Differing user-defined type guard and 'typeof' type guard behaviour when narrowing 'any'

描述

var y: any;

// Built-in type guard
if (typeof y === "string") {
    y.hello = true; // Correct error - 'hello' does not exist on type string
}

// Equivalent user-defined type guard
function f(x: any): x is string {
    return typeof x === "string";
}

if (f(y)) {
    y.hello = true; // No error with user-defined guard
}

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。

4

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。

over 1 week

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

Understanding TypeScript type narrowingFamiliarity with TypeScript compiler architecture

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。

20

研究方向

Investigate the TypeScript compiler source code, particularly the `narrowTypeByTypeOf` and `narrowTypeByExpression` functions in `src/compiler/checker.ts`. The issue is that user defined type guards do not narrow `any` to the guarded type, while `typeof` guards do. Review the logic that applies type predicates to see why `any` is not narrowed. There is a discussion in the comments suggesting that the predicate's type guard might be ignored when the input is `any`. Consider whether the fix should be in the narrowing logic or in the predicate resolution.