René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Callback argument type not inferred for union of interfaces","articleBody":"### 🔎 Search Terms\r\n\r\n`\"implicitly has an\" inference`\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 \"inference\"\r\n\r\n\r\n### ⏯ Playground Link\r\n\r\nhttps://www.typescriptlang.org/play/?target=5\u0026ts=5.5.3#code/KYDwDg9gTgLgBASwHY2FAZgQwMbDgQQGcBPJbAJWEIFcAbGAYU1toCMcBrAHgBUAaOAFE4AXiFQo0AHxwA3gFgAUHBVwAFGigB+AFxC4AHzhI6tAVCp0YuuDwCUegG4QEAEwDcSgL5KloSLCIKGhYuAQkZJQ09ACSqFCYMNC8AuQCwmKCEtJySqrqCKgAtnr8cNjMbJx6RKQUlvRMLOzY3GlCUg5wzm6eij6K-tDwyPGheLWRDTBxaInQAAqSRQiEwClw5DIKyqpqhcAltl1LECtrXFt9A0quwNi0mBZw6NRkMAgQSHBFmGAAMggVjANu0MuJJFApGo8qonlBSgBtAC6fFhKloQMKehMRVYaDRuxUBwSSQRKkm9WiM3i8ygp3O6zKW0M4TqUSss1JyWZ6SkhJOy1W63IKKkfSUvwBWJgakRAEY+AAmPgAZlRcFVAkwEWw6iQdlE23RcAsMGoUG+3wAVNa4ErrnZ3EA\r\n\r\n### 💻 Code\r\n\r\n```ts\r\nexport interface AsyncResultCallback\u003cT, E = Error\u003e {\r\n    (err?: E | null, result?: T): void;\r\n}\r\n\r\nexport interface AsyncResultIterator\u003cT, R, E = Error\u003e {\r\n    (item: T, callback: AsyncResultCallback\u003cR, E\u003e): void;\r\n}\r\nexport interface AsyncResultIteratorPromise\u003cT, R\u003e {\r\n    (item: T): Promise\u003cR\u003e;\r\n}\r\n\r\ndeclare function mapLimit\u003cT, R, E = Error\u003e(\r\n    arr: T[],\r\n    limit: number,\r\n    iterator:  AsyncResultIteratorPromise\u003cT, R\u003e | AsyncResultIterator\u003cT, R, E\u003e,\r\n): Promise\u003cR[]\u003e;\r\n\r\nmapLimit([1,2,3], 3, async (n) =\u003e {\r\n    return n ** 2;\r\n});\r\n```\r\n\r\n\r\n### 🙁 Actual behavior\r\n\r\nThe type of `n` in the `mapLimit` callback is inferred as `any`:\r\n\r\n```\r\nParameter 'n' implicitly has an 'any' type. (7006)\r\n```\r\n\r\n### 🙂 Expected behavior\r\n\r\nI would expect the type of `n` to be inferred as `number`.\r\n\r\n### Additional information about the issue\r\n\r\nThis example is based on the types for the `async` package:\r\n\r\nhttps://github.com/DefinitelyTyped/DefinitelyTyped/blob/b9820410f85f259cbad6065d8cb6f23f403109ac/types/async/index.d.ts#L321-L325\r\n\r\nNote that if the type for `iterator` in `mapLimit` is changed to either just `AsyncResultIteratorPromise\u003cT, R\u003e` or just `AsyncResultIterator\u003cT, R, E\u003e` (that is, no longer a union type), things work as expected and `n` has type `number`.\r\n\r\nI found a very old discussion of what looks to be the same issue in the `DefinitelyTyped` repo: https://github.com/DefinitelyTyped/DefinitelyTyped/issues/24297. The issue described there is exactly the one I'm facing. One commenter claimed that things worked correctly, and another commenter claimed that changing the way that `async` was imported had some effect? Neither of those sounds plausible to me.\r\n\r\nI found some issues that may describe the same or a similar problem:\r\n\r\n- https://github.com/microsoft/TypeScript/issues/56881\r\n- https://github.com/microsoft/TypeScript/issues/55632\r\n- https://github.com/microsoft/TypeScript/issues/52047\r\n- https://github.com/microsoft/TypeScript/issues/47599\r\n\r\nHowever, my knowledge of TypeScript internals/terminology isn't strong enough to say for sure that this isn't a novel issue I'm facing. If this is in fact a duplicate, please close it as such and I'll add this example to the relevant issue. Thank you!","author":{"url":"https://github.com/nwalters512","@type":"Person","name":"nwalters512"},"datePublished":"2024-07-16T21:16:34.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/59309/TypeScript/issues/59309"}