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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Incorrect overload is used for `pipe` when first function has optional parameter","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:**  3.3.1\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\n\r\n**Code**\r\n\r\n- when `strictFunctionTypes` is disabled\r\n- when first composed function parameter is optional\r\n- when first `pipe` overload is zero parameters for first function\r\n\r\n```ts\r\ndeclare const pipe: {\r\n    // 0-argument first function\r\n    // Workaround: disable this overload\r\n    \u003cA\u003e(a: () =\u003e A): () =\u003e A;\r\n\r\n    // 1-argument first function\r\n    \u003cA, B\u003e(ab: (a: A) =\u003e B): (a: A) =\u003e B;\r\n};\r\n\r\n// Expected type: `(a: {} | undefined) =\u003e number`\r\n// Actual type: `() =\u003e number`\r\nconst fn = pipe((_a?: {}) =\u003e 1);\r\n```\r\n\r\n**Related Issues:** \u003c!-- Did you find other bugs that looked similar? --\u003e\r\nhttps://github.com/Microsoft/TypeScript/issues/29904","author":{"url":"https://github.com/OliverJAsh","@type":"Person","name":"OliverJAsh"},"datePublished":"2019-02-14T10:24:26.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/29913/TypeScript/issues/29913"}