Microsoft/TypeScript

No type inferrence of callback arguments inside a tuple union type

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#55,632 opened on Sep 5, 2023

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BugDomain: check: Type InferenceHelp Wanted

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Description

🔎 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

Contributor guide

Research direction
Study the TypeScript type inference algorithm for tuple types and discriminated unions. Focus on how the compiler narrows callback parameter types when the first element of the tuple is a literal type. Look at the checker.ts file, specifically the part where contextual types are assigned to callback parameters. Investigate if the current implementation only supports single element discrimination and not nested unions within tuples.
Tech stack
typescript
Domain
compilers
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
TypeScript
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30