Get size from array if not available in model

If the size of the array is not provided by the model, get the size
directly from the array.

Note that the non-constant case now returns an empty optional rather
than an unknown exprt.

Author: Joel Allred

src/solvers/strings/string_refinement.cpp

@@ -952,6 +952,9 @@ void string_refinementt::add_lemma(
 }
 
 /// Get a model of the size of the input string.
+/// First ask the solver for a size value. If the solver has no value, get the
+/// size directly from the type. This is the case for string literals that are
+/// not part of the decision procedure (e.g. literals in return values).
 /// If the size value is not a constant or not a valid integer (size_t),
 /// return no value.
 /// \param super_get: function returning the valuation of an expression
@@ -970,19 +973,22 @@ static optionalt<exprt> get_valid_array_size(
   const array_poolt &array_pool)
 {
   const auto &size_from_pool = array_pool.get_length_if_exists(arr);
-  const exprt size = size_from_pool.has_value()
-                       ? size_from_pool.value()
-                       : exprt(ID_unknown, arr.length_type());
-
-  exprt size_val = super_get(size);
-  size_val = simplify_expr(size_val, ns);
-
-  if(size_val.id() != ID_constant)
+  exprt size_val;
+  if(size_from_pool.has_value())
   {
-    stream << "(sr::get_valid_array_size) string of unknown size: "
-           << format(size_val) << messaget::eom;
-    return {};
+    const exprt size = size_from_pool.value();
+    size_val = simplify_expr(super_get(size), ns);
+    if(size_val.id() != ID_constant)
+    {
+      stream << "(sr::get_valid_array_size) string of unknown size: "
+             << format(size_val) << messaget::eom;
+      return {};
+    }
   }
+  else if(to_array_type(arr.type()).size().id() == ID_constant)
+    size_val = simplify_expr(to_array_type(arr.type()).size(), ns);
+  else
+    return {};
 
   auto n_opt = numeric_cast<std::size_t>(size_val);
   if(!n_opt)