Use control flow analysis for type guards

[jednano]

TS 1.4

Type guard fails to narrow the union type when returning early.

function foo(x: number | string) {
    if (typeof x === 'string') {
        return;
    }
    // x should now be a number
    if (2 % x !== 0) {
        // do something
    }
}

Unfortunately, I get the following compiler error:

The right-hand side of an arithmetic operation must be of type 'any', 'number' or an enum type.

Clearly, the compiler should know at this point that it can't possibly be dealing with a string, as that scenario has already been returned.

[RyanCavanaugh]

This would be nice. We should probably have a single issue for "Use control flow analysis for type guards" to unify all the various permutations here.

[vilicvane]

+1 for this feature.

[jednano]

@RyanCavanaugh I've updated the issue title, as you suggested. Happy to discuss it here.

[zspitz]

+1
Suggestion: an if block containing a throw statement should be treated the same way:

function foo(x: number | string) {
    if (typeof x === 'string') { throw 'No strings attached';}
    return x * 2;
}

[blakeembrey]

👍 I prefer writing code without using else so this would be pretty amazing

[zpdDG4gta8XKpMCd]

👍