User-defined type guards cannot narrow discriminated union types

[yenshih]

Bug Report

🔎 Search Terms

• user-defined type guards

• narrowing

• discriminated union

🕗 Version & Regression Information

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

⏯ Playground Link

Playground link with relevant code

💻 Code

type FooBar<T = 'foo' | 'bar'> = T extends string ? { type: T; value: Capitalize<T> } : never;

declare const fooBar: FooBar;

const isFoo = (type: 'foo' | 'bar'): type is 'foo' => type === 'foo';

if (fooBar.type === 'foo') {
    // 'Foo'
    fooBar.value
}

if (isFoo(fooBar.type)) {
    // 'Foo' | 'Bar' ???
    fooBar.value
}

🙁 Actual behavior

typeof fooBar.value in the second if block is 'Foo' | 'Bar'

🙂 Expected behavior

typeof fooBar.value in the second if block should be 'Foo' instead of 'Foo' | 'Bar'

[MartinJohns]

Duplicate of #42384.

You can work around this issue by narrowing the object, not the property:

const isFoo = (val: FooBar): val is FooBar<'foo'> => val.type === 'foo';

[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.