switch (true) cannot be used to implement type guards

[kujon]

TypeScript Version: 2.4.0

Code

// Compiles
const typeGuardsOne = (value: number | string | Date): string => {
    if (typeof value === 'number') {
        return value.toFixed(0);
    } else if (value instanceof Date) {
        return value.getTime().toString();
    } else if (typeof value === 'string') {
        return value;
    }
};

const typeGuardsTwo = (value: number | string | Date): string => {
    switch (true) { 
        case typeof value === 'number':
            // Property 'toFixed' does not exist on type 'string | number | Date'.
            return value.toFixed(0);
        case value instanceof Date:
            // Property 'getTime' does not exist on type 'string | number | Date'.
            return value.getTime().toString();
        case typeof value === 'string':
            // Type 'string | number | Date' is not assignable to type 'string'.
            return value;
    }
};

Expected behavior:
I personally find the switch (true) trick to be a really readable and concise way of implementing type guards/pattern matching (especially if a few cases need to be treated in the same way). Typescript doesn't seem to evaluate the cases and the typeGuardsTwo example doesn't compile. It treats the value inside of each case as if it still was number | string | Date.

Actual behaviour
The code doesn't compile with the errors listed in the Code section.

[RyanCavanaugh]

See #8934