René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"The evaluation of the Keyof Type Operator produces different results to the Pick built-in tool.","articleBody":"# Bug Report\r\n\r\n\u003c!--\r\n  Please fill in each section completely. Thank you!\r\n--\u003e\r\n\r\n### 🔎 Search Terms\r\nPick\r\n\u003c!--\r\n  What search terms did you use when trying to find an existing bug report?\r\n  List them here so people in the future can find this one more easily.\r\n--\u003e\r\n\r\n### 🕗 Version \u0026 Regression Information\r\nv4.5.4\r\n\r\n\r\n\r\n\u003c!-- When did you start seeing this bug occur?\r\n\r\n\"Bugs\" that have existed in TS for a long time are very likely to be FAQs; refer to\r\n  https://github.com/Microsoft/TypeScript/wiki/FAQ#common-bugs-that-arent-bugs\r\n\r\nIf possible, please try testing the nightly version of TS to see if it's already been fixed.\r\nFor npm: `typescript@next`\r\nThis is also the 'Nightly' version in the playground: http://www.typescriptlang.org/play/?ts=Nightly\r\n\r\nNote: The TypeScript Playground can be used to try older versions of TypeScript.\r\n\r\nPlease keep and fill in the line that best applies:\r\n--\u003e\r\n\r\n\r\n### ⏯ Playground Link\r\n[Link](https://www.typescriptlang.org/play?ts=4.5.4#code/JYOwLgpgTgZghgYwgAgKoGdrIN4ChnIhwC2EA-AFzLphSgDm+yc95VIArsQEbRNwATAVAjp0larQa4AvrlxgAngAcUAMQD2GgDwAVADRpkAXmQBrCIo0xkugHwmcTANoAFZKGQBRAB61EYNqohhZWNvYAulS6bhGy8goqKABCcFB6+qjGoda2DqYAShAAjhzAIgLarsAIZhneflABQSGWufZ2dglKqshF6I6aOhjQhgDkRKRjDgQEAPRzkxBUNHQg9MgAPsgcIAIQMKAQAgDcib39AEyOqekjUONL07MLSytS60A)\r\n\u003c!--\r\n  A link to a TypeScript Playground \"Share\" link which shows this behavior\r\n\r\n  The TypeScript Workbench can be used for more complex setups, try\r\n  https://www.typescriptlang.org/dev/bug-workbench/\r\n\r\n  As a last resort, you can link to a repo, but these will be slower for us to investigate.\r\n--\u003e\r\n\r\n\r\n### 💻 Code\r\n```lang=ts\r\ninterface User {\r\n  name?: string\r\n  age?: number\r\n  address?: string\r\n}\r\n\r\ntype Foo\u003cT, U = keyof T\u003e = {\r\n  [P in Extract\u003cU, keyof T\u003e]: T[P]\r\n}\r\n\r\ntype Bar\u003cT,U=keyof T\u003e = Required\u003cPick\u003cT, Extract\u003cU, keyof T\u003e\u003e\u003e\r\n\r\ntype Res = Foo\u003cUser, 'name'\u003e     //name: string | undefined;\r\ntype Res2 = Bar\u003cUser, 'name'\u003e   //name: string\r\n```\r\n\u003c!-- Please post the relevant code sample here as well--\u003e\r\n```ts\r\n// We can quickly address your report if:\r\n//  - The code sample is short. Nearly all TypeScript bugs can be demonstrated in 20-30 lines of code!\r\n//  - It doesn't use external libraries. These are often issues with the type definitions rather than TypeScript bugs.\r\n//  - The incorrectness of the behavior is readily apparent from reading the sample.\r\n// Reports are slower to investigate if:\r\n//  - We have to pare too much extraneous code.\r\n//  - We have to clone a large repo and validate that the problem isn't elsewhere.\r\n//  - The sample is confusing or doesn't clearly demonstrate what's wrong.\r\n```\r\n\r\n### 🙁 Actual behavior\r\n```\r\ntype Res = Foo\u003cUser, 'name'\u003e     //name: string | undefined;\r\n```\r\nThe value evaluated for the name contains undefined\r\n\u003c!-- What happened, and why it was wrong --\u003e\r\n\r\n### 🙂 Expected behavior\r\n\r\n```\r\ntype Foo\u003cT, U = keyof T\u003e = {\r\n  [P in Extract\u003cU, keyof T\u003e]: T[P]\r\n}\r\n```\r\n[P in Extract\u003cU, keyof T\u003e]: T[P]   should not return undefined because  Extract only narrows down the range of the keys\r\n\r\n\u003c!-- What you expected to happen instead, and why --\u003e\r\n","author":{"url":"https://github.com/someBrown","@type":"Person","name":"someBrown"},"datePublished":"2022-03-08T08:09:19.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":7},"url":"https://github.com/48173/TypeScript/issues/48173"}