Comment on std::clamp

dune/solvers/common/interval.hh

@@ -61,6 +61,9 @@ public:
      */
     field_type projectIn(const field_type& x) const
     {
+        // NB: C++17 has std::clamp(x, data_[0], data_[1]).
+        //     Note, however, that we need to guarantee neither
+        //     data_[0] nor data_[1] is NaN then.
         return std::max(std::min(x,data_[1]), data_[0]);
     }