Checking if Enum | falsely in if condition if value of Enum might be zero

Suggestion

🔍 Search Terms

checker, enum, undefined, dx

✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

Check if a value has enum | falsely types, and one member of the enum is zero. And the value In top level of If/while/etc's conditions.

It's a common bug case. Even after condition, CFA could strip the zero value. But If we does not use the enum value, we will never find it out.

📃 Motivating Example

enum E {
    Foo,
    Bar
}

function foo (v: E | undefined) {
    if (!v) { // Should be suggestion or error?
        throw new Error("Hoops")
    }

    console.log("Acceptable") // But without E.foo
}

💻 Use Cases

Start with the motivating enum E | undefined example and review the checker behavior for truthiness narrowing in if conditions. Determine whether the result should be a suggestion or an error, then define tests covering a zero-valued enum member and the narrowed type; done when the intended diagnostic behavior is specified and covered.