`asserts` keyword does not enhance hoisted function type, works on functions assigned to a variable

Description

interface LabelledFunction { label: string; }

function assignLabel(fn: () => unknown, label: string): asserts fn is LabelledFunction {
  Object.assign(fn, { label });
}

function a() { }
assignLabel(a, 'a');
console.info(a.label); // Property 'label' does not exist on type '() => void'.(2339)

const b = function () { };
assignLabel(b, 'b');
console.info(b.label); // works as expected

const c = () => { };
assignLabel(c, 'c');
console.info(c.label); // works as expected

Contributor guide

Tech stack

typescript

Domain

backenddeveloper experience

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.

stale

ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.

clear

Prerequisites

TypeScriptasserts keywordfunction hoisting

Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.

45

Research direction

Investigate the type narrowing behavior for hoisted function declarations in the TypeScript compiler. The relevant code is likely in the checker module (checker.ts) where assertion functions are handled. Compare the handling of function declarations versus variable assignments. Check the existing tests for 'asserts' to see if there are missing test cases.