Bug#834492: clang-3.9: "non-virtual thunk" abi_tag mangling is different from g++

[Sylvain Joubert]

Package: clang-3.9
Version: 1:3.9~+rc1-1~exp1
Severity: important

Dear Maintainer,

The new mangling behavior of clang++ for GCC abi_tag attribute seems to be
different from the g++ one on some edge cases.

Consider the following code:
$ cat classes.hpp
#include <string>

struct Mother
{
  virtual ~Mother() {};
  int bar;
};

struct Father
{
  virtual std::string get_foo()  = 0;
};

class Derived:
  public Mother,
  public Father
{
public:
  std::string get_foo();
};

$ cat classes.cpp
#include "classes.hpp"

std::string Derived::get_foo() { return ""; }

$ cat main.cpp
#include "classes.hpp"

struct Final:
  public Derived
{
};

int main()
{
  Final().get_foo();
}

When compiling the classes with clang++ and trying to link the executable with
g++ one gets the following link error:
$ clang++-3.9 -c classes.cpp && g++-5 main.cpp classes.o
/tmp/cchpctTt.o:main.cpp:vtable for Final : erreur : référence à « non-
virtual thunk to Derived::get_foo() » non définie
collect2: error: ld returned 1 exit status

And it seems this symbol is mangled differently by clang++ and g++:
$ clang++-3.9 -c classes.cpp && nm -C classes.o | grep get_foo
0000000000000000 T Derived::get_foo[abi:cxx11]()
0000000000000070 T non-virtual thunk to Derived::get_foo[abi:cxx11]()

$ g++-5 -c classes.cpp && nm -C classes.o | grep get_foo
0000000000000000 T Derived::get_foo[abi:cxx11]()
0000000000000065 T non-virtual thunk to Derived::get_foo()



-- System Information:
Debian Release: stretch/sid
  APT prefers testing
  APT policy: (990, 'testing'), (800, 'stable-updates'), (800, 'stable'), (700, 'unstable'), (90, 'experimental')
Architecture: amd64 (x86_64)

Kernel: Linux 4.6.0-1-amd64 (SMP w/8 CPU cores)
Locale: LANG=fr_FR.UTF-8, LC_CTYPE=fr_FR.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages clang-3.9 depends on:
ii  binutils                 2.26.1-1
ii  libc6                    2.23-4
ii  libc6-dev                2.23-4
ii  libclang-common-3.9-dev  1:3.9~+rc1-1~exp1
ii  libclang1-3.9            1:3.9~+rc1-1~exp1
ii  libgcc-6-dev             6.1.1-10
ii  libgcc1                  1:6.1.1-10
ii  libllvm3.9               1:3.9~+rc1-1~exp1
ii  libobjc-6-dev            6.1.1-10
ii  libstdc++-6-dev          6.1.1-10
ii  libstdc++6               6.1.1-10

Versions of packages clang-3.9 recommends:
ii  llvm-3.9-dev  1:3.9~+rc1-1~exp1
ii  python        2.7.11-2

Versions of packages clang-3.9 suggests:
pn  clang-3.9-doc  <none>
pn  gnustep        <none>
pn  gnustep-devel  <none>

-- no debconf information