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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Inference of constrained generic causes open generic to \"escape\"","articleBody":"\u003c!-- 🚨 STOP 🚨 𝗦𝗧𝗢𝗣 🚨 𝑺𝑻𝑶𝑷 🚨\r\n\r\nHalf of all issues filed here are duplicates, answered in the FAQ, or not appropriate for the bug tracker. Even if you think you've found a *bug*, please read the FAQ first, especially the Common \"Bugs\" That Aren't Bugs section!\r\n\r\nPlease help us by doing the following steps before logging an issue:\r\n  * Search: https://github.com/Microsoft/TypeScript/search?type=Issues\r\n  * Read the FAQ: https://github.com/Microsoft/TypeScript/wiki/FAQ\r\n\r\nPlease fill in the *entire* template below.\r\n--\u003e\r\n\r\n\u003c!-- Please try to reproduce the issue with `typescript@next`. It may have already been fixed. --\u003e\r\n**TypeScript Version:**  3.5.0-dev.20190514\r\n\r\n\u003c!-- Search terms you tried before logging this (so others can find this issue more easily) --\u003e\r\n**Search Terms:**\r\nconstrained generic inference infer keyword\r\ninfer open generic\r\ninfer constrained generic\r\n\r\n**Code**\r\n\r\n```ts\r\ninterface Synthetic\u003cA, B extends A\u003e {}\r\ntype SyntheticDestination\u003cT, U\u003e = U extends Synthetic\u003cT, infer V\u003e ? V : never;\r\ntype TestSynthetic = // Resolved to T\r\n    SyntheticDestination\u003cnumber, Synthetic\u003cnumber, number\u003e\u003e;\r\n\r\nconst w: TestSynthetic = undefined // ✓\r\nconst x: TestSynthetic = null; // ✓\r\n\r\nconst y: TestSynthetic = 3; // Type '3' is not assignable to type 'T'.\r\nconst z: TestSynthetic = '3'; // Type '\"3\"\"' is not assignable to type 'T'.\r\n```\r\n\r\n**Expected behavior:**\r\n- TestSynthetic resolved as number\r\n- w, x, and z invalid with strict\r\n- y valid\r\n\r\n**Actual behavior:**\r\n- TestSynthetic resolved as T -- an open generic from the underlying conditional type\r\n- w and x valid, even with strict\r\n- y and z error with commented errors\r\n\r\n**Playground Link:** \u003c!-- A link to a TypeScript Playground \"Share\" link which demonstrates this behavior --\u003e\r\nhttps://bit.ly/30lTSqk\r\n\r\n**Related Issues:** \u003c!-- Did you find other bugs that looked similar? --\u003e\r\nhttps://github.com/microsoft/TypeScript/issues/31326\r\n\r\n**Notes:**\r\n\r\n- Removing the `extends A` constraint causes expected behavior, so that's likely involved somehow.\r\n- Turning on all strict options in typescript playground causes all four example cases to be errors -- unclear why this is stricter than `strict: true` in my local tsconfig with @next.\r\n- I'm not 100% sure if a conditional type that resolves to an open generic is allowed -- I was certainly surprised to see it. If it is, this still seems to be a bug in conditional type resolution.","author":{"url":"https://github.com/ddurschlag6river","@type":"Person","name":"ddurschlag6river"},"datePublished":"2019-05-14T21:54:39.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/31402/TypeScript/issues/31402"}