Type predicate using generic not working implicitly - Signature must be a type predicate

[jroru]

🔎 Search Terms

• type predicate
• generic
• must be a type predicate

🕗 Version & Regression Information

• TS 5.2.2
• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.2.2#code/MYewdgzgLgBAbgQwDYEsAmCogE4DEHBbYCeMAvDADwAqANAHwAUAUDPMupjgFwyPAIwaTlACmvAK5gA1mBAB3MAEpy9GAKEjRMFBBjVmKsmoDeAXwDczZgHobMUQA8ADqMKi0MALQxsoqBLYYDBQxK4wIABm7KgYRA4uqMAoUEikztggcOgezIixXHgERMSUYBIAtgBGothMrHwawnGiSrxNWjp65dW1qiFholHqgs2Y2mSTMABEPTXY04ZWtvZysE6u7p4+fgFBA+HD+SI4MBAAFiASSJ6gFdqRmRUh59oA5qJgtSjAMM4I2AQz0YogAdG9QTA5rUVBksjlPOAYKgvjAAIxovIcOI4fCEHClaF1FhsfijLRGNShVzDDotchTWaVeaLJQWIA

💻 Code

const validatorFactory = <T,>(
  validator: (candidate: unknown) => candidate is T
) => {};

// expected - return type of validator explicitly provided
validatorFactory<number>(
  (candidate): candidate is number => typeof candidate === "number"
);

// not expected - return type of validator should come from the generic param (e.g. number)
validatorFactory<number>(
  (candidate) => typeof candidate === "number"
);

🙁 Actual behavior

Argument of type '(candidate: unknown) => boolean' is not assignable to parameter of type '(candidate: unknown) => candidate is number'.
  Signature '(candidate: unknown): boolean' must be a type predicate.(2345)

🙂 Expected behavior

The return type of the type predicate should be based on the generic parameter implicitly.

Additional information about the issue

No response

[MartinJohns]

Related / duplicate of #38390.

Arrow functions are never inferred as type guards.

[jroru]

Is this achievable somehow without using arrow functions?

[typescript-bot]

This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.