react-redux libdef causes errors with maybe-typed props

I have some presentational components that expect a maybe-typed boolean readOnly. This is purposefully optional so consumers don’t have to say readOnly={false}.

I have some container components that sometimes pass down a readOnly boolean, but theirs is not a maybe type. For example:

https://flow.org/try/#0C4TwDgpgBACgTgezAZygXigbynCBDAEwHkA7AGxAH4AuKAIwQTPxKgF8AoUSKAZWDzAI8JKgzZchUhVoMmLdhw4BjBCWTAoAWxD9BEWnqEiU6LDnzFyIWgDM8ZZNE6r1mnSeS1PZiZek22roCQgB0klYU7EA

This pattern breaks in my codebase:

Error --------------------------------------------------- 

QuestionSelect.js:41:13
undefined [1] is incompatible with boolean [2].
   41| 	readOnly?: boolean
                   ^^^^^^^ [1]

References:
 QuestionSelectContainer.js:9:45
    9| type Props = { value: QuestionId, readOnly: boolean };
                                                   ^^^^^^^ [2]

Passing readOnly to a component expecting readOnly? should not error.

[thenanyu]

I think this is the same issue as https://github.com/flowtype/flow-typed/issues/1997 We’re running into an issue where doing $Diff causes the flow error to look like it’s being evaluated backwards.

Example

This may be related to the disclaimer here: https://flow.org/en/docs/types/utilities/#toc-diff

Note that $Diff<A, B> will error if the object you are removing properties from does not have the property being removed, i.e. if B has a key that doesn’t exist in A

This has been filed as an issue in the main flow repo https://github.com/facebook/flow/issues/6149

[villesau]

Ok now I got it, thanks! Looks like the flow check is somewhat other way around in this case. It might be possible to solve with a bit of reordering of the types in the libdefs. I could check it out once I have time, but can’t promise anything.