Segfault when casting array of Interface types

[Klaus via Digitalmars-d-learn]

On Tuesday, 16 September 2014 at 03:05:57 UTC, Franz wrote:
> On Tuesday, 16 September 2014 at 03:03:16 UTC, Franz wrote:
>> On Tuesday, 16 September 2014 at 02:21:42 UTC, rcor wrote:
>>> I'm back for another round of "is this a bug, or am I doing 
>>> something stupid?".
>>>
>>> C and D implement interface I, and I have an array of each. 
>>> I'd like to combine these into one I[], but eventually I'd 
>>> like to cast an element back to its original type.
>>>
>>> interface I {}
>>> class C : I {}
>>> class D : I {}
>>>
>>> void main() {
>>> C[] c = [new C, new C];
>>> D[] d = [new D, new D];
>>> auto i = cast(I[]) c ~ cast(I[]) d;
>>> assert(cast(C) i[0]); // segfault
>>> }
>>>
>>> casting each array to I[] is fine, but casting an element 
>>> back to C segfaults (even if it weren't a C, it should just 
>>> return null, right?)
>>
>> look at what 's auto has created:
>>
>> import std.stdio;
>>
>> interface I {}
>> class C : I {}
>> class D : I {}
>>
>> void main() {
>>  C[] c = [new C, new C];
>>  D[] d = [new D, new D];
>>  auto i = cast(I[]) c ~ cast(I[]) d;
>>  writeln(typeof(i).stringof);
>> }
>>
>> Your issue comme from auto.
>
> i is a I[]

writing

    auto i = cast(I[]) c ~ cast(I[]) d;

is just a horrible way of shortcuting the static typing. You 
write this thinking that i "has to be..." and then you complain 
latter because the cast does not work.
D is a strongly typed lang. in your example you use "auto" 
because your brain doesnt give you what the type of i has to be, 
which is an error. D is not a scripting lang. You made a wrong 
usage of "auto".