No type inferrence of callback arguments inside a tuple union type

🔎 Search Terms

• discriminated union inference
• union argument inference

🕗 Version & Regression Information

• This is a compilation error in tsserver 3.3.2

⏯ Playground Link

https://www.typescriptlang.org/play?#code/C4TwDgpgBAkgdgMwgJwILIOZQLwCgpQA+UA2gIwA0UAFAIYBcUcArgLYBGKAlDgHxQA3APYBLACYBdfEVIAmKtXaMAzsGQi4GHtn7DxEgNy4AxkLiqotTI3hI0mHKUo1a2-gG8oAX0O4gA

💻 Code

type InferArg =
  | [1, (a: number) => void]
  | [2, (b: string) => void];
const arg: InferArg = [1, (a) => { }];

🙁 Actual behavior

I get a compilation error:

Parameter 'a' implicitly has an 'any' type.(7006)

🙂 Expected behavior

a should be inferred to be a number because I already passed 1 as the first tuple element. Type inference should be identical to this code:

type InferArg = [1, (a: number) => void];
const arg: InferArg = [1, (a) => { }];

Additional information about the issue

No response

Start with the linked TypeScript Playground reproduction and confirm the behavior using the tuple-union example under tsserver 3.3.2. Trace the compiler's contextual typing and union argument inference for the callback parameter, then add or update coverage for the example so the parameter is inferred as number and the regression is verified.