Type inference fails when using combined boolean variable in if condition

[ddubrava]

🔎 Search Terms

"typescript destructuring losses types", "typescript destructuring losses type object is possibly undefined", "typescript type alias destructuring loses type", "typescript inference with union types"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ

⏯ Playground Link

https://www.typescriptlang.org/play?#code/C4TwDgpgBAqgzhAigVwgJxAJQnZAbYAHgBUA+KAXigG8AoKBqASzgGVkBjDnOALigBmAQzwIA3PUYATIcCH9kAOykQBTRRCkSAvlAA+NSQxbsuPfsDSoJjKDLn9itbRNoClHYEwD2iqMgQUdBAACgBKfngkVAxsXAJCRWQAWwAjdHI6WzQIYGQ0PyzbRhNObjg+QREEABojYvt5f2VVdU062xdnWloOXzhgGjtZJsaARhrmNjLzKdNyuDGoXSoA6ODwiT7FAaHG-kaAJknSswr+U4XD5cp-QJjQsK3+wephh3ehAGYT6bPKy48L43Vb3DZPHoAekhUAAkoMAO7eNAAazgUCE6IgAA9IJ5NLQmAIoCFARUlgAyClzGYVa5Umn-L5hQy2cYAOmA3gAorjfBBFF4RJt6kdOTy+RpBUxhRC2SMvuLeWB+dLZToettdkIcvMeLcyYsoAzDfTqYavq5oXDBlJvDhFAByRHIlFQAAW6AgkwR7qYeGgTGAjvRACsAoMhFA+ml1LIfH5vMSQ7SEHBHYTiSEdRA9RUWUUGByucrVUK8CL5XJDkrJQLy5XpArayqpQ2IdogA

💻 Code

type UseQueryResult<T> = {
    isSuccess: false;
    data: undefined;
} | {
    isSuccess: true;
    data: T
};

function useQuery(): UseQueryResult<number> {
    return {
        isSuccess: false,
        data: undefined,
    };
}

const { data: data1, isSuccess: isSuccess1 } = useQuery();
const { data: data2, isSuccess: isSuccess2 } = useQuery();
const { data: data3, isSuccess: isSuccess3 } = useQuery();

// It works as expected
if (isSuccess1 && isSuccess2 && isSuccess3) {
    data1.toExponential();
    data2.toExponential();
    data3.toExponential();
}

const areSuccess = isSuccess1 && isSuccess2 && isSuccess3;

// It doesn't work here, while it's just a combination of 'successes'
if (areSuccess) {
    data1.toExponential();
    data2.toExponential();
    data3.toExponential();
}

🙁 Actual behavior

The issue occurs when using the combined boolean variable areSuccess in the second if condition. Although isSuccess1, isSuccess2, and isSuccess3 are all true, TypeScript cannot narrow down the types of data1, data2, and data3 within the if block, leading to a type error

🙂 Expected behavior

areSuccess is a combination of the individual isSuccess variables, it should correctly infer the types of data1, data2, and data3 as the specific data type (number) defined for the queries. This should allow the code to call the toExponential method on each data variable without any issues.

Additional information about the issue

No response

[fatcerberus]

For performance reasons, IIRC indirect narrowing only works one level down.

[andrewbranch]

I think #44730 and #46266 aren’t talking to each other. I’m not sure if that’s a known limitation. @ahejlsberg?

[fatcerberus]

I thought it was just that

const typeGuard1 = ...
const typeGuard2 = ...
const check = typeGuard1 && typeGuard2;
if (check) {
    // not a type guard
}

doesn't work in general. But maybe I'm wrong about that, I haven't actually tested it thoroughly.

[andrewbranch]

Seems like it goes through quite a few aliases and logical operators: https://www.typescriptlang.org/play?#code/CYUwxgNghgTiAEYD2A7AzgF3gDwFz0xgEsUBzeAH3gFcVQAzEkYAbgChRJYFl0sBPfIRLkqtBk1Zs2vTDgCSaACIhGKZvAC8OeAEJN28asntZAxSrUbt-PQZp1j6qWyL14ACmwWnGgGR+8Pw+VsAAlPAA3mzwsTgAdBhIADJIAO4gMADCUGggHmHscUGJKemZOXkF7AC+0mbwAEZIGAAWAIJwlpJaCsq+wPABQSEmru4ezW2dIN3OEdHF2KWpGdm5+YUxcfwr5etVW3XwMZzQcIiocgBeQhjEZJQOEs5FDdejzr3XdoaOoewYg0oBAIDM5tYmi0Ol0BkNAh9+gCYm5PCCwbDQgttrFlklVhUNtUcSV8ftKpsinFrns1hTifBjkA