Flow: Objects with indexer properties lose type information after object spread

[swac]

Flow seems to think the result of an object spread on a map object is just Object, rather than the original type. This makes it hard to safely merge multiple map objects into one.

/* @flow */

type A = { [string]: number };

const foo: A = { a: 5 };

foo.b = '5'; // Error, as expected.

const bar = { ...foo };

bar.b = '5'; // No error, despite not matching A's definition.

See https://flow.org/try/#0PQKgBAAgZgNg9gdzCYAoVAXAngBwKZgCCYAvGAN5gDaAzhgE4CWAdgOYC6AXGMwK4C2AIzz0wAXwDc6AMZxmdMFDhxuxMpQCG3AKzipqJXAB0g0mADk28xLDBgYAKL16cegBowGmmDwAPfNIYeAAmRjJyCoIaoupgRvGGeuhR9CZmlta29gBycD7Orh7BeDQ4jEE8cBhg-BoY0gAWLKxE5t7FUCzljHJGQA.

I think this may be a distinct issue from https://github.com/facebook/flow/issues/6505, since this one deals with map objects rather than exact objects.

[wchargin]

Agreed that this a bug, and different from #6505.

These are called objects with _indexer properties_ (commenting with this
keyword to improve searchability). A “Map object” may quite reasonably
be interpreted as an instance of Map.

[swac]

@wchargin good point, I've updated the title of this issue to be more precise.

[NilSet]

Now that the Object type degenerates to any this behavior is even worse, and flow now thinks the spread object is maybe a string or a number or void.

Combined with #6656 this now means that

// @flow

type objmap = {[string]: number};

function bar(o: objmap): objmap {
  return [''].reduce({...o});
}

fails after v0.87.0

[nmote]

This now errors as expected in master