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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Return type of partially applied bound generic function","articleBody":"# Bug Report\r\n\r\n### 🔎 Search Terms\r\n\r\npartial application generic bind return type\r\n\r\n### 🕗 Version \u0026 Regression Information\r\n\r\n- This is the behavior in every version I tried, and I reviewed the FAQ for entries about bind, generics, partial application\r\n\r\n### ⏯ Playground Link\r\n\r\n[Playground link with relevant code](https://www.typescriptlang.org/play?ts=4.6.0-dev.20220211#code/GYVwdgxgLglg9mABAQwDwBVEFMAeUtgAmAzogIIBOFyAnqsmDQHxMAUDl1NAXIugJS9MAbwBQiCYgpYoICkg5VaAblEBfUaIA2MxACNEAXhQA6PTCKtwhLMAtZCAGkQBtAIwBdfssQB6X-qIABbIpFA0AA5YiKz8RkwojC4e2roQRjFxhgnIrO5ePpJFkv6I6SFhkdGx8YhgIAC2elgUyZqipQBiclBBLYjEMA0RWjB2DpqgkLAIiDA2YLDhGFVsyBQA5kJVgnxViGIS0rLyKJuqGqlQiIQZ8wRLNGYWhFZEtvZOiABE+MRQ328fgCtwqiHCUUytXAAGswHAAO5gK7YDI1bJzBaPVi-LD-QGFYpFUrRMEQ6pZBL-CgWDZAA)\r\n\r\n### 💻 Code\r\n\r\n```ts\r\nfunction a\u003cT extends Array\u003cany\u003e\u003e(anArray: T): T {\r\n    return anArray;\r\n}\r\n\r\nlet b = a.bind(undefined, [1]); // b has type () =\u003e any[]\r\nlet c = () =\u003e a([1]);           // c has type () =\u003e number[]\r\n\r\n\r\n// Further simplified\r\n\r\nfunction identity\u003cType\u003e(arg: Type): Type {\r\n  return arg;\r\n}\r\n\r\nlet d = identity.bind(undefined, \"test\"); // d has type () =\u003e unknown\r\nlet e = () =\u003e identity(\"test\");           // e has type () =\u003e string\r\n```\r\n\r\n### 🙁 Actual behavior\r\n\r\nThe types of b and c are different, and the types of d and e are different.\r\n\r\n### 🙂 Expected behavior\r\n\r\nI'd expect them to both have the same type (b that of c, ```() =\u003e number[])```, d that of e ```() =\u003e string```), as the expressions are doing the same thing (arrow function vs bind for partial application of a function).\r\n\r\nThis limitation is noted in a [comment](https://github.com/microsoft/TypeScript/pull/27028#issuecomment-429334450) to the \"Strict bind, call, and apply methods on functions\" PR, but I couldn't find an actual issue referencing this behaviour.\r\nThat comment is also mentioned as a known limitation in a [similar issue](https://github.com/microsoft/TypeScript/issues/28900#issuecomment-445305567) which was closed in #28920, but unfortunately that pr only fixes the issue when binding only this, and not other parameters.\r\n","author":{"url":"https://github.com/thscott","@type":"Person","name":"thscott"},"datePublished":"2022-02-14T23:34:52.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/47899/TypeScript/issues/47899"}