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Type spread doesn't work correctly with optional properties #7697

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mrtnzlml opened this issue May 3, 2019 · 0 comments

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commented May 3, 2019

Hi, I am using the latest Flow version and I am getting unexpected behavior. Example:

type AAA = { headers?: boolean }
type BBB = { ...$Exact<AAA>, headers?: string }

({ headers: true }: BBB) // string and boolean are allowed here ❌, I expected only string

type CCC = { headers: boolean }
type DDD = { ...$Exact<CCC>, headers: string }

({ headers: 'true' }: DDD) // only string is allowed here ✅

The $Exact is not significant, I just wasn't sure if it's related to this issue (seems like it's not). Why is the behavior different with optional props? Am I missing something? 🤔 Thanks for checking it.

flow.org/try

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