[lmi] libstdc++ concept checks vs std::regex

[Greg Chicares]

Apparently we can either abandon libstdc++'s concept checks (which
do have some value), or not use some set of C++11 features that I
cannot today enumerate beyond demonstrating that it includes <regex>.

https://gcc.gnu.org/onlinedocs/libstdc++/manual/concept_checking.html
| Please note that the checks are based on the requirements in the
| original C++ standard, many of which were relaxed in the C++11
| standard and so valid C++11 code may be incorrectly rejected by
| the concept checks. Additionally, some correct C++03 code might be
| rejected ... There are no plans to address these shortcomings.
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Does this mean it has gone the way of '-fcheck-memory-usage'?
  https://gcc.gnu.org/ml/gcc/2002-02/msg00666.html
| Basically, it just didn't work any more, and it was decided to take it
| out to not create false expectations.
Is an alternative tool available to perform concept checking?

Or should we even consider defining the concept-checking macros
only for code that doesn't use any of this as-yet-unknown set of
modern C++ features?

Demonstration:

----------8<----------
Index: sandbox_test.cpp
===================================================================
--- sandbox_test.cpp(revision 6591)
+++ sandbox_test.cpp(working copy)
@@ -28,8 +28,12 @@

 #include "test_tools.hpp"

+#include <regex>
+
 int test_main(int, char*[])
 {
+    std::regex("abc");
+
     return 0;
 }

---------->8----------

make unit_tests unit_test_targets=sandbox_test.exe build_type=safestdlib

C:/opt/lmi/MinGW-4_9_1/i686-w64-mingw32/include/c++/bits/boost_concept_check.h:206:11:
 error: use of deleted function 
'std::__detail::_StateSeq<std::regex_traits<char> >& 
std::__detail::_StateSeq<std::regex_traits<char> >::operator=(const 
std::__detail::_StateSeq<std::regex_traits<char> >&)'

       __a = __a;                        // require assignment operator

           ^


Et cetera, et cetera, et cetera...