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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Can't dereference array type in conditional type","articleBody":"\u003c!-- Please try to reproduce the issue with `typescript@next`. It may have already been fixed. --\u003e\r\n**TypeScript Version:**  2.8.1 and 2.9.0-dev.20180329\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\n\r\nI tried actually a **lot** of terms around mapped types, arrays, conditional types, array member dereferencing in mapped types etc..\r\n\r\n**Code**\r\n\r\n```ts\r\nexport type RecursivePartial\u003cT\u003e = {\r\n  [P in keyof T]?: T[P] extends Array\u003cany\u003e ? {[index: number]: RecursivePartial\u003cT[P][0]\u003e} :\r\n    T[P] extends object ? RecursivePartial\u003cT[P]\u003e : T[P];\r\n};\r\n\r\nvar a = {o: 1, b: 2, c: [{a: 1, c: '213'}]}\r\nfunction assign\u003cT\u003e(o: T, a: RecursivePartial\u003cT\u003e) { }\r\nassign(a, {o: 2, c: {0: {a: 2, c: '213123'}}})\r\n```\r\n\r\n**Expected behavior:**\r\n\r\n`T[P][0]` is usable and doesn't produce an error, since T[P] extends any[] and should thus be indexable on [0] (it seems this is the way to dereference an array)\r\n\r\n**Actual behavior:**\r\nI am getting `type 0 cannot be used to index type T[P]`.\r\n\r\nThe odd part is that behaviour-wise it seems to be working ! I can't change the type of the nested `c` to anything other than string and I can't create random properties or what.\r\n\r\nAm I missing something ?\r\n","author":{"url":"https://github.com/ceymard","@type":"Person","name":"ceymard"},"datePublished":"2018-03-29T15:45:58.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":7},"url":"https://github.com/22985/TypeScript/issues/22985"}