Nullable types, lifted operators and ReSharper

Recently I investigated why ReSharper marks some pieces of my project code as invalid however compiler does not throw any errors.

Finally I narrowed down the problem.

class Program  
{  
    static void Main(string\[\] args)  
    {  
       int? i = 10;  

       A a = i;  
    }  

    class A  
    {  
      A(int i)  
      {  
        Value = i;  
      }  

      public int Value { get; private set; }  

      public static implicit operator A(int i)  
      {  
        return new A(i);  
      }  
    }  
}  

ReSharper highlights an error on line 7. However as you can imagine on a runtime corresponding implicit operator is called.

I investigated this behaviour and found that actually it is according to the C# spec

Nullable conversions and lifted conversions permit predefined and user-defin ed conversions that operate on non-nullable value types also to be used with nullable forms of those types. Likewise, lifted operators permit predefined and user-defined operators that work for non-nullable value types also to work for nullable forms of those types.
For every predefined conversion from a non-nullable value type S to a non-nullable value type T , a predefined nullable conversion automatically exists from S? to T?. This nullable conversion is a null propagating form of the underlying conversion: It converts a null source value directly to a null target value, but otherwise performs the underlying non-nullable co nversion. Nullable conversions are furthermore provided from S to T? and from S? to T , the latter as an explicit conversion that throws an exception if the source value is null.

So finally I know that it is a ReSharper bug and I will report it on their bug-tracker system.

Potentially it is a good question for juniors on interviews :)

UPD: Raised bug on ReSharper bug tracker

http://youtrack.jetbrains.net/issue/RSRP-287221?projectKey=RSRP&query=lifted