René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Inferred type changes from 5.5 to 5.6","articleBody":"### 🔎 Search Terms\n\n- variance\n\n### 🕗 Version \u0026 Regression Information\n\n- This changed in commit or PR #57909 (bisected via `every-ts`)\r\n\n\n### ⏯ Playground Link\n\nhttps://www.typescriptlang.org/play/?ts=5.6.2#code/CYUwxgNghgTiAEYD2A7AzgF3gMwFzwB4AFAPgAoAoeHFfMgBxiXrXyIEp4BeE+ANyQBLYABoq8QSkEYA-GzGce8IgG4KFSRhAxsUMAiJMW8AN7i4AR3yYYkgOZrqzWdYy2UDigF91ydFkZmNG4cSmoGNiM0RV4AclixahN4S3wAIjT4LwU1CkCWADpnCgB6EvgAURgmGAkUeAAVAGV4AFYCgDYReBQkLEl4EFgIQW1+bTRBVDQKIA\n\n### 💻 Code\n\n```ts\r\ndeclare const f: \u003cP\u003e(\r\n  fn: (props: P) =\u003e void,\r\n  init?: P,\r\n) =\u003e P;\r\n\r\ninterface Props {\r\n  req: string;\r\n  opt?: string;\r\n}\r\n\r\nconst props = f(\r\n  (p: Props) =\u003e '',\r\n  { req: \"\" },\r\n);\r\n\r\nprops.opt\r\n// Error in TS 5.6, not in earlier versions\r\n```\r\n\n\n### 🙁 Actual behavior\n\n`P` is inferred as `{req: string}`, so accessing the optional property causes a TypeScript error.\n\n### 🙂 Expected behavior\n\n`P` should be inferred as `Props`, as it is in TS 5.5 (and earlier). If you drop the `init` parameter from the call to `f`, then `P` is also inferred to be `Props` in TS 5.6.\n\n### Additional information about the issue\n\nThis seems related to #59764 but I'm not sure it's the same, so I figured I'd file an issue. I tried the same code on the [playground] for #59709 and it does not fix this issue.\r\n\r\ncc @Andarist for whether TS 5.6 is right and I'm wrong 😄 \r\n\r\n[playground]: https://www.typescriptlang.org/play/?ts=5.7.0-pr-59709-8#code/CYUwxgNghgTiAEYD2A7AzgF3gMwFzwB4AFAGngCUA+ACgCh4cV9qAHGJFtfIgSngF5KFEvXgBLFGIwB+biL6D4RANy1aEjCBjYoYBEXad4Ab1FwAjvkwwJAc1UMOMqxhsp7tAL5rk6LGw40ARw6BlZuQzQFIQByGJEGY3gLfAAiVPhPeVVaAM4AOidaIA","author":{"url":"https://github.com/danvk","@type":"Person","name":"danvk"},"datePublished":"2024-09-26T21:00:47.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":5},"url":"https://github.com/60077/TypeScript/issues/60077"}