Narrowing not works on discriminated unions when not using literal type as discriminator property

[k8w]

Bug Report

🔎 Search Terms

Narrowing Union literal

🕗 Version & Regression Information

• This changed between versions 4.0.5 and 4.6.2

⏯ Playground Link

Playground link with relevant code

💻 Code

// ------- EXPECTED -------
type Data1 = {
    type: 'string',
    a: string,
    b: string
} | {
    type: 'number',
    a: number,
    b: number
}

function test1(data: Data1) {
    if (data.type === 'string') {
        // Work as Expected: data.b is `string`
        data.b
    }
}

// ------- NOT EXPECTED -------

type Data2 = {
    type: string,
    a: string,
    b: string
} | {
    type: number,
    a: number,
    b: number
}

function test2(data: Data2) {
    if (typeof data.type === 'string') {
        // Expect: data.b is `string`
        // Actual: data.b is `string | number
        data.b
    }
}

🙁 Actual behavior

When discriminator property is not literal type in discriminated unions, type of Data2 is not narrowed as expected.

🙂 Expected behavior

Type of Data2 can be narrowed just like Data1

[jcalz]

This changed between versions 4.0.5 and 4.6.2

Really? Looks the same for TS3.9.7

...which makes sense because this is working as intended. Discriminated unions in TS require the discriminant property to be of a literal/unit/singleton type in at least one of the members of the union. Neither string nor number are literal types, so the type property of Data2 is not a discriminant property.

[RyanCavanaugh]

Most recently discussed at #48500

[typescript-bot]

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