René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Contravariant inference gets picked when a a better covariant candidate exists","articleBody":"### 🔎 Search Terms\n\ncovariant covariance contravariant contravariance inference candidates subtyping assignable subtype\n\n### 🕗 Version \u0026 Regression Information\n\n- This is the behavior in every version I tried\n\n### ⏯ Playground Link\n\nhttps://www.typescriptlang.org/play?ts=5.5.0-dev.20240322#code/C4TwDgpgBAShCOBXCBnYAeAKgZQMYAsIBbAQyggA9gIA7AExSj0NID4oBeKAbwCgooSCACcQALig4CxEgG0ARENHyAugG5eAXw29QkJtNKceg5KID8ExDQDWNAPYB3GmqgAje3RCWo1u05oobV5eOghcABsSYWgAM2tcYABLe0Dhe0RqLGYZcipaBgMWElYACns3ACsJPgEwGIlSkgkpYoBKTnYAN3skug0BFENmyRzSAah8EnoIiEaY+Ak4ITRs4dYOjm7e-q02iR6+nVxUtCgukgi+kmpjUoB9GtMRbytbB2dXDy8fPw-AzSbdjcYK8dKZCClWpQepzc6Xa7UAA0-CgQ2KNVRAiU4ig8nkKIEmkJk2mdFm8wQQJ4WKgJxoZ2oaAkaGESRoAHNjAsAHQ4ibEvY6ShgezCYA8YJAA\n\n### 💻 Code\n\n```ts\r\ntype Request\u003cTSchema extends Schema\u003e = {\r\n  query: TSchema[\"query\"];\r\n};\r\n\r\ntype Schema = { query?: unknown; body?: unknown };\r\n\r\ndeclare function route\u003cTSchema extends Schema\u003e(obj: {\r\n  pre: (a: TSchema) =\u003e void;\r\n  schema: TSchema;\r\n  handle: (req: Request\u003cTSchema\u003e) =\u003e void;\r\n}): void;\r\n\r\nconst validate = (_: { query?: unknown; body?: unknown }) =\u003e {};\r\n\r\nroute({\r\n  pre: validate,\r\n  schema: {\r\n    query: \"\",\r\n  },\r\n  handle: (req) =\u003e {\r\n    const test: string = req.query;\r\n  },\r\n});\r\n\r\nexport {};\r\n```\r\n\n\n### 🙁 Actual behavior\n\nassignment to `test` fails because `{ query?: unknown; body?: unknown; }` was inferred there\n\n### 🙂 Expected behavior\n\nI'd expect `{ query: string }` to be inferred here and the assignment to `test` to succeed\n\n### Additional information about the issue\n\nThis is a reduced version of [this failure](https://github.com/microsoft/TypeScript/pull/54183#issuecomment-2008420338).","author":{"url":"https://github.com/Andarist","@type":"Person","name":"Andarist"},"datePublished":"2024-03-22T18:30:04.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/57908/TypeScript/issues/57908"}