// (13,11): warning CS8620: Nullability of reference types in argument of type 'A<object?>' doesn't match target type 'A<object>' for parameter 'b' in 'void Program.F(B b)'.
// (13,11): warning CS8620: Nullability of reference types in argument of type 'A<object>' doesn't match target type 'A<object?>' for parameter 'a' in 'void Program.F(A<object?> a)'.