Overloaded function args not inferred correctly

Bug Report

🔎 Search Terms

overload functions

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about overloading and different function arguments behavior

⏯ Playground Link

Playground link with relevant code

💻 Code

function a1(arg1: unknown): 1;
function a1(arg1: unknown, arg2: unknown): 2;
function a1(...args: ([arg1: unknown] | [arg1: unknown, arg2: unknown])): 1 | 2 {
    return args.length;
}

const b1: typeof a1 = function f1(...args) {
    return a1(...args);
}

even simpler is using an interface

interface a1 {
    (arg1: unknown): 1;
    (arg1: unknown, arg2: unknown): 2;
}

// errors
const b1: a1 = function f1(...args) {
    return args.length;
}

🙁 Actual behavior

Error: Target signature provides too few arguments. Expected 2 or more, but got one., given the single signature of b1 above is variadic, and in fact matches the implementation signature of a1, one would assume the two functions are equivalent (from a signature perspective)

🙂 Expected behavior

The type of args in b1 should be inferred as the union of all overloads of a1 since it is the only way the variadic args would satisfy the assigned typeof a1 requirement.

Start with the linked TypeScript Playground and the two overload examples to reproduce the diagnostic. Trace how the assigned function's rest parameter is inferred against the overloads, then verify that args receives the required overload union and the assignment is accepted without the reported error.