[Issue 20208] New: extern (C++) copy constructor bad mangling for dmd

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20208

          Issue ID: 20208
           Summary: extern (C++) copy constructor bad mangling for dmd
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: edi33416 at gmail.com

DMD is incorrectly mangling the copy constructor of an `extern (C++) struct`.
Please see the details below

c.d
```
module c;

extern (C++) struct A
{
    int x;
    this(int x) { this.x = x; }
    this(const ref A rhs) { this.x = rhs.x; }
    ~this() { }
}
```

c.h
```
struct A
{
    _d_int x;
    A(_d_int x);
    A(const A& rhs);
    ~A();
};
```

tc.cpp
```
#include "c.h"                                                                  

int main(int argc, char *argv[])
{
    A x = A(42);
    A y = x;
    return 0;
}
```

I'm running on Ubuntu 18.04.1 LTS

Compiling `.d` with DMD64 D Compiler v2.086.0-1241-g08360654e

custdmd2 -c -betterC c.d

Compiling `.cpp` with g++ (Ubuntu 7.4.0-1ubuntu1~18.04.1) 7.4.0

g++ -c tc.cpp

Linking the two objects together

`g++ tc.o c.o` results in the linker error

tc.o: In function `main':
tc.cpp:(.text+0x3f): undefined reference to `A::A(A const&)'
collect2: error: ld returned 1 exit status

Results of `nm` on objects

nm c.o
[...]
0000000000000000 W _ZN1AC2ERKS_

nm tc.o
[...]
                 U _ZN1AC1ERKS_

Compiling the `.d` module with LDC - the LLVM D compiler (1.14.0) based on DMD
v2.084.1 and LLVM 7.0.1

ldc2 -betterC -c c.d

The resulted `c.o` object correctly links with the g++ generated one.

The output of `nm c.o` on the ldc2 generated object is
[...]
0000000000000000 T _ZN1AC1ERKS_

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