Libstdc++ exports the following symbols:

_ZNSt15basic_streambufIcSt11char_traitsIcEEC1ERKS2_@@GLIBCXX_3.4
_ZNSt15basic_streambufIcSt11char_traitsIcEEC2ERKS2_@@GLIBCXX_3.4
_ZNSt15basic_streambufIcSt11char_traitsIcEEaSERKS2_@@GLIBCXX_3.4
_ZNSt15basic_streambufIwSt11char_traitsIwEEC1ERKS2_@@GLIBCXX_3.4
_ZNSt15basic_streambufIwSt11char_traitsIwEEC2ERKS2_@@GLIBCXX_3.4
_ZNSt15basic_streambufIwSt11char_traitsIwEEaSERKS2_@@GLIBCXX_3.4

These are the copy constructor and copy assignment operator for std::streambuf and std::wstreambuf. Although those functions are defined they don't have sensible definitions e.g. the assignment operator doesn't update any members:

      __streambuf_type&
      operator=(const __streambuf_type&) { return *this; };

Prior to gcc-5 those functions are 'private' and are never used, so the symbols are redundant, and it's not a problem that the functions have the wrong semantics (in retrospect it would have been better to declare them private but not define them at all, so the symbols would not have been in libstdc++.so).

In C++11 basic_streambuf is copyable, so when using gcc-5 from DTS-4 code might require the copy constructor and assignment operator. However, that code will find the exported symbols in the system libstdc++.so, and those symbols don't have sensible definitions.

By applying a patch to the RHEL system GCC we can ensure that the symbols defined in the library have the correct semantics for C++11 code built with DTS-4. That change would have no impact on code built with the system compiler or DTS-3, because such code never uses those symbols anyway.

Changing the definition of these symbols would fix these failing tests when using devtoolset:

27_io/basic_fstream/assign/1.cc execution test
27_io/basic_ifstream/assign/1.cc execution test
27_io/basic_istringstream/assign/1.cc execution test
27_io/basic_istringstream/cons/move.cc execution test
27_io/basic_ofstream/assign/1.cc execution test
27_io/basic_ostringstream/assign/1.cc execution test
27_io/basic_streambuf/cons/57394.cc execution test
27_io/basic_stringstream/assign/1.cc execution test