5.6.1-rc Incorrectly assumes "this" is not nullish even when declared as such

[Knagis]

🔎 Search Terms

ts2869
Right operand of ?? is unreachable because the left operand is never nullish.

🕗 Version & Regression Information

Feature introduced in 5.6
reproduced in 5.6.1-rc and v5.7.0-dev.20240826

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.6.1-rc#code/GYVwdgxgLglg9mABMOcAUUAWMDOAuRAeQCMArAU2kQB9FwATc4GMc+gSkQG8AoRfxACdyUEIKRZciAPzTEABgDcPAL48gA

💻 Code

function foo(this: Object | undefined) {
    return this ?? 0;
}

🙁 Actual behavior

Complains that this is never nullish.

🙂 Expected behavior

Since this is explicitly declared as | undefined, the error should not be shown.

In non-strict mode, this indeed could not be nullish, but in strict mode it can.

new Function(`"use strict"
function foo() {
    console.log(this);
}
foo.apply(undefined)`)()

Additional information about the issue

No response