Construct plus_exprt in a non-deprecated way

The default constructor is deprecated. To facilitate construction with an
arbitrary number of operands a new constructor taking a initializer list is
added.

Author: tautschnig

src/solvers/smt2/smt2_conv.cpp

@@ -3076,13 +3076,13 @@ void smt2_convt::convert_plus(const plus_exprt &expr)
     // add component-by-component
     for(mp_integer i = 0; i != size; ++i)
     {
-      plus_exprt tmp(vector_type.subtype());
-      forall_operands(it, expr)
-        tmp.copy_to_operands(
-          index_exprt(
-            *it,
-            from_integer(size - i - 1, index_type),
-            vector_type.subtype()));
+      exprt::operandst summands;
+      summands.reserve(expr.operands().size());
+      for(const auto &op : expr.operands())
+        summands.push_back(index_exprt(
+          op, from_integer(size - i - 1, index_type), vector_type.subtype()));
+
+      plus_exprt tmp(std::move(summands), vector_type.subtype());
 
       out << " ";
       convert_expr(tmp);

src/util/std_expr.h

@@ -1076,6 +1076,11 @@ class plus_exprt:public multi_ary_exprt
     multi_ary_exprt(_lhs, ID_plus, _rhs, _type)
   {
   }
+
+  plus_exprt(operandst &&_operands, const typet &_type)
+    : multi_ary_exprt(ID_plus, std::move(_operands), _type)
+  {
+  }
 };
 
 /// \brief Cast an exprt to a \ref plus_exprt