Development builds at LZ are done with clang-3.8, but there is an alternate nightly build done with the older RHEL7 GCC-4.8.3 compiler (gcc is up to 6.1 now, so the RHEL7 default is truly _ancient_). This bit of code didn’t compile with gcc:

   template <typename mutex_type>
   class shared_lock
      mutex_type &      m_mutex ;


      /** construct and acquire the mutex in shared mode */
      explicit shared_lock( mutex_type & mutex )
         : m_mutex{ mutex }

The error is:

error: invalid initialization of non-const reference of type ‘lz::shared_mutex&’ from an rvalue of type ‘<brace-enclosed initializer list>’

This seems like a compiler bug to me, one that I’d seen when doing my scinet scientific computing course, which mandated the use of at least -std=c++11. In the scinet assignments, I fixed all such issues by using -std=c++14, which worked fine, but I was using gcc-5.3 for those assignments.

It appears that this is a compiler bug, and not just an issue with the c++11 language specification, as I initially thought while doing my scinet assignments. If I rebuild this code with g++-6.1, explicitly specifying -std=c++11 (GCC 6.1 defaults to c++14), then the issue goes away, so specification of -std=c++14 is not required to allow uniform initialization to work in this situation.

Because of being forced to use the older compiler, it looks like I have to fix this by using pre-c++11 syntax:

      explicit shared_lock( mutex_type & mutex )
         : m_mutex( mutex )

My conclusion is that gcc-4.8.3 is not truly up to the job of building c++11 compliant code. I’ll have to be more careful with the language features that I use in the future.