Type predicates only work with shorthand syntax in if statements

[ngregory-rbi]

Bug Report

🔎 Search Terms

predicate
type predicate
predicate syntax

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about type predicates.

⏯ Playground Link

Playground link with relevant code

💻 Code

function a() {
    const validFlags = ['-a', '-b', '-c'] as const;
    function isFlagValid(flag: string): flag is typeof validFlags[number] {
        return validFlags.includes(flag as typeof validFlags[number]);
    }
    const flag = Math.random().toString();
    if (isFlagValid(flag) === false) {
        flag; // string
        return;
    }
    flag; // string

    if (!isFlagValid(flag)) {
        flag; // string
        return;
    }
    flag; // typeof validFlags[number]
}

function b() {
    const validFlags = ['-a', '-b', '-c'] as const;
    function isFlagValid(flag: string): flag is typeof validFlags[number] {
        return validFlags.includes(flag as typeof validFlags[number]);
    }
    const flag = Math.random().toString();
    if (isFlagValid(flag) === true) {
        flag; // string
        return;
    }
    flag; // string
    
    if (isFlagValid(flag)) {
        flag; // typeof validFlags[number]
        return;
    }
    flag; // string
}

🙁 Actual behavior

The function with a type predicate only limits the type definition of flag if it's used in shorthand syntax !isFlagValid(flag) or isFlagValid(flag) and not the more explicit isFlagValid(flag) === false or isFlagValid(flag) === true.

🙂 Expected behavior

The function with a type predicate limits the type definition of flag regardless of the syntax uses and they both mean the same thing.

[MartinJohns]

Duplicate of #44366.

[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.