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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Conditional types are evaluated too eagerly","articleBody":"\r\n**TypeScript Version:**  3.7.x-dev.201xxxxx\r\n\r\n**Search Terms:** conditional type eager, conditional type narrowing\r\n\r\n**Code**\r\n\r\n```ts\r\ntype FooHelper\u003cX\u003e = (arg1: X, arg2: X extends 0 ? number : string) =\u003e void;\r\ntype Foo = FooHelper\u003c0 | 1\u003e;\r\n\r\nlet x: number;\r\nconst foo: Foo = (t, d) =\u003e {\r\n    if(t === 0) { x = d; }\r\n}\r\n```\r\n\r\n**Expected behavior:**\r\nNo errors, `t` has type `0 | 1` in the function body which gets narrowed to just `0` in the body of the if statement. In the if statement, `d` should have type number, because we know that `t` has to have type `0`.\r\n\r\n**Actual behavior:**\r\nType 'string | number' is not assignable to type 'string'.\r\n\r\n**Playground Link:** https://www.typescriptlang.org/play/?ts=3.7-Beta#code/C4TwDgpgBAYg9nAEhANpATgHgBoD4oC8UAFAIboDmAjAFxTYA0U5FATHdlBAB7AQB2AEwDOUAAxQA-FGHB0AS34UodfgFcAtgCMI6AJSF8ANzjzBAbgBQoSLASE7SVBkwSAPlCq4rllBGBQ3HSyCkpWAMZw-LJQAGYIdPBwDsTATIIGBPgA3pZQ+VDysamEBERiBtmBDhZQAL6WdUA\r\n\r\n**Related Issues:** not really\r\n","author":{"url":"https://github.com/jvanbruegge","@type":"Person","name":"jvanbruegge"},"datePublished":"2019-10-29T21:13:40.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":6},"url":"https://github.com/34810/TypeScript/issues/34810"}