D and C++ undefined reference when namespace

[Markus]

On Thursday, 8 March 2018 at 17:04:02 UTC, Steven Schveighoffer 
wrote:
> On 3/8/18 11:35 AM, Markus wrote:
> When I do this locally on my mac, I get a similar error. When I 
> nm the main.o file vs. the lib.o file, I see different mangled 
> names.
>
> It appears that the D mangled name is not doing back 
> references. This is probably why the change to another 
> namespace for the function works (there isn't a back reference)
>
> In order to demonstrate this better, I did namespace 
> thenamespace instead of ns_a.
>
> The symbol I see in the D file:
> U __ZN12thenamespace13some_functionEPN12thenamespace7class_aE
>
> And in the C++ file:
> T __ZN12thenamespace13some_functionEPNS_7class_aE
>
> Note the difference is instead of 12thenamespace, it's S_, 
> which probably is a back reference. Googled...
>
> Yep, I'm right: 
> https://itanium-cxx-abi.github.io/cxx-abi/abi.html#mangle.seq-id
>
> I'd recommend filing a bug.
>
> -Steve

You are right.
$ dmd -c main.d
$ nm main.o | grep some
                  U _ZN4ns_a13some_functionEPN4ns_a7class_aE
$ nm lib.o | grep some
0000000000000000 T _ZN4ns_a13some_functionEPNS_7class_aE

But when i merge the main.d and other.d I get
$ nm main.o | grep some
                  U _ZN4ns_a13some_functionEPNS_7class_aE

I tested dmd (2.079.0), gdc and ldc2. All got the same result. 
Which makes me think, that it's not a bug, but a "feature" :)