[Issue 18582] New: C++ namespace mangling from multiple modules doesn't use sequence ids

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18582

          Issue ID: 18582
           Summary: C++ namespace mangling from multiple modules doesn't
                    use sequence ids
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Keywords: C++, mangling
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: schveiguy at yahoo.com

discussion on the forum:
https://forum.dlang.org/post/tgzafcnjrdtkklefrsbz@forum.dlang.org

If there is a namespace that is declared in 2 different files, and a function
in the namespace in one file uses a type from the same namespace from another
file, instead of using a backreference (sequence id) in the name mangling for
the namespace of the parameter, it is incorrectly spelled out.

example:

lib.cpp:
namespace thenamespace
{
    class class_a {
    };
    void some_function(class_a*) {;}
} 

other.d:
extern (C++, thenamespace) {
        class class_a {}
}

main.d:
import other;
extern (C++, thenamespace) {
        void some_function(class_a);
}
void main() {
        class_a instance_a;
            thenamespace.some_function(instance_a);
} 

build via:

g++ -c lib.cpp
dmd main.d other.d lib.o

Linker error:
Undefined symbols for architecture x86_64:
  "thenamespace::some_function(thenamespace::class_a*)", referenced from:
      __Dmain in main.o


If I use nm I see:
in main.o:
U __ZN12thenamespace13some_functionEPN12thenamespace7class_aE

in lib.o:
T __ZN12thenamespace13some_functionEPNS_7class_aE

Note the back reference S_ near the end of the symbol. I believe the reason it
doesn't happen for D's C++ mangler is because even though the namespace is
identical, it's a different D symbol from a different module.

This is shown by moving the declaration of class_a from other.d into main.d,
and compilation succeeds.

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