opAssign: cannot set reference of object to null

[Steven Schveighoffer]

"Mahe" wrote
> Hi,
> in the following code the statement "b = null" calls "B opAssign( A v )" 
> and causes an access violation. I don't understand why! How can I set b to 
> null?
> I'm using dmd 1.028.
>
> class A
> {
> this( int i )
> {
> a = i;
> }
> int a;
> }
>
> class B
> {
> this( int i )
> {
> a = i;
> }
> int a;
>
> B opAssign( A v )
> {
> a = v.a;
> return this;
> }
> }
>
> int main(char[][] args)
> {
>
> A a = new A( 4 );
> assert( a.a == 4 );
> B b ;
> assert( b is null );
> b = new B( 5 );
> b = a;
> assert( b.a == 4 );
> b = null;
> assert( b is null );
> }

That is interesting... I know that you cannot overload opAssign with 
something that implicitly casts to the lvalue type, but what does the 
compiler do in the case of null?  It casts both to the lvalue type, and 
possibly the argument that is passed to opAssign.

IMO, it should not call opAssign, but set the implicit reference.  I'd file 
it as a bug.

You may be able to workaround by doing:

b = (B)null;

But that seems like a lot of extra work.

-Steve