René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Homomorphic mapped generic array type sometimes no longer seen as an array","articleBody":"### 🔎 Search Terms\r\n\r\nconditional, mapped, array\r\n\r\n### 🕗 Version \u0026 Regression Information\r\n\r\n- This changed in PR #57122 (I think... definitely between 5.3 and 5.4)\r\n\r\n\r\n### ⏯ Playground Link\r\n\r\n[Playground link](https://www.typescriptlang.org/play?ts=5.5.0-dev.20240312#code/C4TwDgpgBAsgrgZ2AIQgQQE4YIYgDwAqUEAHsBAHYAmCU2FIA2gLoB8UAvFAQFA+iQoACQC2IwsTKUadBi3ZcipctVoU4IgEYQMLKAH4eUY7EQp0WXHgDeURgEkoASwpQA1hBAB7AGbdmAFxQAIxQAL7sAUYmFBAAbjoA3HwC0AAanMJieIwAzAA0UAAshQCsbDwA9JUmAHr6QA)\r\n\r\n### 💻 Code\r\n\r\n```ts\r\ntype MustBeArray\u003cT extends any[]\u003e = T\r\n\r\ntype Hmm\u003cT extends any[]\u003e = T extends number[] ?\r\n    MustBeArray\u003c{ [I in keyof T]: 1 }\u003e : // \u003c-- error!\r\n    //          ~~~~~~~~~~~~~~~~~~~~~\r\n    // Type '{ [I in keyof T]: 1; }' does not satisfy the constraint 'any[]'.\r\n    // Types of property 'toString' are incompatible.\r\n    // Type 'number' is not assignable to type '() =\u003e string'.(2344)\r\n    never;\r\n\r\ntype X = Hmm\u003c[3, 4, 5]\u003e\r\n//   ^? type X = [1, 1, 1]\r\n```\r\n\r\n\r\n### 🙁 Actual behavior\r\n\r\n`{[I in keyof T]: 1}` is not seen as an array type inside of `Hmm\u003cT\u003e`, causing a compiler error.  Something about the conditional type re-constraining `T` to another array type causes it to fail.\r\n\r\n### 🙂 Expected behavior\r\n\r\nThe code should compile without error.\r\n\r\n### Additional information about the issue\r\n\r\nComes from [this Stack Overflow question](https://stackoverflow.com/q/78148685/2887218).\r\n\r\nI'm a bit concerned that I can't seem to work around this without `//@ts-ignore`, since the normal approaches of `Extract\u003cT, any[]\u003e` don't help (they end up just doing the same conditional type thing again which break things) and `T \u0026 any[]` kind of works but you end up with intersections of arrays which nobody likes.","author":{"url":"https://github.com/jcalz","@type":"Person","name":"jcalz"},"datePublished":"2024-03-12T17:28:52.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":6},"url":"https://github.com/57744/TypeScript/issues/57744"}