{"action":"created","comment":{"url":"https://api.github.com/repos/Microsoft/TypeScript/pulls/comments/65283642","id":65283642,"diff_hunk":"@@ -119,9 +119,9 @@ namespace ts {\n         const voidType = createIntrinsicType(TypeFlags.Void, \"void\");\r\n         const undefinedType = createIntrinsicType(TypeFlags.Undefined | nullableWideningFlags, \"undefined\");\r\n         const nullType = createIntrinsicType(TypeFlags.Null | nullableWideningFlags, \"null\");\r\n-        const emptyArrayElementType = createIntrinsicType(TypeFlags.Undefined | TypeFlags.ContainsUndefinedOrNull, \"undefined\");\r\n         const unknownType = createIntrinsicType(TypeFlags.Any, \"unknown\");\r\n         const neverType = createIntrinsicType(TypeFlags.Never, \"never\");\r\n+        const wideningNeverType = createIntrinsicType(TypeFlags.Never | TypeFlags.ContainsWideningType, \"never\");\r","path":"src/compiler/checker.ts","position":16,"original_position":16,"commit_id":"0f05c64a77d0836f572306d6162118dfff539b9e","original_commit_id":"0f05c64a77d0836f572306d6162118dfff539b9e","user":{"login":"DanielRosenwasser","id":972891,"avatar_url":"https://avatars.githubusercontent.com/u/972891?v=3","gravatar_id":"","url":"https://api.github.com/users/DanielRosenwasser","html_url":"https://github.com/DanielRosenwasser","followers_url":"https://api.github.com/users/DanielRosenwasser/followers","following_url":"https://api.github.com/users/DanielRosenwasser/following{/other_user}","gists_url":"https://api.github.com/users/DanielRosenwasser/gists{/gist_id}","starred_url":"https://api.github.com/users/DanielRosenwasser/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/DanielRosenwasser/subscriptions","organizations_url":"https://api.github.com/users/DanielRosenwasser/orgs","repos_url":"https://api.github.com/users/DanielRosenwasser/repos","events_url":"https://api.github.com/users/DanielRosenwasser/events{/privacy}","received_events_url":"https://api.github.com/users/DanielRosenwasser/received_events","type":"User","site_admin":false},"body":"Leave a comment on what a *widening* never implies over an unwidened never. From what I understand, it's kind of a similar concept to freshness.","created_at":"2016-06-01T00:01:47Z","updated_at":"2016-06-01T00:01:47Z","html_url":"https://github.com/Microsoft/TypeScript/pull/8907#discussion_r65283642","pull_request_url":"https://api.github.com/repos/Microsoft/TypeScript/pulls/8907","_links":{"self":{"href":"https://api.github.com/repos/Microsoft/TypeScript/pulls/comments/65283642"},"html":{"href":"https://github.com/Microsoft/TypeScript/pull/8907#discussion_r65283642"},"pull_request":{"href":"https://api.github.com/repos/Microsoft/TypeScript/pulls/8907"}}},"pull_request":{"url":"https://api.github.com/repos/Microsoft/TypeScript/pulls/8907","id":72103654,"html_url":"https://github.com/Microsoft/TypeScript/pull/8907","diff_url":"https://github.com/Microsoft/TypeScript/pull/8907.diff","patch_url":"https://github.com/Microsoft/TypeScript/pull/8907.patch","issue_url":"https://api.github.com/repos/Microsoft/TypeScript/issues/8907","number":8907,"state":"open","locked":false,"title":"Use 'never' type for empty array literals","user":{"login":"ahejlsberg","id":4226954,"avatar_url":"https://avatars.githubusercontent.com/u/4226954?v=3","gravatar_id":"","url":"https://api.github.com/users/ahejlsberg","html_url":"https://github.com/ahejlsberg","followers_url":"https://api.github.com/users/ahejlsberg/followers","following_url":"https://api.github.com/users/ahejlsberg/following{/other_user}","gists_url":"https://api.github.com/users/ahejlsberg/gists{/gist_id}","starred_url":"https://api.github.com/users/ahejlsberg/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/ahejlsberg/subscriptions","organizations_url":"https://api.github.com/users/ahejlsberg/orgs","repos_url":"https://api.github.com/users/ahejlsberg/repos","events_url":"https://api.github.com/users/ahejlsberg/events{/privacy}","received_events_url":"https://api.github.com/users/ahejlsberg/received_events","type":"User","site_admin":false},"body":"This PR introduces a widening form of the `never` type that we use for empty array literals in `--strictNullChecks` mode. This fixes the following issue reported in #8878:\r\n\r\n```typescript\r\n// Compile with --strictNullChecks\r\nfunction concat<T>(xs: T[], ys: T[]): T[] {\r\n    return [...xs, ...ys];\r\n}\r\nconst y = concat([], [\"a\"]);  // Should be type string[]\r\n```\r\n\r\nBefore this fix the inferred type above was `(string | undefined)[]` because we were using type `undefined[]` for empty array literals, and `undefined` isn't a bottom type in strict null checking 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