Type unexpectedly not narrowed after check

[frankdejonge]

🔎 Search Terms

Type narrowing, type narrowing with conditionals, type not narrowed after check

🕗 Version & Regression Information

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

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.5.0-dev.20240402#code/C4TwDgpgBAsghiARhAEnAzgSQHYDMD2ATgLZzACW+2UAvFAN4BQULUEp5ANgPwBcU6YIXLYA5gBpmrbHGIQ+DKa1a5yhQQsHCxk5cs4ZgmoSIlKoAX0ZXGoSFDToAoh04BBbABNMuTMEcAcrIQfo4AYmqCQXK0isrscFz8Wqa60sEKTHoqkcDJJjrm+obG2mbKVja4AK7YAMYUVFBCcNjoBCQAFLiE+MT88EioGDgdpI3YAJT8ji6J7l4+oRjRIf4YEerAq3GsdVSCbK6xPX0AdAlcANyM5uS4UJ2XnLQ0dLWeEKrYEJ6Tu9lgAALXoAdygP3BTkIvUInQA5AEAPIABSc8MmNwqt2UhAgwGqhGop2IZxkcm4Z1UWyKrG4DGe4ghwX4JLJwSs2RY-HozwsNwsQA

💻 Code

type MaybeHasInformation = {
    email?: string,
    name?: {
        first?: string,
        last?: string,
    }
}

type HasEmailAndIfItHasNameItHasFirstName = {
    email: string,
    name?: {
        first: string,
        last?: string,
    }
}

function transform(from: MaybeHasInformation): HasEmailAndIfItHasNameItHasFirstName {
    const email = from.email;

    if (email === undefined) {
        throw new Error('NOPE');
    }

    return from.name?.first
        ? {email, name: from.name}
        : {email};
}```


### 🙁 Actual behavior

There is a compilation error:

Type '{ email: string; name: { first?: string | undefined; last?: string | undefined; }; } | { email: string; }' is not assignable to type 'HasEmailAndIfItHasNameItHasFirstName'.
Type '{ email: string; name: { first?: string | undefined; last?: string | undefined; }; }' is not assignable to type 'HasEmailAndIfItHasNameItHasFirstName'.
The types of 'name.first' are incompatible between these types.
Type 'string | undefined' is not assignable to type 'string'.
Type 'undefined' is not assignable to type 'string'.

### 🙂 Expected behavior

No error was expected.

### Additional information about the issue

_No response_