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Issue narrowing tagged unions with temporary const #31037

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@kyleshay

Description

@kyleshay

TypeScript Version: 3.4.0-dev.201xxxxx

Search Terms: Narrowing, "Tagged Unions"

Code

interface Square {
    kind: 'square';
    size: number;
}
interface Circle {
    kind: 'circle';
    radius: number;
}
type Shape = Square | Circle;

// works as expected (s.kind) does the correct narrowing
function areaWorking(s: Shape) {
    switch (s.kind) {
        case 'square': return s.size * s.size;
        case 'circle': return Math.PI * s.radius ** 2;
    }
}

// When saving s.kind to a const variable, narrowing does not work
function area(s: Shape) {
    const {kind} = s; // same as const kind = s.kind;
    switch (kind) {
        case 'square': return s.size * s.size; // Property 'size' does not exist on type [Shape|Circle].
        case 'circle': return Math.PI * s.radius ** 2; // Property 'radius' does not exist on type [Shape|Square].
    }
}

Expected behavior:
I would expect to be able to pull the kind property from the Shape object into a const and the narrowing to work as expected.

Actual behavior:
Narrowing does not work, I need have the switch() go off of the variable passed in.

Playground Link: TS Playground

Related Issues: #18758

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