namedGenericParameters.ts

type Generics = Partial<{
  asdf:string;
  qwer : number;
}>

type Defaults = Required<{
  asdf: 'default';
  qwer: 1234;
}>;

type identity<T> = {[k in keyof T ]:T[k]}

type MergeDefaults<Inputs extends Generics> = identity<
  {
    [k in keyof Defaults]: k extends keyof Inputs
      ? Inputs[k]
      : Defaults[k];
  }
>;

const myGenericFunc = <Inputs extends Generics>(args: string): MergeDefaults<Inputs> => {
  return args as any;
};

myGenericFunc<{asdf: 'override'}>('asdf');
// => { asdf: "override"; qwer: 1234; }