Flutter DART：如何判断有时可以为空的泛型类型

给出下面的DART 3片段，是否可以匹配Switch子句中可为空的A，并找出哪种可为空的A是？(String?、int?等) 类型判断器错误地假设A永远不能为空，结果我得到了一个似乎无法匹配的大小写.

void main() {
  String? x = test(null);
  String y = test("hi");
}

A test<A extends Object?>(A a) {
  switch((A,a.runtimeType)){
    case (String,Null) :
      print("found miscasted nullable string");
    case (String,_) : 
      print("found non-null string: $a");
    // This case should match, but something is weird.
    case (String?,_) :
      print("found nullable string");
    case (var x,Null):
      print("found something null: $x $a");
      if(x is String?)
        print("$x is String?");
      else
        print("$x is not String?");
    case (_,_):
      print("failed matching completely" ); 
  };
  return a;
}

输出将变为

found something null: String? null
    String? is not String?
found non-null string: hi

而且我似乎找不到一种方法来匹配输入类型和String?.

我需要这个在Flutter 中工作，所以dart :镜子不能解决问题.

Edit:多亏了@jamesdlin的回答，我现在的代码中有了以下函数:

Type typeOf<A>() => A;
bool isSomeKindOf<T,S>() => S == typeOf<T>() || S ==  typeOf<T?>();

.这意味着我现在可以做像这样的事情

bool isIntOrNullableInt<A>(A a) => isSomeKindOf<int,A>();
print(isIntOrNullableInt(3)); // true
print(isIntOrNullableInt(null as Int?)); // true
print(isIntOrNullableInt("3")); // false

这意味着我现在可以做一些事情，比如

A getOrDefault<A>(String token, A defaultVal) =>
    (
    isSomeKindOf<int,A>() ? source.getInt(token) ?? defaultVal :
    isSomeKindOf<String,A>() ? source.getString(token) ?? defaultVal :
    defaultVal
    ) as A; 

which works whether A is a Foo or a Foo?, which was my secret agenda all along.
This lets me finally have the following working:

String prop1 = getOrDefault("prop1", "property missing");
int? prop2 = getOrDefault("prop2", null);