Narrowing a type union with user-defined type guard #12235

getResults1([]);
getResults1({data: []});

getResults2([]);
getResults2({data: []});


type Result = {value: string}
type Results = Result[];


function isResponseInData<T>(value: T | { data: T}): value is { data: T } {
    return value.hasOwnProperty('data');
}
function getResults1(value: Results | { data: Results }): Results {
    // error TS2322: Type 'Results | {data: Results}' is not assignable to 'Results'
    return isResponseInData(value) ? value.data  // value: {data: Results}
                                   : value;      // value: Results | {data: Results}   !!
}


function isPlainResponse<T>(value: T | { data: T}): value is T {
    return !value.hasOwnProperty('data');
}
function getResults2(value: Results | { data: Results }): Results {
    // Good
    return isPlainResponse(value) ? value        // value: Results
                                  : value.data;  // value: {data: Results}
}