Type guard of Number.isNaN is wrong

[saschanaz]

TypeScript Version: 2.1 RC bundled in VS2017 RC

Code

let num = 8;
if (!Number.isNaN(num)) { // NumberConstructor.isNaN(value: any): value is number
    printNumber(num + 1); // `num` should still be a number but TS thinks it has `never` type
}

function printNumber(num: number) {
    console.log(`Got ${num} - 1`);
}

Expected behavior: No error

Actual behavior: TS2356 An arithmetic operand must be of type 'any', 'number' or an enum type.

[akarzazi]

This code would raise an other error on num++ since it was declared with const.

Javascript :

const num = 0;
num++;

VM173:2 Uncaught TypeError: Assignment to constant variable

[RyanCavanaugh]

Introduced via #11670. Seemed like a good idea at the time!