Why can't I return a specific subclass from a generic class' factory constructor?-CodePudding

The following example of a factory constructor (using a static method) compiles fine:

class Base<T> {
  Base();
  
  static Base myFactory(bool isInt) {
    if (isInt) { return A(); }
    else { return B(); }
  }
}

class A extends Base<int> {}

class B extends Base<String> {}

However, if I use the syntax for a factory constructor:

class Base<T> {
  Base();
  
  factory Base.myFactory(bool isInt) {
    if (isInt) { return A(); }
    else { return B(); }
  }
}

class A extends Base<int> {}

class B extends Base<String> {}

I get:

A value of type 'A' can't be returned from the method 'myStatic' because it has a return type of 'Base<T>'.

Why is this an error? A is a subtype of Base<int>, so isn't it a subtype of the generic Base<T>?

CodePudding user response：

If you don't specify a type, it is dynamic by default. Thus, in the first case, the return type of the myFactory method is Base<dynamic>, while in the other case, the return type of Base.myFactory method is Base<T>.