Avoid bison compiler warning for yynerrs

src/parser/parser.yy

@@ -28,6 +28,12 @@ refptr<Scope> parser_scope;
 %locations
 %define parse.error verbose
 
+%initial-action {
+    /* Reference yynerrs so the generated parser does not warn that it is
+       set but never used; this grammar has no error-recovery rules. */
+    (void) yynerrs;
+}
+
 %token PLUS;
 %token MINUS;
 %token TIMES;