stobix

void main() {
  String? x = test(null);
}
A test<A>(A a) =>
  switch((A,a.runtimeType)){
    (String,Null) => (){print("found nullable string"); return a;}(),    
    (String,_) => (){print("found nullable string"); return a;}(),
    (String?,_) => (){print("found nullable string"); return a;}(),
    (Type(),_) => (){print("failed to find nullable string for $A"); return a;}(),
  };

stobix

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

A test<A extends Object?>(A a) =>
  switch((A,a.runtimeType)){
    (String,Null) => (){print("found miscasted nullable string"); return a;}(),    
    (String,_) => (){print("found non-null string"); return a;}(),
    (String?,_) => (){print("found nullable string"); return a;}(),