//setup
type QueryField<T> = keyof T; 

interface FunctionField<T> {
    fn: 'count' | 'avg';
    alias?: string;
    field: QueryField<T>;
}

class fieldBuilder<T>{
    public select<F extends FunctionField<T>, P extends QueryField<T>>(...args: Array<P | F>) {}
}


//example error
interface Foo{
    x: string;
    y: number;
    z: boolean;
}

let builder = new fieldBuilder<Foo>();

//works
builder.select('x', 'y', 'z'); 

//Type Error: Argument of type '"y"' is not assignable to parameter of type '"x" | FunctionField<Foo>'.
builder.select({ fn: 'count', field: 'x' }, 'x', 'y');