How to correctly type generic UseFormRegister<T> and FieldErrors<T> when dealing with sub form?

[Quanderr]

I have this ThingForm.tsx, which is a separated part of the bigger form for reusability purposes.

type Thing = {
  nestedProp: number
}

interface ThingFormValues {
  thing: Thing;
}

interface ThingFormProps<T extends ThingFormValues> {
  register: UseFormRegister<T>;
  errors: FieldErrors<T>;
  }

export const ThingForm = <T extends ThingFormValues>({
  register,
}: ThingFormProps<T>) => {
  return (
      <Input
        label="Thing"
        inputProps={{
          type: 'number',
          ...register('thing.nestedProp'),
        }}
        error={errors.thing?.nestedProp?.message}
      />
  );
};

I want to type the register prop to accept register function from any form as long as this form has thing: Thing property. In my understanding the way above should work:

• We have a generic type constraint what is ensuring that the form must have the thing prop
• This generic type T is passed to type the register function using UseFormRegister type
• This generic type T is passed to type the FieldErrors type with generic
• register function is correctly typed, so the name argument must be a keyof of the Thing type

However, this is not working. When I'm trying to use register('thing.nestedProp'), the following ts error is being thrown:

Argument of type '"thing.nestedProp"' is not assignable to parameter of type 'Path<T>'.ts(2345)

The same goes for the error. Using errors.thing?.nestedProp?.message gives standard Prop Does Not Exist error.

Property 'nestedProp' does not exist on type 'FieldError'.ts(2339)

where Path<T> should evaluate to Path<{} & thing: Thing>

definition of Path

Type which eagerly collects all paths through a type

@typeParam — T - type which should be introspected

@example

Path<{foo: {bar: string}}> = 'foo' | 'foo.bar'

I am able to resolve these errors by casting exactly to ThingFormValues.

  const parsedErrors = errors as FieldErrors<ThingFormValues>;

However, I'm wondering - why this is not working in the first place? How (except casting) can I achieve that?