Partial type fails to narrow in spite of check for undefined

The code below works in TS 3.0.

TypeScript Version: 3.2.1.0

Search Terms: is:issue 3.2 Partial

Code

declare function never(value: never): never;

    const enum BarEnum {
        bar1 = 1,
        bar2 = 2,
    }

    type UnionOfBar = TypeBar1 | TypeBar2;

    type TypeBar1 = {

        type: BarEnum.bar1;
    };

    type TypeBar2 = {

        type: BarEnum.bar2;
    };

    const value: Partial<UnionOfBar> = {} as any;

    if (value.type !== undefined) {

        switch (value.type) {

            case BarEnum.bar1: break;
            case BarEnum.bar2: break;
            default: never(value.type); // Error: Type 'undefined' is not assignable to parameter of type 'never'
        }
    }

Expected behavior:
No compiler error

Actual behavior:
Type 'undefined' is not assignable to parameter of type 'never'

Related Issues:
#28748

Update: this particular version of the bug is fixed in 3.3RC. But the original bug (default: return never(value.type); in the code below) is still present.

A similar error in a slightly different context:

    type GeoType2 = GeoPoint2 | GeoRoute2;

    type GeoPoint2 = {

        type: 'Point';
        coordinates: {};
        id: number;
    };

    type GeoRoute2 = {

        type: 'LineString';
        coordinates: {}[];
        start: { id: number, name: string };
        end: { id: number, name: string };
    };

    function foo(value: Partial<GeoType2>) {

        switch (value.type) {

            case 'Point':

                return;

            case 'LineString':

                return !!value.start && !!value.end && !!value.start.id && !!value.end.id && !!value.coordinates
                    && value.start.name && value.end.name; // HERE

            default: return never(value.type);
        }
    }

The compiler reports an error for undefined access at the point marked "HERE". The start and end properties should narrow out undefined.

Your first case is correct behaviour according to the definition of Partial.

type can still be undefined because Partial<T> means each key of T can be undefined.

undefined is not assignable to never. never type has no inhabitants.

EDIT:
Here's the "fixed" version of the first example. Note that I've added a new case to your switch statement to handle the undefined case, which makes value to be correctly inferred as never.

Proof of this is

const value: Partial<UnionOfBar> = { type: undefined }

This is valid and type checks correctly

Dhruv Rajvanshi (@dhruvrajvanshi)

Dhruv Rajvanshi (@dhruvrajvanshi) there is a check for undefined before the switch statement, so value.type cannot be undefined within the switch.

Welp. I missed the undefined check. It does seem like a genuine bug to me. Sorry about that.

Converting the switch case to equivalent if else statements does seem to work so I don't see why this wouldn't work.

The playground is actually working correctly for both reported bugs. But following the download link from the playground for Visual Studio 2017 and installing version 3.2.2.0 causes the same error.

Daniel Rosenwasser (@DanielRosenwasser)

I think this is fixed in TypeScript 3.2.4 but there is no download link to this version.

https://www.microsoft.com/en-us/download/details.aspx?id=55258

Daniel Rosenwasser (@DanielRosenwasser) is there a download link for TS 3.2.4 so that I can test and close this issue? Thanks!

The bug is still present in TS 3.3RC.

Anders Hejlsberg (@ahejlsberg)

I'm afraid I can't offer up a solution, but in terms of tracing the problem this looks to be something affected by this fix here #27612. Existing narrowings on properties, such as value.type !== undefined, are getting reset when a switch on a discriminant happens, such as switch (value.type). The example looks to be the same as the one in this comment.

This is the comment by Anders Hejlsberg (@ahejlsberg) regarding the issue:

Yes, that is due to #27612. We weren't correctly resetting our narrowing assumptions in switch statements. It happened to work in some cases, including yours, but generally was wrong and inconsistent with corresponding if statements.

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Partial type fails to narrow in spite of check for undefined #29496

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Partial type fails to narrow in spite of check for undefined #29496

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NoelAbrahams commented on Jan 20, 2019

dhruvrajvanshi commented on Jan 20, 2019

NoelAbrahams commented on Jan 20, 2019

dhruvrajvanshi commented on Jan 20, 2019

NoelAbrahams commented on Jan 20, 2019

NoelAbrahams commented on Jan 20, 2019

NoelAbrahams commented on Jan 23, 2019

jack-williams commented on Jan 24, 2019

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