c# - Guid == null should not be allowed by the compiler

Question

The behaviour described below is specific to .net-3.5 only

I just ran across the most astonishing behavior in the C# compiler;

I have the following code:

Guid g1 = Guid.Empty;
bool b1= (g1 == null);

Well, Guid is not nullable therefore it can never be equal to null. The comparison i'm making in line 2 always returns false.

If you make the same thing for an integer, the compiler issues an warning saying the result will always be false:

int x=0;
bool b2= (x==null);

My question is: Why does the compiler lets you compare a Guid to null?
According to my knowledge, it already knows the result is always false.
Is the built-in conversion done in such a way that the compiler does assume null is a possible value?
Am I missing anything here?

Answer 1

Mark is correct. Value types that define their own equality operators automatically get lifted-to-nullable versions defined as well, for free. The nullable equality operator that takes two nullable guids is applicable in this situation, will be called, and will always return false.

In C# 2, this produced a warning, but for some reason, this stopped producing a warning for guid-to-null but continues to produce a warning for int-to-null. I don't know why; I haven't had time to investigate yet.

I apologize for the error; I probably screwed up one of the warning-detection code paths when rewriting the nullable logic in C# 3. The addition of expression trees to the language majorly changed the order in which nullable arithmetic operations are realized; I made numerous mistakes moving that code around. It's some complicated code.