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

TypeScript Version: 4.0.2, 4.1.0-beta

Search Terms:
asserts function

Code

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

Expected behavior:

At least in runtime code coming after assignLabel(a, 'a');, a.label should be accessible.

Actual behavior:

Property 'label' does not exist on type '() => void'.(2339)

Playground Link: link

Start with the linked TypeScript Playground reproduction and compare the hoisted function a with the variable-assigned functions b and c after assignLabel is called. Trace the type-checker behavior for assertion functions and confirm completion when a.label is accepted in code after the call without regressing the working cases.