Optional properties of function parameter not detected when default parameter created indirectly

[dummdidumm]

Bug Report

When calling a function with an object which has a default parameter and is destructured with fallback values, optional properties are only inferred correctly if that default parameter is created ad-hoc. If it's a function call or variable reference, the optional properties of the parameter are no longer detected.

🔎 Search Terms

function default parameter optional

🕗 Version & Regression Information

• This is the behavior in every version I tried (3.3.3 to Nightly), and I reviewed the FAQ for entries about function calls

⏯ Playground Link

https://www.typescriptlang.org/play?ts=4.2.3&ssl=13&ssc=1&pln=1&pc=1#code/GYVwdgxgLglg9mABAdzgJwNYGcAUBvRAQwBpEAjRAXkSjRAFNEBfKxAwgLhrvtLK9oNmASjZMAUKky48nbgybCA3OPGhIsBIgAm9YIRAAbKAAVCaQgFsAjDlF5xiJ4jT0oINEnYCefH0KYVCXVoeCRtOHosMCgAdXQMW3Y+VkFGFmpdfSNTcytbYXsJCKiY+MwkuTTFJUQAejryFDQEAHNQQ0MATxd6AEcQGFdtVQgELCgdPQNjMwtLACZWb3lecn90lRDNcMjouISF-CIU6jTmViyZ3PmFovES-fKMI9kNmvrGimQWsHajbq9AZDegjIA

💻 Code

function works({ a, b = true } = { a: true, b: true }) {}
works({a: true});

function defaultParam1() {
    return { a: true, b: true };
}
function doesntWork1({ a, b = true } = defaultParam1()) {}
doesntWork1({a: true}); // b wrongfully required

const defaultParam2 = { a: true, b: true };
function doesntWork2({ a, b = true } = defaultParam2) {}
doesntWork2({a: true}); // b wrongfully required

🙁 Actual behavior

Destructuring a function parameter with a default value doesn't mark properties as optional that have a fallback value when the default parameter isn't created ad-hoc.

🙂 Expected behavior

It doesn't make a difference if the default parameter is created ad-hoc or indirectly.

[RyanCavanaugh]

This is the intended behavior. We always use the type of the parameter default to determine the type of the parameter; destructuring defaults are considered "part of the function implementation" and not used to influence the type of the parameter. What you see here is that the type of the fresh object literal in the works example gets contextually typed by the binding pattern and thus acquires a different type than it has a non-contextually-typed position like in defaultParam2.

[dummdidumm]

Thanks for the explanation.