The following code compiles with gcc 4.7.1. Clang 3.1 reports an error   
   reading "constexpr variable 'x' must be initialized by a constant   
   expression".  The problem is independent of ini_list being passed by   
   const-ref or by value.   
      
   Which compiler conforms to the C++11 standard?   
      
   Thanks for your time.   
      
      
      
   #include    
   #include    
      
   struct foo   
   {   
      constexpr   
      foo( const std::initializer_list &ini_list ) : val(   
   *(ini_list.begin()) )   
      {}   
      const int val;   
   };   
      
   int main()   
   {   
      constexpr foo  x{ 4, 5 };   
      std::cout << x.val << "\n";   
   }   
      
      
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