Boolean() cannot be used to perform a null check

TypeScript Version: 2.4.0

Apologies for today's issue raising binge.

Code

// Compiles
const nullCheckOne = (value?: number) => {
    if (!!value) {
        return value.toFixed(0);
    }

    return '';
}

const nullCheckTwo = (value?: number) => {
    if (Boolean(value)) {
        // Object is possibly 'undefined'
        return value.toFixed(0);
    }

    return '';
}

Expected behavior:
Both examples compile.

Actual behavior:
The latter example fails w/ Object is possibly 'undefined'.

Explanation
To my knowledge !!value and Boolean(value) are equivalent. I'm wondering what is the reason behind not supporting the second case. One reason I can think of would be an imported, non-typescript module, globally overriding it to something like: Boolean = (value) => !value.

首先编译提供的 TypeScript 2.4 示例，并比较 !!value 与 Boolean(value) 的控制流收窄行为。由于 issue 中没有指定源文件或测试路径，因此在添加回归测试之前，先追踪类型检查和收窄的入口点。完成标准是：两种形式都能将 value 收窄到足以调用 toFixed(0)，且不会削弱无关的检查。