microsoft/TypeScript

Partial Discriminated Union cannot always be narrowed

Closed this issue · 3 comments

CakeWithDivinity commented

🔎 Search Terms

narrow partial discriminated union

🕗 Version & Regression Information

  • This is the behavior in every version I tried, and I reviewed the FAQ for entries about union types

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=6.0.0-dev.20251022#code/KYOwrgtgBAKgngB2AZygbwFBW1AYge3ygF4oByAM0LIBoscAhAQwCcTyAjV2jAXwwwBLEABdgLCkwDGwPIQAKrJhGBiWqTDihMECADaCpTEYPwh4SAFyxEKAHQF8AbnrYq+a+AgdxL-kNFxSRkoZhZFFmVVcQ1XbV0DIxMzC2BrVOQ7MJctLhZPSB8WPwERWygI5VRSR0qVNVQAH1DWOuj1FwEKMBApZJAoMWQRAH0QMxGEVhMmPRGAE3wUMdYWfAB3AAoEazrkAEp0OMEKKG27HX1DY1NzcuIHqB754AphYHmoRuaEC4Tr-qpEiPDIOQiHTRaHAsVRgFggHI4fxaKRmZD4PTAOx6fAAc3OeX2iKhJIA9KSoAA9AD8fC6PT6t0GKFGUxYMzmi2W41GIFWG22u2mglmAB49gA+CHHU7nS6JG4pe6PZ6vd6fb5QX7ygG3IEPUigxzSknYGEiOEIuLInCokDozHYvEE1hEuKm7Dkqm0-wYbq9frM4aTYWzBZLZArFhrLY7CqhvTipQQZBSo5aE5nbX-JJ68oAQhBtkyYRNpvNluJUBt2DtDqxOPxv0JVY9UC9NLpQA

💻 Code

enum Types {
    Foo = 'foo',
    Bar = 'bar',
}

interface FooParameters {
    applicationType: Types.Foo;
    foo: number;
}

interface BarParameters {
    applicationType: Types.Bar;
    bar: number;
}

type Params = FooParameters | BarParameters;


function test_non_partial_does_narrow(p: Params) {
    if (p.applicationType === undefined || p.applicationType === Types.Foo) {
        return;
    }

    console.log(p.bar);
             // ^? BarParameters
}

function test_partial_does_not_narrow(p: Partial<Params>) {
    if (p.applicationType === undefined || p.applicationType === Types.Foo) {
        return;
    }

    console.log(p.bar);
             // ^? Partial<Params>
}


function test_partial_does_narrow(p: Partial<Params>) {
    if (p.applicationType !== Types.Bar) {
        return;
    }

    console.log(p.bar);
             // ^? Partial<BarParameters>
}

🙁 Actual behavior

The Partial<Params> discriminated union can only be narrowed if you return when it does not equal the expected value. When ruling out all other possibilities it does not narrow the type.

🙂 Expected behavior

It also narrows the type, when ruling out all other possibilities

Additional information about the issue

No response

MartinJohns commented

I believe this is a duplicate of #31404.

  • 👍2
RyanCavanaugh commented

🤖 Thank you for your issue! I've done some analysis to help get you started. This response is automatically generated; feel free to 👍 or 👎 this comment according to its usefulness.

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typescript-bot commented

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