Use non-truncating check for char-is-unsigned

Author: tautschnig

src/util/config.cpp

@@ -9,6 +9,7 @@ Author: Daniel Kroening, kroening@kroening.com
 #include "config.h"
 
 #include <cstdlib>
+#include <limits>
 
 #include "namespace.h"
 #include "symbol_table.h"
@@ -949,7 +950,10 @@ bool configt::set(const cmdlinet &cmdline)
     assert(ansi_c.pointer_width==sizeof(void *)*8);
     assert(ansi_c.single_width==sizeof(float)*8);
     assert(ansi_c.double_width==sizeof(double)*8);
-    assert(ansi_c.char_is_unsigned==(static_cast<char>(255)==255));
+    assert(
+      ansi_c.char_is_unsigned ==
+      (static_cast<unsigned char>(std::numeric_limits<char>::max()) ==
+       std::numeric_limits<unsigned char>::max()));
 
     #ifndef _WIN32
     // On Windows, long double width varies by compiler