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.

Start by compiling the provided TypeScript 2.4 example and compare control-flow narrowing for !!value with Boolean(value). No source file or test path is named in the issue, so trace the type-checking and narrowing entry points before adding a regression test. Done means both forms narrow value sufficiently for toFixed(0) without weakening unrelated checks.