Is there any good reason why C++ namespaces are "closed" in D?

[Atila Neves]

On Monday, 30 July 2018 at 20:23:25 UTC, Walter Bright wrote:
> On 7/30/2018 9:45 AM, Manu wrote:
>> Sure it 'works', but it's an annoying point of friction, and it
>> doesn't need to be that way.
>
> You can reduce it to:
>
>     mixin(cppNamespace("ns", "void foo();"));
>
>
>>> You haven't explained why you can't just move the namespace 
>>> ns declarations in
>>> one file together.
>> 
>> Are you serious?
>
> Yes, please explain why you can't coalesce the namespace 
> declarations in one C++ file into one namespace declaration.

Because one C++ file != one C++ translation unit. All of them 
will #include other files and need the symbols in them, which, 
for anything using namespaces (the standard library, boost, ...), 
will mean several instances of opening and closing the same 
namespace.

The only good way (I don't think the mixin template and struct 
solutions count) to link to any of that today would be to have 
one enormous D file with _everything_ in it, including nested 
namespaces.