typescript - Cannot be dynamically inferred after passing some generic parameters

Question

const a = <
  T extends any = any,
  B extends boolean = true,
  R extends T | undefined = B extends true ? T : T | undefined
>(
  v: B = true as B // not use "as" will trigger type argument assign error
): R => '' as R;

const x = a<string>(); // ok! type is string
const y = a<string>(false); // error! `TS2345: Argument of type 'false' is not assignable to parameter of type 'true | undefined'.`, but the type inferred by webstorm is string | undefined

Is this because generics can only be passed parameters or dynamically inferred and cannot be use simultaneously the two?

Answer 1

等待大神解答