René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"When trying to use mapped tuples as rest parameters error 'A rest parameter must be of an array type' given","articleBody":"\u003c!-- 🚨 STOP 🚨 𝗦𝗧𝗢𝗣 🚨 𝑺𝑻𝑶𝑷 🚨\r\n\r\nHalf 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!\r\n\r\nPlease help us by doing the following steps before logging an issue:\r\n  * Search: https://github.com/Microsoft/TypeScript/search?type=Issues\r\n  * Read the FAQ: https://github.com/Microsoft/TypeScript/wiki/FAQ\r\n\r\nPlease fill in the *entire* template below.\r\n--\u003e\r\n\r\n\u003c!-- Please try to reproduce the issue with `typescript@next`. It may have already been fixed. --\u003e\r\n**TypeScript Version:**  3.2\r\n\r\n\u003c!-- Search terms you tried before logging this (so others can find this issue more easily) --\u003e\r\n**Search Terms:** mapped tuples rest \r\n\r\n**Code**\r\n\r\n```ts\r\ntype FuncParams\u003cT\u003e = T extends (...args: infer P) =\u003e any ? P : never;\r\ntype Stringify\u003cT\u003e = {\r\n    [K in keyof T]: string;\r\n};\r\ntype Optional\u003cT\u003e = {\r\n    [K in keyof T]?: T[K];\r\n};\r\n\r\ntype ThreeParamFunc = (paramOne: string, paramTwo: number, paramThree: boolean) =\u003e void;\r\n\r\ntype Params = FuncParams\u003cThreeParamFunc\u003e; // [string, number, boolean]\r\ntype StringParams = Stringify\u003cFuncParams\u003cThreeParamFunc\u003e\u003e; // [string, string, string]\r\ntype OptionalParams = Optional\u003cFuncParams\u003cThreeParamFunc\u003e\u003e; // [string?, number?, boolean?]\r\n\r\nfunction doStuff\u003cT\u003e(func: T, ...params: FuncParams\u003cT\u003e) { // works fine\r\n}\r\n\r\nfunction doOptionalStuff\u003cT\u003e(func: T, ...params: Optional\u003cFuncParams\u003cT\u003e\u003e) { // A rest parameter must be of an array type.\r\n}\r\n\r\nfunction doStringStuff\u003cT\u003e(func: T, ...params: Stringify\u003cFuncParams\u003cT\u003e\u003e) { // A rest parameter must be of an array type.\r\n}\r\n```\r\n\r\n\r\n**Expected behavior:**\r\nI should be able to use a mapped tuple as a rest param in a function\r\n\r\n**Actual behavior:**\r\nI get the error `A rest parameter must be of an array type.`\r\n\r\n**Playground Link:** \r\n[Link](https://www.typescriptlang.org/play/#src=%0D%0Atype%20FuncParams%3CT%3E%20%3D%20T%20extends%20(...args%3A%20infer%20P)%20%3D%3E%20any%20%3F%20P%20%3A%20never%3B%0D%0Atype%20Stringify%3CT%3E%20%3D%20%7B%0D%0A%20%20%20%20%5BK%20in%20keyof%20T%5D%3A%20string%3B%0D%0A%7D%3B%0D%0Atype%20Optional%3CT%3E%20%3D%20%7B%0D%0A%20%20%20%20%5BK%20in%20keyof%20T%5D%3F%3A%20T%5BK%5D%3B%0D%0A%7D%3B%0D%0A%0D%0Atype%20ThreeParamFunc%20%3D%20(paramOne%3A%20string%2C%20paramTwo%3A%20number%2C%20paramThree%3A%20boolean)%20%3D%3E%20void%3B%0D%0A%0D%0Atype%20Params%20%3D%20FuncParams%3CThreeParamFunc%3E%3B%20%2F%2F%20%5Bstring%2C%20number%2C%20boolean%5D%0D%0Atype%20StringParams%20%3D%20Stringify%3CFuncParams%3CThreeParamFunc%3E%3E%3B%20%2F%2F%20%5Bstring%2C%20string%2C%20string%5D%0D%0Atype%20OptionalParams%20%3D%20Optional%3CFuncParams%3CThreeParamFunc%3E%3E%3B%20%2F%2F%20%5Bstring%3F%2C%20number%3F%2C%20boolean%3F%5D%0D%0A%0D%0Afunction%20doStuff%3CT%3E(func%3A%20T%2C%20...params%3A%20FuncParams%3CT%3E)%20%7B%20%2F%2F%20works%20fine%0D%0A%7D%0D%0A%0D%0Afunction%20doOptionalStuff%3CT%3E(func%3A%20T%2C%20...params%3A%20Optional%3CFuncParams%3CT%3E%3E)%20%7B%20%2F%2F%20A%20rest%20parameter%20must%20be%20of%20an%20array%20type.%0D%0A%7D%0D%0A%0D%0Afunction%20doStringStuff%3CT%3E(func%3A%20T%2C%20...params%3A%20Stringify%3CFuncParams%3CT%3E%3E)%20%7B%20%2F%2F%20A%20rest%20parameter%20must%20be%20of%20an%20array%20type.%0D%0A%7D)\r\n","author":{"url":"https://github.com/Roaders","@type":"Person","name":"Roaders"},"datePublished":"2019-02-14T16:42:48.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":18},"url":"https://github.com/29919/TypeScript/issues/29919"}