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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Mapped conditional type leaks unbound type parameter","articleBody":"**TypeScript Version:**  3.5.0-dev.20190509\r\n\r\n**Search Terms:** generic type parameter, scope leak, scope escape, conditional type, mapped type\r\n\r\n**Code**\r\n\r\n```ts\r\ntype Hmm\u003cT, U extends T\u003e = U extends T ? { [K in keyof U]: number } : never;\r\ntype What = Hmm\u003c{}, { a: string }\u003e\r\n// type What = { [K in keyof (T \u0026 { a: string; })]: number; } 🤪\r\nconst w: What = { a: 4 };  // error! '{ a: number; }' not assignable to What\r\n```\r\n**Expected behavior:**\r\n`What` should be the concrete object type `{ a: number }` and therefore the assignment to `w` should compile without errors.\r\n\r\n**Actual behavior:**\r\nAn unbound type parameter `T` has escaped from its scope and is terrorizing the surrounding countryside.  Please stay indoors with windows and doors locked until further notice.\r\n\r\n**Playground Link:** \u003c!-- A link to a TypeScript Playground \"Share\" link which demonstrates this behavior --\u003e [👨‍💻🔗](https://www.typescriptlang.org/play//#src=type%20Hmm%3CT%2C%20U%20extends%20T%3E%20%3D%20U%20extends%20T%20%3F%20%7B%20%5BK%20in%20keyof%20U%5D%3A%20number%20%7D%20%3A%20never%3B%0D%0Atype%20What%20%3D%20Hmm%3C%7B%7D%2C%20%7B%20a%3A%20string%20%7D%3E%0D%0A%2F%2F%20type%20What%20%3D%20%7B%20%5BK%20in%20keyof%20(T%20%26%20%7B%20a%3A%20string%3B%20%7D)%5D%3A%20number%3B%20%7D%20%F0%9F%A4%AA%0D%0Aconst%20w%3A%20What%20%3D%20%7B%20a%3A%204%20%7D%3B%20%20%2F%2F%20error!%20'%7B%20a%3A%20number%3B%20%7D'%20not%20assignable%20to%20What%0D%0A)\r\n\r\n**Related Issues:** \r\nProbably caused by: #29437\r\nPrior escapes: #31099 \r\nAlso: [Relevant SO question](https://stackoverflow.com/questions/56050214/typescript-nested-map-type-gives-confusing-type-hints)","author":{"url":"https://github.com/jcalz","@type":"Person","name":"jcalz"},"datePublished":"2019-05-09T15:07:56.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/31326/TypeScript/issues/31326"}