Type narrowing with union types create impossible case with else statement about typescript HOT 7 CLOSED

Not a bug. Types are narrowed on assignment, so TS knows it's impossible for that else to execute and test gets narrowed to never. Hence the error message

Property 'c' does not exist on type 'never'.

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The simplest form looks like this:

type Thing<T> =
    | { op: 'a', a: T }
    | { op: 'c' | 'd', cd: T };

function test(value: Thing<number | string>): void {
    if (value.op === 'c' || value.op === 'd') {
        // this is fine
        value.op
        value.cd
    } else {
        // ts knows value.op can only be 'a'
        value.op
        //    ^?
        // but no a property
        value.a
    }
}

This is a duplicate of #31404 but I'll ping some folks to take another bite at the apple; maybe it's more tractable these days

from typescript.

The TL;DR is that from the control flow analyzer's perspective, the code looks like this

type Thing<T> =
    | { op: 'a', a: T }
    | { op: 'c' | 'd', cd: T };

function test(value: Thing<number | string>): void {
    if (value.op === 'c') {
        // this is fine
        value.op
        value.cd
    } else if (value.op === 'd') {
        // this is fine
        value.op
        value.cd
    } else {
        // ts knows value.op can only be 'a'
        value.op
        //    ^?
        // but no a property
        value.a
    }
}

By exclusion, value.op went from the total possible union 'a' | 'c' | 'd' to a, but the object union itself couldn't undergo any narrowing because there's never a distinct point at which we can remove { op: 'c' | 'd', cd: T } from the list of possible inhabitants of value

from typescript.

Oh, I've provided a bad example here. It still happens when the value is not known (in a function) for me in some places

It seems to happen when there are nested type unions?

https://www.typescriptlang.org/play/?#code/C4TwDgpgBAKgFgSwHYHMA8MB8UC8UDeAUFCVAPZgBcUA5AIY1QA+tARjcaQG7UyEC+zApxIVqNAMaMWNACYdSULgEZeIpQCY1gljCEwA2gF0A3IUIAzAK5IJwBGSRRgEAM7AAFBLIAbKwFskV2p8JTo-CF5EVDQkANYIACchd0TkFGwWG1kIC2QIWSE4nx8ofmMASmouMgRCokULMmSvR3cCMIiy8gsobz9A1wrhRUUEXo8ucKtoZHc6WwgyXoBBRMS6EGGG0d2Aej3nRFcoBBO8pAh1RUEIH1dZidBIZc6Z3DwaMlYAKwg7RgAMkBb2gAEI8MUfNtrrtThMphEAHQUD60KTMFiImYosBouQ0GFw4lQA5HM6nc75WEk0G4mkk7EQJEqBnEpksjRs0i3e7QHa0xRk4AnADWSDIAHcThzURIFuQkD4QFAErQGEIaOxubtxlBJtNmaicJ8MUwsYbcfj5MMmTrRmTWFZgEpKVB3AgSlAJS6chcCvbFByuPb+AzeQ94fqmVAId6rCUiYKyal0kV4kl7XbiWGbgIgA

type Thing<T> = {
    op: 'a' | 'b'
    v: T
} | {
    op: 'c' | 'd'
    v1: T
    v2: T
} | T | T[];

function test(columns: { value: Thing<number | string> | undefined | null }[]): void {
    for (const { value } of columns) {
        if (value instanceof Array) {
            // this is fine
        } else if (typeof value == 'object' && value != null) {
            if (value.op == 'c' || value.op == 'd') {
                // this is fine
                value.op
                value.v1
                value.v2
            } else {
                // ts knows value.op can only be 'a' | 'b'
                if (value.op == 'c' || value.op == 'd') value
                // but v is still not defined
                value.v
            }
        } else if (value != null) {
            // string | number
            value
        }
    }
}

from typescript.

That's a much more instructive example, thanks. I'd recommend replacing the example in the OP with it since it perfectly illustrates the actual problem you're facing.

It seems like the addition of extra union members causes TS to no longer treat Thing as a discriminated union: from the example it's easy to see that TS narrows value.op to "a" | "b" (hence the first error) but not value itself (hence the second).

edit: That second paragraph is probably wrong. See Ryan's reply below.

from typescript.

Ah, so it's just the union-typed discriminant that's throwing it off? I figured it was because value was initially typed as <discriminated union> | T | T[] | null | undefined that it wasn't triggering the discriminated union checks.

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This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.

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