Comparing class references with null or other classes causes an error in CTFE, e.g.:

module test;

class C 
{ 
	int x;
}

int getA()
{
	C c = new C;
	if(c is null)
		return -1;
	return c.x;
}

static assert(getA() == 0);


Compiling with "dmd -c test.d" yields:
test.d(11): Error: cannot compare C(0) at compile time
test.d(16):        called from here: getA()
test.d(16):        while evaluating: static assert(getA() == 0)

if you use "if(c)" instead of "if(c is null)", the error itself disappears, there is only the call stack left:
test.d(16):        called from here: getA()
test.d(16):        while evaluating: static assert(getA() == 0)

Using "if(!c)" seems to work.