`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

从链接的 TypeScript Playground 复现开始，并在调用 assignLabel 后，将 hoisted 函数 a 与赋值给变量的函数 b 和 c 进行比较。跟踪 type-checker 对 assertion function 的行为，并确认在调用后的代码中接受 a.label，且不会使正常工作的案例出现回归后即可完成。