Narrow the parent type of a singleton type to never in the false branch

[weswigham]

If the type of the member checked is not falsy (and so is never in the false branch).

TypeScript Version: 2.7.0-dev.201xxxxx

Code

const x: {a: 1, b: 2} = { a: 1, b: 2 };
if (x.a) {

}
else {
    x.a;
}

Expected behavior:
type of x in the else branch is never.

Actual behavior:
type of x is {a: 1. b: 2}, and x.a is (counterintuitively) never.

This would bring the behavior of truthiness guards in line with our new behavior for in type guards.

[WORMSS]

I am not sure if this is exactly similar, but I am getting property doesn't exist on never

abstract class Guards {
    public static isBasic(obj: Base): obj is Basic {
        return obj.type === Type.Basic;
    }
    public static isChainage(obj: Base): obj is Chainage {
        return obj.type === Type.Chainage;
    }
}

enum Type {
    Basic,
    Chainage,
}

interface Base {
    type: Type,
}

interface Basic extends Base {}
interface Chainage extends Base {
    start: number;
    end: number;
}

const ref: Chainage = {
    type: Type.Chainage,
    start: 0,
    end: 1,
}

if ( Guards.isBasic(ref)) {
    // No-op
} else if ( Guards.isChainage(ref)) {
    ref.start // Property 'start' does not exist on 'never'.
}

https://www.typescriptlang.org/play/#code/IYIwzgLgTsDGEAJYBthjAg4gV2FAJhgN4BQC5CADtiMgJawKTAQMJ1gBCaDAFAPYgAVgC4E3MAFMAlGMFD2GCW1IU1CKJIjYoAOwTyAdBACelSQgC81hABUzkw8tgBuMhQC+78tVptmrIwcAMIAFsB0usAA5pICwmISMnLCighhEVGxCKrq5JraegbCxg5WNvbmhhmRMZJual5eJJK62AC2dmW55M4ANN7p4bWxA82REJJQAGZwFkk5g6bmYpWSYyQkE1OzsPM8jJIAHpO6hOJoFkRe2zNzQ5l1CMen5ws9TBB4EGJt7SBTBoUVr4X4dAFQNzNWD8XSQDSSaZiGpZCyWRZqZaSVYOarDVEDNTMKA-BAABkJwLOYgAjBsttMELwsLgCGBDBxnLxNNNpNIMRQAPSChAAOX4AFp+JQSB5nsgpOxGcycHhCBywCi6tzEXyBflEYZiYhhQgAApQaVTUwIADkxttCHw-EkGF0-EQxw4iFhdt0kgAblNbYZZUA