SMT2 back-end: fix inconsistent array flattening

Whether we flatten or don't (when datatype support is not available)
depends on the expression, but we may need to unflatten when the context
expects an array.

Fixes: #8399

Author: tautschnig

src/solvers/smt2/smt2_conv.cpp

@@ -4565,57 +4565,50 @@ void smt2_convt::unflatten(
   }
   else if(type.id() == ID_array)
   {
-    if(use_datatypes)
+    PRECONDITION(use_as_const);
+
+    if(where == wheret::BEGIN)
+      out << "(let ((?ufop" << nesting << " ";
+    else
     {
-      PRECONDITION(use_as_const);
+      out << ")) ";
 
-      if(where == wheret::BEGIN)
-        out << "(let ((?ufop" << nesting << " ";
-      else
-      {
-        out << ")) ";
+      const array_typet &array_type = to_array_type(type);
 
-        const array_typet &array_type = to_array_type(type);
+      std::size_t subtype_width = boolbv_width(array_type.element_type());
 
-        std::size_t subtype_width = boolbv_width(array_type.element_type());
+      DATA_INVARIANT(
+        array_type.size().is_constant(),
+        "cannot unflatten arrays of non-constant size");
+      mp_integer size =
+        numeric_cast_v<mp_integer>(to_constant_expr(array_type.size()));
 
-        DATA_INVARIANT(
-          array_type.size().is_constant(),
-          "cannot unflatten arrays of non-constant size");
-        mp_integer size =
-          numeric_cast_v<mp_integer>(to_constant_expr(array_type.size()));
+      for(mp_integer i = 1; i < size; ++i)
+        out << "(store ";
 
-        for(mp_integer i = 1; i < size; ++i)
-          out << "(store ";
+      out << "((as const ";
+      convert_type(array_type);
+      out << ") ";
+      // use element at index 0 as default value
+      unflatten(wheret::BEGIN, array_type.element_type(), nesting + 1);
+      out << "((_ extract " << subtype_width - 1 << " "
+          << "0) ?ufop" << nesting << ")";
+      unflatten(wheret::END, array_type.element_type(), nesting + 1);
+      out << ") ";
 
-        out << "((as const ";
-        convert_type(array_type);
-        out << ") ";
-        // use element at index 0 as default value
+      std::size_t offset = subtype_width;
+      for(mp_integer i = 1; i < size; ++i, offset += subtype_width)
+      {
+        convert_expr(from_integer(i, array_type.index_type()));
+        out << ' ';
         unflatten(wheret::BEGIN, array_type.element_type(), nesting + 1);
-        out << "((_ extract " << subtype_width - 1 << " "
-            << "0) ?ufop" << nesting << ")";
+        out << "((_ extract " << offset + subtype_width - 1 << " " << offset
+            << ") ?ufop" << nesting << ")";
         unflatten(wheret::END, array_type.element_type(), nesting + 1);
-        out << ") ";
-
-        std::size_t offset = subtype_width;
-        for(mp_integer i = 1; i < size; ++i, offset += subtype_width)
-        {
-          convert_expr(from_integer(i, array_type.index_type()));
-          out << ' ';
-          unflatten(wheret::BEGIN, array_type.element_type(), nesting + 1);
-          out << "((_ extract " << offset + subtype_width - 1 << " " << offset
-              << ") ?ufop" << nesting << ")";
-          unflatten(wheret::END, array_type.element_type(), nesting + 1);
-          out << ")"; // store
-        }
-
-        out << ")"; // let
+        out << ")"; // store
       }
-    }
-    else
-    {
-      // nop, already a bv
+
+      out << ")"; // let
     }
   }
   else if(type.id() == ID_struct || type.id() == ID_struct_tag)
@@ -4767,19 +4760,22 @@ void smt2_convt::set_to(const exprt &expr, bool value)
         {
           out << "(define-fun " << smt2_identifier;
           out << " () ";
+          convert_type(equal_expr.lhs().type());
+          out << ' ';
           if(
             equal_expr.lhs().type().id() != ID_array ||
             use_array_theory(prepared_rhs))
           {
-            convert_type(equal_expr.lhs().type());
+            convert_expr(prepared_rhs);
           }
           else
           {
-            std::size_t width = boolbv_width(equal_expr.lhs().type());
-            out << "(_ BitVec " << width << ")";
+            unflatten(wheret::BEGIN, equal_expr.lhs().type());
+
+            convert_expr(prepared_rhs);
+
+            unflatten(wheret::END, equal_expr.lhs().type());
           }
-          out << ' ';
-          convert_expr(prepared_rhs);
           out << ')' << '\n';
         }