3e48064effd82e

I was getting stumped when this code wasn't throwing any Flow errors:

import { fromJS, Map as IMap } from 'immutable';

export async function createClientIdAndSecret() : Promise<IMap<string, string>> {
  const clientId: string = uuid.v1();
  const clientSecret: string = crypto.randomBytes(48).toString('hex');

  const shasum = crypto.createHash('sha512');
  shasum.update(clientSecret);
  const clientSecretToStore = shasum.digest('hex');

  await OAuthClient.forge({
    client_id: clientId,
    client_secret: clientSecretToStore
  }).save();

  return 'hello';
}

But this code was throwing the correct error (that "number" and "string" weren't compatible):

import { fromJS, Map as IMap } from 'immutable';

export async function createClientIdAndSecret() : Promise<number> {
  const clientId: string = uuid.v1();
  const clientSecret: string = crypto.randomBytes(48).toString('hex');

  const shasum = crypto.createHash('sha512');
  shasum.update(clientSecret);
  const clientSecretToStore = shasum.digest('hex');

  await OAuthClient.forge({
    client_id: clientId,
    client_secret: clientSecretToStore
  }).save();

  return 'hello';
}

After discussing with @jeffmo on IRC, it seems that the reason this is happening is because Flow can't reason about the Immutable.js build properly, and since it isn't tagged with @flow, treats it's exports as any, and ignores the immutable library def.

Sure enough, when I ignore node_modules/immutable in my .flowconfig, it uses my Immutable interface definition and throws errors (seemingly correct ones too!).

[jeffmo]

cc @bhosmer

[Macil]

Seems like a duplicate of #447

[unscriptable]

Any idea if/when this will be fixed?

[Macil]

I only today discovered that Flow actually supports type-checking of props passed to React components, but you have to add a line like .*/node_modules/react\(-dom\)?/.* to your .flowconfig [ignore] to make it work, because otherwise the React library (without type definitions) overrides Flow's built-in React definitions. I'm really surprised there's such a useful feature hidden by an issue like this.

[JoeStanton]

I can't get this working, even with the above line in .flowconfig