Why are extern(C/C++) definitions and references mangled differently in separately compiled modules?

[Max Samukha]

On Friday, 6 September 2019 at 15:52:46 UTC, Stefan Koch wrote:
> On Friday, 6 September 2019 at 15:09:22 UTC, Max Samukha wrote:
>> Consider the following two modules:
>>
>> 1. test.d:
>>
>> module test;
>>
>> import lib.a;
>>
>> void main() {
>>     foo();
>> }
>>
>>
>> 2. lib/a.d:
>>
>> module lib.a;
>>
>> extern(C) void foo() {}
>>
>>
>> When compiled separately (dmd -c lib/a.d; dmd test.d a.o), the 
>> function in 'a.o' is mangled as 'foo', but the reference in 
>> 'test.o' is mangled as '_D3lib1a3fooFZv', which leads to a 
>> link error. I would expect both of them to be either plain C 
>> mangling, or fully qualified D (better). What is the reason 
>> for current behavior?
>
> If that is happening you hit a bug.
> It seems unlikely though.

Could you elaborate a bit? How should extern(C/C++) definitions 
be mangled - fully qualified or not, and why is the reference to 
extern(C/C++) D-mangled? The spec seems to say nothing about it.