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Conditional types fail to distribute in properties of mapped types

microsoft/typescript#33669·111137·Go·2533 天未动·5 条评论·上游最近活跃 ·池内状态:可认领
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<!-- 🚨 STOP 🚨 𝗦𝗧𝗢𝗣 🚨 𝑺𝑻𝑶𝑷 🚨 Half of all issues filed here are duplicates, answered in the FAQ, or not appropriate for the bug tracker. Even if you think you've found a *bug*, please read the FAQ first, especially the Common "Bugs" That Aren't Bugs section! Please help us by doing the following steps before logging an issue: * Search: https://github.com/Microsoft/TypeScript/search?type=Issues * Read the FAQ: https://github.com/Microsoft/TypeScript/wiki/FAQ Please fill in the *entire* template below. --> <!-- Please try to reproduce the issue with `typescript@next`. It may have already been fixed. --> **TypeScript Version:** 3.7.0-dev.20190928 <!-- Search terms you tried before logging this (so others can find this issue more easily) --> **Search Terms:** conditional mapped property union **Code** ```ts type NullifyStrings<T> = T extends string ? null : T type NullifyStringsInPropsWorking<T> = { [K in keyof T]: NullifyStrings<T[K]> } type NullifyStringsInPropsBroken<T> = { [K in keyof T]: T[K] extends string ? null : T[K] } type TestType = { a: number | string } // { a: number | null } type WorkingReplaceProps = NullifyStringsInPropsWorking<TestType> // { a: string | number } type BrokenReplaceProps = NullifyStringsInPropsBroken<TestType> ``` **Expected behavior:** `NullifyStringsInPropsWorking` and `NullifyStringsInPropsBroken` should be functionally identical - expanding the `NullifyStrings` type alias in `NullifyStringsInPropsWorking` results in the same definition as `NullifyStringsInPropsBroken`. **Actual behavior:** `NullifyStringsInPropsWorking` and `NullifyStringsInPropsBroken` have different behaviour - `BrokenReplaceProps` has type `{ a: string | number }` instead of the expected `{ a: number | null }`. **Playground Link:** <!-- A link to a TypeScript Playground "Share" link which demonstrates this behavior --> [Link](https://www.typescriptlang.org/play/?ts=Nightly#code/C4TwDgpgBAcgrgGwQSwGYgMrAE7IHYDmAzgDwAqAfFALxRlQQAewEeAJkVETvgVAPxQ8iBFA…
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