Add a default back in for a case where gcc (erroneously) claims

control flow reaches end without return.  Must also locally disable the
clang covered-switch-default warning.

src/util/ext_numeral.h

@@ -21,6 +21,7 @@ Revision History:
 
 #include<ostream>
 #include "util/debug.h"
+#include "util/manage_warnings.h"
     
 enum ext_numeral_kind { EN_MINUS_INFINITY, EN_NUMERAL, EN_PLUS_INFINITY };
 
@@ -280,6 +281,16 @@ bool lt(numeral_manager & m,
             return m.lt(a, b);
         case EN_PLUS_INFINITY:
             return true;
+
+	    // The default case below is not necessary: the cases above cover all the
+	    // elements of the ext_numeral_kind enum.  But gcc complains with a warning
+	    // if this default case is absent.  So we leave it in, but disable Clang's
+	    // (correct) warning that the default is unnecessary.
+	    START_DISABLE_COVERED_SWITCH_DEFAULT;
+        default:
+            UNREACHABLE();
+            return false;
+	    END_DISABLE_WARNING_STMT;
         }
     case EN_PLUS_INFINITY:
         return false;

src/util/manage_warnings.h

@@ -42,15 +42,22 @@ Revision History:
   DO_PRAGMA(clang diagnostic ignored #s)
 
 
+// This version should be used in decl contexts.
 #define END_DISABLE_WARNING			\
   _Pragma("clang diagnostic pop")		\
   DUMMY_DECL
 
+// This version should be used in statement contexts.
+#define END_DISABLE_WARNING_STMT		\
+  _Pragma("clang diagnostic pop")
+
 #define START_DISABLE_EXTRA_SEMI_WARNING START_DISABLE_WARNING(-Wextra-semi)
+#define START_DISABLE_COVERED_SWITCH_DEFAULT START_DISABLE_WARNING(-Wcovered-switch-default)
 
 #else
 
 #define START_DISABLE_EXTRA_SEMI_WARNING
+#define START_DISABLE_COVERED_SWITCH_DEFAULT
 #define END_DISABLE_WARNING
 
 #endif