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
}

贡献者指南

技术栈

typescript

领域

developer experience

议题类型

bug

难度面向新贡献者的预计实现难度，1 表示很小改动，5 表示专家级工作。

4

预计时间有经验贡献者完成调查、实现、测试并准备 pull request 的粗略时间范围。

over 1 week

活动状态议题当前的可参与程度：新鲜、活跃、陈旧、阻塞或等待维护者输入。

stale

清晰度议题是否清楚说明期望改动、验收标准和下一步。

clear

前置要求

Understanding TypeScript type narrowingFamiliarity with TypeScript compiler architecture

新手友好度1-100 的估计分数，表示该议题对首次贡献者的友好程度。

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.