Here is generics infer(not infer keyword) demo:

function foo<A>(a: A): A {}

const result = foo({ a: 1 });
//  const result: {
//    a: number;
//  }

But it seems i can't add some "extra" types to function foo, otherwise infer will not working:

function foo<Extra, A>(a: A): A & Extra {}

const result = foo<{ b: number }>({ a: 1 });
//  const result: {
//    b: number;
//  }

So, what i want is the function foo can both has type a and b in the above example, can anyone help?