Flow: Regression: union of string literals can't be used as computed properties

[kkaefer]

Flow version: v0.111 and up have this regression. v0.110 and lower work fine

Expected behavior

Flow doesn't report an error.

Actual behavior

Flow reports an error for the following code:

// @flow
const key: 'a' | 'b' = 'a';
const obj = {[key]: 42};

Error:

    3: const obj = {[key]: 42};
                     ^ Cannot use `key` [1] as a computed property. Computed properties may only be primitive literal values, but union type [2] is a union. Can you add a literal type annotation to `key` [1]? See https://flow.org/en/docs/types/literals/ for more information on literal types.
        References:
        2: const key: 'a' | 'b' = 'a';
                 ^ [1]
        2: const key: 'a' | 'b' = 'a';
                      ^ [2]

[dsainati1]

This is deliberate as part of the changes described here: https://medium.com/flow-type/spreads-common-errors-fixes-9701012e9d58

[kkaefer]

Thanks for pointing me to this article. From reading the section on unions in computed properties, it sounds like this decision was made for performance considerations in a contrived scenario, and not because the code is actually faulty. Therefore, I'd disagree with the classification as "not a bug", since Flow is now complaining about valid code that was working prior to this change.

[jbrown215]

It's not a contrived scenario-- it makes Flow time out in our codebases.

[danielo515]

I wonder who has 10k members sum types ...