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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Extending empty tuple in generic functions causes compiler to infer arrays as tuples","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:**  3.4.5\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:**\r\nempty tuple, type inference, generic function\r\n\r\n**Code**\r\nBasically I'm writing a generic function with dynamic type mapping. I think this use case is pretty common for database drivers and JSON validators and I found some related issues. There is a workaround and I want to confirm whether it is intended behavior or a bug.\r\n```ts\r\ninterface TypeMap { STRING: string; NUMBER: number;}\r\ntype MapTypes\u003cT\u003e = {\r\n  [P in keyof T]: T[P] extends keyof TypeMap ? TypeMap[T[P]] : never\r\n};\r\n\r\ndeclare function fromArray\u003cT extends ReadonlyArray\u003ckeyof TypeMap\u003e\u003e(\r\n  types: T, params: MapTypes\u003cT\u003e): void;\r\n\r\nfromArray(['STRING', 'NUMBER', 'STRING'], ['abc', 123, 'xyz']);  // ok\r\n// the default behavior is to infer types in array as type union\r\nfromArray(['STRING', 'NUMBER', 'STRING'], ['abc', '123', 'xyz']);  // no error\r\nfromArray(['STRING', 'NUMBER', 'STRING'] as const, ['abc', '123', 'xyz']);  // error\r\n\r\ndeclare function fromTuple\u003cT extends([] | ReadonlyArray\u003ckeyof TypeMap\u003e)\u003e(\r\n  types: T, params: MapTypes\u003cT\u003e): void;\r\n\r\nfromTuple(['STRING', 'NUMBER', 'STRING'], ['abc', 123, 'xyz']);  // ok\r\n// T extends([]|MyType) cases the compiler to infer array as tuple\r\nfromTuple(['STRING', 'NUMBER', 'STRING'], ['abc', '123', 'xyz']);  // error\r\nfromTuple(['STRING', 'NUMBER', 'STRING'] as const, ['abc', '123', 'xyz']);  // error\r\n```\r\n\r\n**Expected behavior:**\r\nType of the array literal is inferred as array of union type.\r\n\r\n**Actual behavior:**\r\nType of the array literal is inferred as tuple when `T extends []|MyType` is used in generic function.\r\nI'd prefer it to be the expected behavior, but I didn't find any tests for that use case.\r\n\r\n**Playground Link:** \u003c!-- A link to a TypeScript Playground \"Share\" link which demonstrates this behavior --\u003e\r\n[You can see the same behavior in the playground](https://www.typescriptlang.org/play/#src=interface%20TypeMap%20%7B%20STRING%3A%20string%3B%20NUMBER%3A%20number%3B%7D%0D%0Atype%20MapTypes%3CT%3E%20%3D%20%7B%0D%0A%20%20%5BP%20in%20keyof%20T%5D%3A%20T%5BP%5D%20extends%20keyof%20TypeMap%20%3F%20TypeMap%5BT%5BP%5D%5D%20%3A%20never%0D%0A%7D%3B%0D%0A%0D%0Adeclare%20function%20fromArray%3CT%20extends%20ReadonlyArray%3Ckeyof%20TypeMap%3E%3E(%0D%0A%20%20types%3A%20T%2C%20params%3A%20MapTypes%3CT%3E)%3A%20void%3B%0D%0A%0D%0AfromArray(%5B'STRING'%2C%20'NUMBER'%2C%20'STRING'%5D%2C%20%5B'abc'%2C%20123%2C%20'xyz'%5D)%3B%20%20%2F%2F%20ok%0D%0A%2F%2F%20the%20default%20behavior%20is%20to%20infer%20types%20in%20array%20as%20type%20union%0D%0AfromArray(%5B'STRING'%2C%20'NUMBER'%2C%20'STRING'%5D%2C%20%5B'abc'%2C%20'123'%2C%20'xyz'%5D)%3B%20%20%2F%2F%20no%20error%0D%0AfromArray(%5B'STRING'%2C%20'NUMBER'%2C%20'STRING'%5D%20as%20const%2C%20%5B'abc'%2C%20'123'%2C%20'xyz'%5D)%3B%20%20%2F%2F%20error%0D%0A%0D%0Adeclare%20function%20fromTuple%3CT%20extends(%5B%5D%20%7C%20ReadonlyArray%3Ckeyof%20TypeMap%3E)%3E(%0D%0A%20%20types%3A%20T%2C%20params%3A%20MapTypes%3CT%3E)%3A%20void%3B%0D%0A%0D%0AfromTuple(%5B'STRING'%2C%20'NUMBER'%2C%20'STRING'%5D%2C%20%5B'abc'%2C%20123%2C%20'xyz'%5D)%3B%20%20%2F%2F%20ok%0D%0A%2F%2F%20T%20extends(%5B%5D%7CMyType)%20cases%20the%20compiler%20to%20infer%20array%20as%20tuple%0D%0AfromTuple(%5B'STRING'%2C%20'NUMBER'%2C%20'STRING'%5D%2C%20%5B'abc'%2C%20'123'%2C%20'xyz'%5D)%3B%20%20%2F%2F%20error%0D%0AfromTuple(%5B'STRING'%2C%20'NUMBER'%2C%20'STRING'%5D%20as%20const%2C%20%5B'abc'%2C%20'123'%2C%20'xyz'%5D)%3B%20%20%2F%2F%20error)\r\n\r\n**Related Issues:** \u003c!-- Did you find other bugs that looked similar? --\u003e\r\nhttps://github.com/microsoft/TypeScript/issues/13151\r\nhttps://github.com/microsoft/TypeScript/issues/22679\r\n\r\n**Related PR**\r\nThis PR includes a similar test, but it there is no test for empty tuple and array combination https://github.com/microsoft/TypeScript/pull/26676\r\n","author":{"url":"https://github.com/ipotuzhnov","@type":"Person","name":"ipotuzhnov"},"datePublished":"2019-05-16T19:53:27.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/31434/TypeScript/issues/31434"}