Work around spurious warning raised up to GCC 6.5

GCC complains:

../util/irep.h:247:21: error: ‘*((void*)(& __val)+8).irept::data’ may be used
uninitialized in this function [-Werror=maybe-uninitialized]
     remove_ref(data);

GCC 7 and later are fine without this hack.

Author: tautschnig

src/goto-programs/remove_virtual_functions.cpp

@@ -399,7 +399,7 @@ void remove_virtual_functionst::get_child_functions_rec(
       }
     }
     functions.push_back(function);
-    entry_map.insert({child, function});
+    entry_map.emplace(child, function);
 
     get_child_functions_rec(
       child,

src/goto-programs/remove_virtual_functions.h

@@ -54,13 +54,34 @@ enum class virtual_dispatch_fallback_actiont
 class dispatch_table_entryt
 {
  public:
-  dispatch_table_entryt() = default;
-  explicit dispatch_table_entryt(const irep_idt &_class_id)
-    : class_id(_class_id)
-  {}
-
-  optionalt<symbol_exprt> symbol_expr;
-  irep_idt class_id;
+   explicit dispatch_table_entryt(const irep_idt &_class_id)
+     : symbol_expr(), class_id(_class_id)
+   {}
+
+#if defined(__GNUC__) && __GNUC__ < 7
+   // GCC up to version 6.5 warns about irept::data being used uninitialized upon
+   // the move triggered by std::sort; using operator= works around this
+   dispatch_table_entryt(dispatch_table_entryt &&other)
+   {
+     symbol_expr = other.symbol_expr;
+     class_id = other.class_id;
+   }
+
+   dispatch_table_entryt &operator=(const dispatch_table_entryt &other)
+   {
+     symbol_expr = other.symbol_expr;
+     class_id = other.class_id;
+     return *this;
+   }
+
+   dispatch_table_entryt(const dispatch_table_entryt &other)
+     : symbol_expr(other.symbol_expr), class_id(other.class_id)
+   {
+   }
+#endif
+
+   optionalt<symbol_exprt> symbol_expr;
+   irep_idt class_id;
 };
 
 typedef std::vector<dispatch_table_entryt> dispatch_table_entriest;