Inconsistent inferred types when using array methods map() and reduce()

[eidheim]

From:

// @flow

let v1 = [1, 2, 3];

let v2 = v1.map(e => e);

let sum = v2.reduce((e, acc) => e + acc, 0);

Inferred type of v2 is Array<(any | number)>, and of sum is any | number. One would think the inferred types should instead be Array<(number)> and number respectively.

The above behaviour was introduced in Flow v0.86.

However, the following code infers correct types:

// @flow

let v1 = [1, 2, 3];

let v2 = v1.map(e => e);

let sum = v1.reduce((e, acc) => e + acc, 0);

That is, the inferred types of v2 and sum are Array<number> and number.

Flow version: v0.86-v0.108

Expected behavior

Consistent inferred types of v2 and sum in the above examples.

Actual behavior

Inconsistent inferred types of v2 and sum in the above examples.

Try-Flow

[knu]

This is still an issue in v0.132.0. v2 is Array<number> at the time it is born and adding a piece of code that uses it afterwards turns it to a looser type Array<any | number>, which is quite unexpected.

[eidheim]

Fixed in v0.141.0: https://github.com/facebook/flow/releases/tag/v0.141.0