Assertion functions do not work with arrow functions

[ehmicky]

Bug Report

Assertion functions do not work when using arrow functions.

🔎 Search Terms

ts2775, arrow functions, assertion functions

🕗 Version & Regression Information

Assertion functions were introduced in 3.7. The behavior has been the same since that version.
This is the behavior in every version I tried, and I reviewed the FAQ for entries about it.

⏯ Playground Link

Playground link with relevant code

💻 Code

function assertOne(val: unknown): asserts val is string {
  if (typeof val !== "string") {
    throw new Error("Not a string!");
  }
}

const assertTwo = (val: unknown): asserts val is string => {
  if (typeof val !== "string") {
    throw new Error("Not a string!");
  }
}

function main(str: unknown) {
  assertOne(str); // works
  assertTwo(str); // "Assertions require every name in the call target to be declared with an explicit type annotation. (ts2775)"
  return str;
}

🙁 Actual behavior

assertOne() works. assertTwo() (although nearly identical) shows the following type checking error:

Assertions require every name in the call target to be declared with an explicit type annotation. (ts2775)

🙂 Expected behavior

assertTwo() should not show any type checking error.

[xiBread]

Duplicate of #34523

[ehmicky]

Thanks @xiBread!
This is indeed a duplicate, so I will close.