Type constraints in code block

[kefyru]

public static void Calc<T>(T num) where T: INumber<T>
{
   // do something
}

public static void Do<T>(T value)
where T: ISomeInterface
{
     Calc(value);  // compile error (type constraint)
     if (value is INumber<T>) {} // compile error (type constraint)
     if (value is INumber<int> num) Calc(num); // ok, but boxing
     if (value is int num) Calc(num); // ok, no boxing in RELEASE, boxing in DEBUG
     
     // proposal
     if (T is T1 where T1:  INumber<T1>)  {
         // in this block
         // T1: INumber<T1>
         // T: ISomeInterface, INumber<T>
         Calc(value);  // -> Calc<T>(value)  where T: ISomeInterface, INumber<T>
         T1 num = value; // no boxing
         value++; // ok
     }

     if (T is int)  // the same as typeof(T) == typeof(int)
     {
         Calc(value); // ok -> Calc<int>(value)
     }
 
     // also
     if (T is unmanaged) { }
     if (T is class) {}
     if (T is struct) {}

     // complex
     if (T is IDictionary<T1,T2>  
         where T1: unmanaged, INumber<T1>
         where T2: struct, INumber<T2>
     ) {
         var arr = value.ToArray(); // arr: KeyValuePair<T1,T2>[]
     }
     
}

[KennethHoff]

Duplicates #6308