Undefined not narrowed out of array pop/shift return type despite check

[wbt]

Bug Report

🔎 Search Terms

array pop shift undefined length

🕗 Version & Regression Information

Observed in 3.7.5 - 4.5.5 (latest) + nightly as of issue filing.
Prior versions claim an error but aren't as specific; there's no reason to believe this is a regression.

⏯ Playground Link

Playground link with relevant code

💻 Code

export class MyClass {   
    typescriptCopyExample(): Array<string> {
        const stringsToCopy = ['a', 'b', 'c'];
        let result: Array<string> = [];
        while(stringsToCopy.length>0) {
            result.push(stringsToCopy.pop()); // Error: 'string | undefined' is not assignable to parameter of type 'string'
        }
        return result;
    }   
}

🙁 Actual behavior

An error on the flagged line that the result of pop() could be undefined, which is what happens when the array is empty, even inside a conditional check that this is not the case.

🙂 Expected behavior

Inside the conditional check, stringsToCopy is narrowed to an array of length > 0. The return type of pop() and shift() operations are not flagged as possibly undefined.

[MartinJohns]

Duplicate of #30406.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.