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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Assignability of callback parameter defined with conditional tuple type","articleBody":"**TypeScript Version:**  3.1.0-dev.20180727\r\n\r\n**Code**\r\n\r\n```ts\r\n// ConditionalTuple could also return just [] in the false branch, issue still repros\r\ntype ConditionalTuple\u003cT\u003e = [T] extends [undefined] ? [] : [T];\r\n\r\n// works fine!\r\ndeclare var f1: (...args: any[]) =\u003e void;\r\ndeclare var f2: \u003cT\u003e(...args: ConditionalTuple\u003cT\u003e) =\u003e void;\r\ndeclare var f3: (...args: ConditionalTuple\u003cnumber\u003e) =\u003e void;\r\nf2 = f1;\r\nf3 = f1;\r\n\r\n// but if we move the signatures above into a callback parameter with that signature,\r\n// assignability breaks - the types of cb are incompatible.\r\ndeclare var cb1: (cb: (...args: any[]) =\u003e void) =\u003e void;\r\ndeclare var cb2: \u003cT\u003e(cb: (...args: ConditionalTuple\u003cT\u003e) =\u003e void) =\u003e void;\r\ndeclare var cb3: (cb: (...args: ConditionalTuple\u003cnumber\u003e) =\u003e void) =\u003e void;\r\n\r\ncb2 = cb1; // can't assign to cb2\r\ncb3 = cb1; // this works though\r\n```\r\n\r\n**Expected behavior:**\r\nI am able to construct callback signatures with generic conditional types and tuples that are assignable with the maximally general callback signature (`cb1`).\r\n\r\n**Actual behavior:**\r\n`cb1` cannot be assigned to `cb2`, but removing the generic parameter ala `cb3`  makes assignability work as expected.","author":{"url":"https://github.com/bterlson","@type":"Person","name":"bterlson"},"datePublished":"2018-07-27T18:25:45.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":8},"url":"https://github.com/26013/TypeScript/issues/26013"}