René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Wrongly infers any[] from Array.prototype.flat, Array.prototype.concat","articleBody":"**TypeScript Version:**  3.2.4, 3.3.0-dev.20190126\r\n\r\n**Search Terms:** array, flat, concat, any, unsound\r\n\r\n**Code**\r\n\r\n```ts\r\nconst a: boolean[] = [[17], [\"foo\"]].flat();  // wrongly accepted\r\nconst b: boolean[] = Array.prototype.concat([17], [19], [21]);  // wrongly accepted\r\n```\r\n\r\nUsing `tsc --strict -t esnext`.\r\n\r\n(What I’m really trying to do: find a type safe way to concatenate an array of arrays. It’d be nice if `[].concat(...arrays)` were accepted, which I think would be a consequence of #26976, but that’s not the issue I’m reporting here. `arrays.flat()` and `Array.prototype.concat(...arrays)` are accepted but apparently provide no type safety.)\r\n\r\n**Expected behavior:** `[[17], [\"foo\"]].flat()` should be typed `(number | string)[]` (if it’s accepted at all), and `Array.prototype.concat([17], [19], [21])` should be typed `number[]`. In both cases the assignment to `boolean[]` should be rejected.\r\n\r\n**Actual behavior:** TypeScript infers `any[]` for both right hand sides (even with `--noImplicitAny` implied by `--strict`), and accepts the assignments to `boolean[]` with no complaints.\r\n\r\n**Playground Link:** Playground doesn’t support `esnext`, but here’s the [second line](https://www.typescriptlang.org/play/#src=const%20b%3A%20boolean%5B%5D%20%3D%20Array.prototype.concat(%5B17%5D%2C%20%5B19%5D%2C%20%5B21%5D)%3B).\r\n\r\n**Related Issues:** #19535 for `concat` (but that one may have more to do with the difficulty of typing `concat`’s weird behavior when non-arrays are passed?), #26976","author":{"url":"https://github.com/andersk","@type":"Person","name":"andersk"},"datePublished":"2019-01-26T10:15:32.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":28},"url":"https://github.com/29604/TypeScript/issues/29604"}