[Bug c++/101488] Implement p1042r1 __VA_OPT__ placemarker changes

["jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101488

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
I've tried a WIP (with just 0 && or commenting out parts of current code
instead of removing/cleaning up) that attempts to treat __VA_OPT__ ( ... ) with
## before __VA_OPT__ and/or after ) more similarly to the case where the
__VA_OPT__, ( and ) tokens are missing:
--- libcpp/macro.c.jj   2021-07-16 11:10:08.512925510 +0200
+++ libcpp/macro.c      2021-07-17 15:55:03.178568895 +0200
@@ -2059,7 +2059,7 @@ replace_args (cpp_reader *pfile, cpp_has

              /* Remove any tail padding from inside the __VA_OPT__.  */
              paste_flag = tokens_buff_last_token_ptr (buff);
-             while (paste_flag && paste_flag != start
+             while (0 && paste_flag && paste_flag != start
                     && (*paste_flag)->type == CPP_PADDING)
                {
                  tokens_buff_remove_last_token (buff);
@@ -2184,7 +2184,7 @@ replace_args (cpp_reader *pfile, cpp_has
                 previous emitted token is at the beginning of __VA_OPT__;
                 placemarkers within __VA_OPT__ are ignored in that case.  */
              else if (arg_tokens_count == 0
-                      && tmp_token_ptr != vaopt_start)
+/*                    && tmp_token_ptr != vaopt_start */)
                paste_flag = tmp_token_ptr;
            }
        }
@@ -2197,7 +2197,7 @@ replace_args (cpp_reader *pfile, cpp_has
                                     MACRO_ARG_TOKEN_EXPANDED,
                                     arg, arg->expanded);

-         if (last_token_is (buff, vaopt_start))
+         if (0 && last_token_is (buff, vaopt_start))
            {
              /* We're expanding an arg at the beginning of __VA_OPT__.
                 Skip padding. */
@@ -2215,7 +2215,7 @@ replace_args (cpp_reader *pfile, cpp_has
       /* Padding on the left of an argument (unless RHS of ##).  */
       if ((!pfile->state.in_directive || pfile->state.directive_wants_padding)
          && src != macro->exp.tokens && !(src[-1].flags & PASTE_LEFT)
-         && !last_token_is (buff, vaopt_start))
+         /* && !last_token_is (buff, vaopt_start) */)
        {
          const cpp_token *t = padding_token (pfile, src);
          unsigned index = expanded_token_index (pfile, macro, src, i);
@@ -2301,7 +2301,7 @@ replace_args (cpp_reader *pfile, cpp_has

       /* Avoid paste on RHS (even case count == 0).  */
       if (!pfile->state.in_directive && !(src->flags & PASTE_LEFT)
-         && !last_token_is (buff, vaopt_start))
+         /*&& !last_token_is (buff, vaopt_start)*/)
        {
          const cpp_token *t = &pfile->avoid_paste;
          tokens_buff_add_token (buff, virt_locs,
@@ -3544,6 +3544,8 @@ create_iso_definition (cpp_reader *pfile
   bool varadic = false;
   bool ok = false;
   cpp_macro *macro = NULL;
+  bool vaopt_after_paste = false;
+  unsigned int vaopt_end_idx = 0;

   /* Look at the first token, to see if this is a function-like
      macro.   */
@@ -3700,14 +3702,33 @@ create_iso_definition (cpp_reader *pfile
                token[-1].flags |= SP_DIGRAPH;
              if (token->flags & PREV_WHITE)
                token[-1].flags |= SP_PREV_WHITE;
+             if (macro->count == vaopt_end_idx)
+               token[-2].flags |= PASTE_LEFT;
            }
          following_paste_op = true;
        }
       else
        following_paste_op = false;

-      if (vaopt_tracker.update (token) == vaopt_state::ERROR)
+      vaopt_state::update_type vostate = vaopt_tracker.update (token);
+      if (vostate == vaopt_state::INCLUDE)
+       continue;
+      if (vostate == vaopt_state::ERROR)
        goto out;
+      if (vostate == vaopt_state::BEGIN
+         && macro->count > 1
+         && (token[-1].flags & PASTE_LEFT))
+       vaopt_after_paste = true;
+      if (vostate == vaopt_state::DROP && vaopt_after_paste)
+       {
+         vaopt_after_paste = false;
+         /* Duplicate the PASTE_LEFT flag on ( after __VA_OPT__
+            if ## is before __VA_OPT__.  */
+         if (!vaopt_tracker.stringify ())
+           token->flags |= PASTE_LEFT;
+       }
+      if (vostate == vaopt_state::END)
+       vaopt_end_idx = macro->count;
     }

   /* We're committed to winning now.  */

and that results into identical output on the #c0 testcase above as clang++,
and ditto macro_vaopt_p1042r1.cpp testcase, but macro_vaopt_expand.cpp differs
slightly:
-9: HT_A() C ( ) B ( ) "A()"
+9: TONG C ( ) B ( ) "A()"
-27_1: BA0 11
+27_1: BexpandedA0 11
(- is gcc with #__VA_OPT__ and the above patch, + is clang++).  That actually
matches how the F0 case in
#define LPAREN ( 
#define A0 expandedA0
#define A1  expandedA1 A0
#define A2  expandedA2 A1
#define A3  expandedA3 A2
#define A() B LPAREN )
#define B() C LPAREN )
#define C() D LPAREN )
#define HT_B() TONG
#define F0(x, ...) HT_ ## x x A()  #x
#define F(x, ...) HT_ ## __VA_OPT__(x x A()  #x)
8: F0(A(),1)
9: F(A(),1)
is handled, so I'm getting lost on what is supposed to be special about
__VA_OPT__ here.  In any case, the unpatched libcpp handled that F0 and F cases
the same as clang++.  And we probably need more extensive testsuite coverage...