Question

Please look at simple code below

public class A{}

  public class B: A{}

  public class G<T> where T : A
  {
    public T GetT()
    {
      return new A();
    }
  }

This code is incorrect – compiler error “Cannot convert A to return type T”.
But A is actually T. If I change

return new A(); 

to

 return new A() as T;

everything is ok. What is the reason of that behavior?
Thanks in advance

UPD: there was an error in initial question. Now fixed

Answer 1

Reworked answer based on update

Although A meets the generic constraint where T : A, it’s a concrete type. However, your generic class’s GetT() method has a generic return type of T, so you have to cast your concrete type to your generic type to make the returns compatible.

The old answer holds true for your previous case of returning new B().

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Old answer

The generic type constraint says that T must inherit from A; however, it does not say that T must be B (or a derivation of it), although B happens to itself inherit from A and meet the constraint.

So the return type isn’t compatible (B is always B, but T is not necessarily B), and you get the error.