Improve value_set to_constant()

For multi-valued sets, calculate the interval then, if it's a single
value interval return a constant, otherwise use the base operation.

Author: jezhiggins

src/analyses/variable-sensitivity/value_set_abstract_object.cpp

@@ -166,8 +166,15 @@ value_set_abstract_objectt::value_range_implementation() const
 exprt value_set_abstract_objectt::to_constant() const
 {
   verify();
-  return values.size() == 1 ? (*values.begin())->to_constant()
-                            : abstract_objectt::to_constant();
+
+  if(values.size() == 1)
+    return values.first()->to_constant();
+
+  auto interval = to_interval();
+  if(interval.is_single_value_interval())
+    return interval.get_lower();
+
+  return abstract_objectt::to_constant();
 }
 
 constant_interval_exprt value_set_abstract_objectt::to_interval() const

unit/analyses/variable-sensitivity/constant_abstract_value/merge.cpp

@@ -354,6 +354,19 @@ SCENARIO(
         EXPECT_UNMODIFIED(merged, val1);
       }
     }
+    WHEN("merging 1 with { 1, [1, 1] }")
+    {
+      auto op1 = make_constant(val1, environment, ns);
+      auto op2 = make_value_set(
+        {val1, constant_interval_exprt(val1, val1)}, environment, ns);
+
+      auto merged = merge(op1, op2);
+
+      THEN("result is unmodified 1")
+      {
+        EXPECT_UNMODIFIED(merged, val1);
+      }
+    }
     WHEN("merging 1 with { [ 2, 2 ] }")
     {
       auto op1 = make_constant(val1, environment, ns);