/* Example from Hansen and Sestoft: C# Precisely */

class C<T>{
  // Compiler Error message:
  // Cannot convert null to type parameter 'T' because it could 
  // be a value type. Consider using 'default(T)' instead.
  T f = null;
}

class D<U>{
  // Compiler Error message:
  // The type 'U' must be a non-nullable value type in order to use 
  // it as parameter 'T' in the generic type or method 
  // 'System.Nullable<T>'
  U? f;
}