fix crashes with MSVC 2019

src/math/lp/lar_solver.h

@@ -94,7 +94,6 @@ class lar_solver : public column_namer {
     // these are basic columns with the value changed, so the the corresponding row in the tableau
     // does not sum to zero anymore
     u_set                                               m_incorrect_columns;
-    stacked_value<int>                                  m_crossed_bounds_column_index; // such can be found at the initialization step
     stacked_value<unsigned>                             m_term_count;
     vector<lar_term*>                                   m_terms;
     indexed_vector<mpq>                                 m_column_buffer;

src/util/stacked_value.h

@@ -63,6 +63,10 @@ public:
         m_value = std::move(arg);
     }
 
+    void operator=(const T &arg) {
+        m_value = arg;
+    }
+
     operator T&() {
         return m_value;
     }