From: daniel.kruegler@googlemail.com   
      
   Am 20.12.2011 22:33, schrieb Daniel Krügler:   
      
   > A final comment: The current preconditions of the expression-based   
   > traits are defective (The next LWG issue list will contain a   
   > corresponding new one): They allow too many situations which should   
   > actually lead to an ill-formed instantiation or to undefined behaviour,   
   > depending on the subtlety of the concrete case. One example is an   
   > assignment test of the following form:   
   >   
   > #include    
   >   
   > struct U;   
   >   
   > static_assert(!std::is_copy_assignable::value, "Error"); // ??   
   >   
   > struct U {};   
   >   
   > static_assert(std::is_copy_assignable::value, "Error");   
   >   
   > Now assume that the second test would happen in another translation   
   > unit: We would end up in a silent ODR violation.   
      
   In the above example I forgot to replace   
   std::is_copy_assignable by std::is_copy_assignable instead. As of   
   the current wording above examples violates the preconditions of the   
   trait, but by using a reference type instead it wouldn't (which is not   
   OK, of-course).   
      
   Apologize for the confusion,   
      
   Daniel   
      
      
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