Invalid 'Object is possibly undefined' for variable definitely assigned via compound assignment

[ajafff]

Bug Report

🔎 Search Terms

🕗 Version & Regression Information

• This is the behavior since 4.0 where compound assignments were introduced

⏯ Playground Link

Playground Link: Provided

💻 Code

function foo(a: string[] | undefined, b: string[] | undefined) {
    a ??= b || []; // doesn't work
    a ??= b ?? []; // doesn't work
    a ||= (b || []); // doesn't work
    a ||= b || []; // doesn't work
    // a ??= b!; // works, so it seems to require the BarBar or QuestionQuestion operator on the RHS
    // a ?? (a = b || []); // works
    // a || (a = b || []); // works
    // if (!a) a = b || []; // works
    a.includes('foo'); // 'a' is possibly 'undefined'
}

🙁 Actual behavior

Error Object is possibly 'undefined'. on the last statement.

🙂 Expected behavior

Each of the assignments should remove undefined from the type.

This might be caused by a faulty control flow graph for compound assingments because the same code written as its downlevel equivalent (see commented out lines) works as expected.

/cc @Kingwl

[MartinJohns]

Looks like a duplicate of #40494. Tho according to that issue there's a fix available for 4.2.0 already, which is not working on the nightly in the playground.