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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"On DT, lambda-tester is broken by better alias preservation","articleBody":"lambda-tester is broken by #51152.\r\n\r\nConditional type instantiation seems to proceed differently for an alias when it was constructed without type parameters that are later defaulted:\r\n\r\n```ts\r\ntype Actual = lambdaTester.Verifier\u003clambdaTester.HandlerResult\u003cHandler\u003e\u003e\r\ntype Expected = lambdaTester.Verifier\u003clambdaTester.HandlerResult\u003cHandler\u003cany, any\u003e\u003e\u003e\r\n\r\n// from lambda-tester's index.d.ts:\r\ntype HandlerResult\u003cT extends Handler\u003e = T extends Handler\u003cany, infer TResult\u003e ? TResult : never;\r\ntype Verifier\u003cS\u003e = S extends HandlerError\u003cHandler\u003e\r\n    ? (S extends string ? VerifierFn\u003cstring\u003e\r\n       : S extends Error ? VerifierFn\u003cError\u003e\r\n       : never)\r\n    : VerifierFn\u003cS\u003e;\r\nclass LambdaTester\u003cT extends Handler\u003e {\r\n    // ....\r\n    expectSucceed(verifier: Verifier\u003cHandlerResult\u003cT\u003e\u003e): Promise\u003cany\u003e;\r\n    // ....\r\n}\r\n// from aws-lambda's index.d.ts:\r\nexport type Handler\u003cTEvent = any, TResult = any\u003e = (\r\n    event: TEvent,\r\n    context: Context,\r\n    callback: Callback\u003cTResult\u003e,\r\n) =\u003e void | Promise\u003cTResult\u003e;\r\n```\r\n\r\n`Actual` and `Expected` are both `VerifierFn\u003cstring\u003e | VerifierFn\u003cError\u003e | VerifierFn\u003cany\u003e` before #51152. Afterwards, `Actual` is `VerifierFn\u003cunknown\u003e`.\r\n\r\n","author":{"url":"https://github.com/sandersn","@type":"Person","name":"sandersn"},"datePublished":"2022-11-30T17:44:37.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/51698/TypeScript/issues/51698"}