Strange boolean-discriminant narrowing with strictNullChecks off

描述

type Result = { success: true }
            | { success: false, error: string }

function handleError(res: Result) {
    if (res.success === true) {
        return;
    }

    res.error; // OK
}

function handleError2(res: Result) {
    if (res.success !== false) {
        return;
    }

    res.error; // OK
}

function handleError3(res: Result) {
    if (res.success) {
        return;
    }

    res.error; // Property 'error' does not exist on type 'Result'
               // but should be OK
}

貢獻者指南

技術棧

typescript

領域

developer experience

議題類型

bug

難度面向新貢獻者的預計實作難度，1 表示很小改動，5 表示專家級工作。

3

預計時間有經驗貢獻者完成調查、實作、測試並準備 pull request 的粗略時間範圍。

1-2 days

活動狀態議題目前的可參與程度：新鮮、活躍、陳舊、阻塞或等待維護者輸入。

stale

清晰度議題是否清楚說明預期改動、驗收標準和下一步。

clear

前置要求

TypeScript type system understandingKnowledge of control flow analysis

新手友善度1-100 的估計分數，表示該議題對首次貢獻者的友善程度。

40

研究方向

Investigate the control flow narrowing logic in the TypeScript compiler, specifically in checker.ts. Compare how truthy checks (if (res.success)) are handled versus explicit boolean comparisons (if (res.success === true)). The bug suggests that with strictNullChecks off, truthy checks fail to narrow union types correctly. Look for the narrowing function for boolean discriminants and see why it misses the case where a property is compared to true/false vs. a truthy check. Also consider the effect of strictNullChecks on the narrowing algorithm.