Object.keys return value treated different than other functions returning Array<string>

[kevinbarabash]

Flow version: 0.102.0

Expected behavior

Flow should treat the return type of Object.keys the same way as other functions returning Array<string>. The return type for Object.keys is Array<string> as defined by https://github.com/facebook/flow/blob/master/lib/core.js#L71.

Actual behavior

Currently flow allows pushing non-string elements to the arrays return by Object.keys, but does not allow pushing of non-string to arrays returned by other functions.

Example

If we want the return value of Object.keys to be treated differently it should be typed differently.

[goodmind]

Related #6927

[kevinbarabash]

I'm going close this as a dupe of that ticket. Thanks for linking to that.

[goodmind]

Duplicate of #6927