void fn(out int a) {
  a = 2;
  int a; // shadows/hides parameter 'a', no error, from this point on, you cannot access the out parameter.
  a = 4;
}
void Test() {
  int res;
  fn(res);
  Print(res); // 2
}

void fn(int a) {
  a = 2;
  int a; // error, multi decl of 'a'
  a = 4;
}