Unexpected covariance type inference

unexpected_covariance_type_inference.dart

abstract class Stated<T> {
 const Stated._();
  
  factory Stated.idle() => const _Idle();
  const factory Stated.value(T value) = _Value;

  Type get valueType => T;
}

class _Idle<T> extends Stated<T>{
  const _Idle(): super._();
}

class _Value<T> extends Stated<T>{
  final T value;
  const _Value(this.value): super._();
}

void main(){
  final Stated<String> stated = Stated.idle();
  
  print(stated.valueType);
}