MINGW-packages: Issues when building with clang (duplicate libstdc++.a symbols)

Hello,

I am currently testing clang compilation and ran into several issues. Due to #1669 being unfixed at this moment I mostly relied on 3.8.0 win32 and 3.9.0 rc3 win32, however I tested mingw-w64-i686-clang (3.8) as well. Due to reports getting large I decided to split them.

2. A number of symbols present in libstdc++ headers are exported by libstdc++.a The issue is pretty straight-forward. When performing optimised static linking to libstdc++.a one is likely to face multiple symbol definition issue.

Minimal sample (issue confirmed on any clang): clang++ -target i686-w64-mingw32 -static -std=c++11 -pthread -O1 example2.cpp

#include <string>

int main () {
    auto s = std::string("1234");
    auto ws = std::u16string(u"1234");
    auto s1 = "#" + s;
}

This will produce the following error message:

C:\msys64\mingw32\lib\gcc\i686-w64-mingw32\6.1.0\libstdc++.a(string-inst.o):(.text$_ZStplIcSt11char_traitsIcESaIcEENSt7__cxx1112basic_stringIT_T0_T1_EEPKS5_RKS8_[__ZStplIcSt11char_traitsIcESaIcEENSt7__cxx1112basic_stringIT_T0_T1_EEPKS5_RKS8_]+0x0): multiple definition of `std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > std::operator+<char, std::char_traits<char>, std::allocator<char> >(char const*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
C:\msys64\tmp\example2-b21e71.o:(.text[__ZStplIcSt11char_traitsIcESaIcEENSt7__cxx1112basic_stringIT_T0_T1_EEPKS5_RKS8_]+0x0): first defined here
C:\msys64\mingw32\lib\gcc\i686-w64-mingw32\6.1.0\libstdc++.a(string-inst.o):(.text$_ZNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEE12_M_constructIPKcEEvT_S8_St20forward_iterator_tag[__ZNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEE12_M_constructIPKcEEvT_S8_St20forward_iterator_tag]+0x0): multiple definition of `void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct<char const*>(char const*, char const*, std::forward_iterator_tag)'
C:\msys64\tmp\example2-b21e71.o:(.text[__ZNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEE12_M_constructIPKcEEvT_S8_St20forward_iterator_tag]+0x0): first defined here
clang++.exe: error: linker command failed with exit code 1 (use -v to see invocation)

As it can be seen from the error the bug is caused by exported symbols in libstdc++.a. I do not expect it to export any _M_construct but it does this for char-instantiated one. I.e. u16string has no this bug. One is also required to enable optimisation (e.g. -O1) to get compilation failure.

I could imagine various ways to get this fixed, for example, SFINAE. I used a force_inline attribute.

Suggested changes: msys64\mingw32\include\c++\6.1.0\bits\basic_string.h

      // For forward_iterators up to random_access_iterators, used for
      // string::iterator, _CharT*, etc.
      template<typename _FwdIterator>
#ifdef __clang__
        __attribute__((always_inline))
#endif
        void
        _M_construct(_FwdIterator __beg, _FwdIterator __end,
             std::forward_iterator_tag);

and

  /**
   *  @brief  Concatenate C string and string.
   *  @param __lhs  First string.
   *  @param __rhs  Last string.
   *  @return  New string with value of @a __lhs followed by @a __rhs.
   */
  template<typename _CharT, typename _Traits, typename _Alloc>
#ifdef __clang__
    __attribute__((always_inline))
#endif
    basic_string<_CharT,_Traits,_Alloc>
    operator+(const _CharT* __lhs,
          const basic_string<_CharT,_Traits,_Alloc>& __rhs);

These methods may not be the only ones but are so far the only noticed ones by myself.

Also, I had to pass -pthread to get the sample compile without undefined pthread symbols. This feels undesired but I do not know any details about who is responsible here.