instanceof Array refinement adds Array<any> to type

[postcasio]

https://flow.org/try/#0PTACDMBsHsHcCh4BMCmBjSBDATigBBpgM5F4DC0M2eA3vHnkQC6ZMCWaeA1ACraYA7IgAccKAUwBc5StGwBueAF9ETAJ7D8ABWzRhpALy16eAEZzU2ClQD8063LwAfPAEFs-NQB4H2AHwANCbm2JYA6mxITAAWdngCAK4AtqYo1C7unl6JKWl+yorwaNBCTGYWaUQ8KMxkxPhGABTCuvrSOnpEAJR4Bn7GDJAoZSGWvtKZmN6+-UYA2gC6hQxs4HjNrUQAdKNpvj10DCtrG507FVay1Gylgmgo0GuTagcmR+Whe1e9eC1nu5cqIp3ko8ChIER8Id3h8xt8jAIULA3B4po0ACxdLbgNiQSCnfTnT6AuRdYFHFQMUHgyEDI4A3w-RHI54YrE4vGNXxbPiCERiCRkkwqExDEYXCJRaITVHeHKpfw-RbLPCrdZ-QkAyUxV7vNUE7ZayIxVW3AT3R4ozy6mFGqU-DWGiXG6LkqlgiFQt70532hFIq1ozHY3H4x1E8IuoUg4Ue2nQhh2k3+lmytkhzkABmjeBUSkUQA

This case demonstrates the same code structure working with two different types: void | number | Array<number> and void | Color | Array<Color>.

Flow chokes on the code dealing with the Color class. Inspecting what flow thinks the types are using the vscode plugin, it shows that on line 15, the type is correct: void | Color | Array<Color>. On line 16, it is correct: Color | Array<Color>. On line 17, after the instanceof Array check, it suddenly becomes Array<Color> | Array<any>, which is incompatible with the Array<Color> variable I'm trying to assign it to.

This doesn't happen for the number case - after the instanceof Array check, it correctly infers that the type is only Array<number>.

Where is this Array<any> coming from?

If I use Array.isArray instead of instanceof Array, it type checks correctly.

[jbrown215]

This looks related to #6741, likely the same bug.

[mgreenw]

I think this is related to an issue I'm having with $ObjMap. Using the Official Documentation's $ObjMap Example I am running $ObjMap as such:

type TypeAssertions = { [key: string]: Function };
function run<O: {[key: string]: Function}>(o: O): $ObjMap<O, ExtractReturnType> {
  return Object.keys(o).reduce((acc, k) => Object.assign(acc, { [k]: o[k]() }), {});
}

const o = {
  a: () => true,
  b: () => 'foo',
  c: () => ['hey there'],
};

By itself, c is of type () => Array<string>.
However, when the following code is added, c's type changes to () => Array<any|string>:

const p = run(o).c;

Just running the method causes flow to add the any type to the return type. I'm not sure if this is a bug with $ObjMap or any of the mentioned bugs above.

Tested with Flow 0.93.

[jbrown215]

@mgreenw: That's not related. Feel free to open a separate issue.

[postcasio]

Closing because this appears to have been fixed as of v0.97.0.