This syntax regarding null checking baffles me

[kdevel]

On Sunday, 10 January 2021 at 19:21:47 UTC, Paul Backus wrote:
> On Sunday, 10 January 2021 at 17:28:42 UTC, kdevel wrote:
>> On Sunday, 10 January 2021 at 16:44:29 UTC, Paul Backus wrote:
>>
>> [...]
>>
>>> Your opCast has the wrong signature.
>>
>> "Wrong" in what respect?
>>
>
> It does not match the signature required by the language spec:
>
> https://dlang.org/spec/operatoroverloading.html#cast

Sure, but it is called anyway:

```null3.d
import std.stdio: writeln;
class C {
    int i;
    bool opCast ()
    {
       __PRETTY_FUNCTION__.writeln;
       return false;
    }
}

void main ()
{
    C c = new C;
    cast (bool) c; // print bool null3.C.opCast()
    c = null;
    cast (bool) c; // Segmentation fault
}
```

$ dmd null3 && ./null3
bool null3.C.opCast()
Segmentation fault

As ag0aep6g pointed out

    if (c)

does not invoke the opCast member. Neither one of the signatures

    bool opCast ()
    T opCast (T: bool) ().

This "conversion to bool" [1] of the class variable seems to be 
not
programmatically accessible.

[1] "Class references are converted to bool by checking to see if 
the class reference is null or not." from § 20.2.1