generate the default action only for C++

This commit adds restrictions to what was done in
01898726e2 [1].

Rici Lake [2] has shown that it's risky to disable the pre-action, at
least now.  Also, generating the default $$ = $1 action can have bad
effects in some cases [3].

The original change [1] was prompted for C++.  Let's try it there
only, for a start.  We could restrict it further to lalr1.cc with
variants, but we need to see in the wild how this change behaves.  And
it is not unreasonable to expect grammar files in C++ to behave better
wrt types.

See
[1] https://lists.gnu.org/archive/html/bison-patches/2018-10/msg00050.html
[2] https://lists.gnu.org/archive/html/bison-patches/2018-10/msg00061.html
[3] https://lists.gnu.org/archive/html/bison-patches/2018-10/msg00066.html

* src/getargs.c: Style changes.
* src/reader.c (grammar_rule_check_and_complete): Complete only for
C++.

src/getargs.c

@@ -51,8 +51,9 @@ int report_flag = report_none;
 int trace_flag = trace_none;
 
 static struct bison_language const valid_languages[] = {
-  { "c", "c-skel.m4", ".c", ".h", true },
-  { "c++", "c++-skel.m4", ".cc", ".hh", true },
+  /* lang,  skeleton,       ext,     hdr,     add_tab */
+  { "c",    "c-skel.m4",    ".c",    ".h",    true },
+  { "c++",  "c++-skel.m4",  ".cc",   ".hh",   true },
   { "java", "java-skel.m4", ".java", ".java", false },
   { "", "", "", "", false }
 };
@@ -432,8 +433,7 @@ language_argmatch (char const *arg, int prio, location loc)
 
   if (prio < language_prio)
     {
-      int i;
-      for (i = 0; valid_languages[i].language[0]; i++)
+      for (int i = 0; valid_languages[i].language[0]; ++i)
         if (c_strcasecmp (arg, valid_languages[i].language) == 0)
           {
             language_prio = prio;

src/reader.c

@@ -306,13 +306,20 @@ grammar_rule_check_and_complete (symbol_list *r)
                       lhs_type, rhs_type);
           else
             {
-              /* Install the default action.  */
-              code_props_rule_action_init (&r->action_props, "{ $$ = $1; }",
-                                           r->location, r,
-                                           /* name */ NULL,
-                                           /* type */ NULL,
-                                           /* is_predicate */ false);
-              code_props_translate_code (&r->action_props);
+              /* Install the default action only for C++.  */
+              bool is_cxx =
+                skeleton
+                ? STREQ (skeleton, "glr.cc") || STREQ (skeleton, "lalr1.cc")
+                : STREQ (language->language, "c++");
+              if (is_cxx)
+                {
+                  code_props_rule_action_init (&r->action_props, "{ $$ = $1; }",
+                                               r->location, r,
+                                               /* name */ NULL,
+                                               /* type */ NULL,
+                                               /* is_predicate */ false);
+                  code_props_translate_code (&r->action_props);
+                }
             }
         }
       /* Warn if there is no default for $$ but we need one.  */