[Bug c++/87380] Explicit instantations should use weak symbols on darwin

[mrs at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87380

--- Comment #12 from mrs at gcc dot gnu.org <mrs at gcc dot gnu.org> ---
I changed the test case around, and the linker seems to be able to resolve from
a .a now:

$ ar -q liblib.a lib.o
$ nm -m liblib.a
liblib.a(lib.o):
0000000000000020 (__TEXT,__eh_frame) non-external EH_frame1
0000000000000000 (__TEXT,__text) external __Z5matchPi
0000000000000018 (__DATA,__data) weak external __ZN1AIiE6memberE
$ nm main.o 
0000000000000018 s EH_frame1
                 U __ZN1AIiE6memberE
0000000000000000 T _main
$ ../gcc/xgcc -B../gcc main.cc -L. -llib -o t -lstdc++

so, I think this is now safe to flip with later linkers.  Some type of code
like:

  if (darwin_target_linker
      && (strverscmp (darwin_target_linker, "409.12") >= 0))

I think will do it.  We just need to figure out when the bug was fixed.  I
tested 409.12.  This is the linker included with Xcode 10.0, which is the now
current Xcode.  Jack or Iain might have access to older systems.

up:t mrs$ cat lib.cc
#define LIB

#include "lib.h"

template class A<int>;

bool match(int* p)
{
  return p == &A<int>::member;
}
up:t mrs$ cat lib.h
template<typename T>
struct A {
  static T member;
};
template<typename T>
  T A<T>::member;

bool match(int*);

#ifndef LIB
extern template class A<int>;
#endif
up:t mrs$ cat main.cc
#include "lib.h"

extern char _ZN1AIiE6memberE[3];

int main()
{

  void *vp = _ZN1AIiE6memberE;



  //  if (!match(&A<int>::member))
  //    throw 1;
}

is the type of test case I tried.  This is _with_ the change
TARGET_WEAK_NOT_IN_ARCHIVE_TOC to make it 0.  This is required to get the
symbol weak.