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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Unexpected error/bad inference for generic composed function inside `pipe`","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:**  2.9.2\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:** index signature pipe generic lost dictionary\r\n\r\n**Code**\r\n\r\n```ts\r\ndeclare function pipe\u003cV0, T1, T2, T3\u003e(\r\n    fn0: (x: V0) =\u003e T1,\r\n    fn1: (x: T1) =\u003e T2,\r\n    fn2: (x: T2) =\u003e T3,\r\n): (x: V0) =\u003e T3;\r\n\r\ndeclare function getStringDict(): { [index: string]: string };\r\n\r\ndeclare function stringDictIdentity\u003cT\u003e(stringDict: {\r\n    [index: string]: T;\r\n}): { [index: string]: T };\r\n\r\n/*\r\nWithout strict enabled:\r\nExpected type (x: {}) =\u003e { [index: string]: string; }\r\nActual type (x: {}) =\u003e { [index: string]: {}; }\r\n\r\nWith strict enabled, unexpected type error:\r\nArgument of type '\u003cT\u003e(stringDict: { [index: string]: T; }) =\u003e { [index: string]: T; }' is not assignable to parameter of type '(x: {}) =\u003e { [index: string]: {}; }'.\r\n  Types of parameters 'stringDict' and 'x' are incompatible.\r\n    Type '{}' is not assignable to type '{ [index: string]: {}; }'.\r\n      Index signature is missing in type '{}'.\r\n */\r\nconst fn = pipe(\r\n    getStringDict,\r\n    x =\u003e x,\r\n    stringDictIdentity,\r\n);\r\n\r\n//\r\n// Workarounds\r\n//\r\n\r\ndeclare function identity\u003cT\u003e(a: T): T;\r\nconst fn2 = pipe(\r\n    getStringDict,\r\n    identity,\r\n    stringDictIdentity,\r\n);\r\n\r\nconst fn3 = pipe(\r\n    getStringDict,\r\n    x =\u003e x,\r\n    x =\u003e stringDictIdentity(x),\r\n);\r\n```\r\n","author":{"url":"https://github.com/OliverJAsh","@type":"Person","name":"OliverJAsh"},"datePublished":"2018-07-19T09:51:12.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/25791/TypeScript/issues/25791"}