[Patch] bitsetvectortest fails to compile with clang and libc++ (the default on OSX)

The compiler complains as follows:

.../dune/common/test/bitsetvectortest.cc:23:24: error: no viable conversion from
      'const_reference' (aka '__bit_const_reference<std::__1::vector<bool, std::__1::allocator<bool> > >') to 'bitset'
      (aka 'bitset<4>')
    DUNE_UNUSED bitset x = t[0];
                       ^   ~~~~

The code in question reads

   typedef typename BBF::const_reference const_reference;
 
   void operator()(const_reference t){
     DUNE_UNUSED bitset x = t[0];
   }

Here, BBF is a BitSetVector, so that the const_reference t is a reference to an entry, which is a (4-entry) std::vector<bool>, thus std::vector<bool>::const_reference, which can be converted to an std::bitset<4>.

For some reason, operator() expects t[0], a bool, to be convertible to a bitset as well. Which indeed it is with gcc for some reason. Should this really be t[0] rather than just t, though, which would make clang happy?

@@ -20,7 +20,7 @@ struct ConstReferenceOp
   typedef typename BBF::const_reference const_reference;
 
   void operator()(const_reference t){
-    DUNE_UNUSED bitset x = t[0];
+    DUNE_UNUSED bitset x = t;
   }
 };