type ConditionalType<T> = T extends string ? string : number ;

function ConditionalOrUndefined<T>() : ConditionalType<T> | undefined {
    return 0 as any;
}

function JustConditional<T>() : ConditionalType<T> {
    return ConditionalOrUndefined<T>()!; // Type 'NonNullable<ConditionalType<T>>' is not assignable to type 'ConditionalType<T>'
}


// For comparison...
function genericOrUndefined<T>() : T | undefined {
    return 0 as any;
}

function JustGeneric<T>(): T {
    return genericOrUndefined<T>()!; // no error
}