You need to assign the error to a variable, probably. So it would have to be more something like:

  n := strconv.Atoi(s) or |err| {
      return fmt.Errorf("foo: %w", err)
  }

  n := strconv.Atoi(s) or |err| {
      if !errors.Is(err, pkg.ErrFoo)
          return fmt.Errorf("foo: %w", err)
      }
  }

Just "error" (which shadows the built-in type) won't really work.

I'm just making up syntax here to illustrate the point; doesn't look too brilliant to me. A func might be a bit more "Go-like":

  n := strconv.Atoi(s) or func(n int, err error) {
      return fmt.Errorf("foo: %w", err)
  }

All of this is kind of a moot point at Robert's blog post says that these proposals won't be considered for the foreseeable future, but IMHO any error handling proposal should continue to treat errors as values, which means you should be able to use fmt.Errorf(), errors.Is(), mylogger.Error(), etc.