Dartで嵌ったけど、原因が分かったやつ

map.dart

List<List<U>> deepMap2D_F<T, U>(List<List<T>> src, U Function(T) f) {
  var ret = <List<U>>[];
  for(List<T> innerList in src) {
    ret.add([for(var e in innerList) f(e)]);
  }
  return ret;
}
extension <T,U> on List<List<T>> {
  List<List<U>> deepMap2D_M(U Function(T) f) {
    var ret = <List<U>>[];
    for(List<T> innerList in this) {
      ret.add([for(var e in innerList) f(e)]);
    }
    return ret;
  }
}
extension <T> on List<List<T>> {
  List<List<U>> deepMap2D_M2<U>(U Function(T) f) {
    var ret = <List<U>>[];
    for(List<T> innerList in this) {
      ret.add([for(var e in innerList) f(e)]);
    }
    return ret;
  }
}

void main() {
  List<List<dynamic>> src = [['foo', 'bar'], ['baz', 'foobar']];

  var dst1 = deepMap2D_F(src, (e) => e as String);
  print(dst1.runtimeType);    // List<List<String>>

  var dst2 = src.deepMap2D_M((e) => e as String);
  print(dst2.runtimeType);    // List<List<dynamic>>

  var dst3 = src.deepMap2D_M2((e) => e as String);
  print(dst3.runtimeType);    // List<List<String>>
}