[Bug preprocessor/114007] gcc chokes on __has_cpp_attribute(clang::unsafe_buffer_usage)

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114007

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rsandifo at gcc dot gnu.org

--- Comment #13 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Ah, the thing is that while in -std=gnu* modes or -std=c23 the preprocessor
recognizes CPP_SCOPE as one token, in -std=c{89,99,11,17} modes it doesn't, ::
are 2 CPP_COLONs.
So, we could either:
--- gcc/c-family/c-lex.cc.jj    2024-01-03 12:07:02.171734141 +0100
+++ gcc/c-family/c-lex.cc       2024-02-21 14:30:37.247945782 +0100
@@ -357,7 +357,24 @@ c_common_has_attribute (cpp_reader *pfil
       do
        nxt_token = cpp_peek_token (pfile, idx++);
       while (nxt_token->type == CPP_PADDING);
-      if (nxt_token->type == CPP_SCOPE)
+      if (!c_dialect_cxx ()
+         && flag_iso
+         && !flag_isoc23
+         && nxt_token->type == CPP_COLON)
+       {
+         do
+           nxt_token = cpp_peek_token (pfile, idx++);
+         while (nxt_token->type == CPP_PADDING);
+         if (nxt_token->type == CPP_COLON)
+           {
+             /* __has_attribute (vendor::attr) in -std=c17 etc. modes.
+                :: isn't CPP_SCOPE in there and [[vendor::attr]] will
+                not work, only [[__extension__ vendor::attr]].  */
+             have_scope = true;
+             get_token_no_padding (pfile); // Eat first colon.
+           }
+       }
+      if (nxt_token->type == CPP_SCOPE || have_scope)
        {
          have_scope = true;
          get_token_no_padding (pfile); // Eat scope.
but then on testcase like:
#if __has_c_attribute (gnu::unused)
[[gnu::unused]]
#endif
int i;
#if __has_cpp_attribute (gnu::unused)
[[gnu::unused]]
#endif
int j;
fails to compile with e.g -std=c11:
pr114007.c:2:1: warning: ‘gnu’ attribute ignored [-Wattributes]
    2 | [[gnu::unused]]
      | ^
pr114007.c:2:6: error: expected ‘]’ before ‘:’ token
    2 | [[gnu::unused]]
      |      ^
      |      ]
pr114007.c:6:1: warning: ‘gnu’ attribute ignored [-Wattributes]
    6 | [[gnu::unused]]
      | ^
pr114007.c:6:6: error: expected ‘]’ before ‘:’ token
    6 | [[gnu::unused]]
      |      ^
      |      ]
or we could force always returning 0 from __has_attribute/__has_cpp_attribute
in that case, like:
--- gcc/c-family/c-lex.cc.jj    2024-01-03 12:07:02.171734141 +0100
+++ gcc/c-family/c-lex.cc       2024-02-21 14:41:33.768992572 +0100
@@ -357,7 +357,33 @@ c_common_has_attribute (cpp_reader *pfil
       do
        nxt_token = cpp_peek_token (pfile, idx++);
       while (nxt_token->type == CPP_PADDING);
-      if (nxt_token->type == CPP_SCOPE)
+      if (!c_dialect_cxx ()
+         && flag_iso
+         && !flag_isoc23
+         && nxt_token->type == CPP_COLON)
+       {
+         do
+           nxt_token = cpp_peek_token (pfile, idx++);
+         while (nxt_token->type == CPP_PADDING);
+         if (nxt_token->type == CPP_COLON)
+           /* __has_attribute (vendor::attr) in -std=c17 etc. modes.
+              :: isn't CPP_SCOPE in there but 2 CPP_COLON tokens.  */
+           have_scope = true;
+       }
+      if (have_scope)
+       {
+         /* [[vendor::attr]] will not work, only
+            [[__extension__ vendor::attr]] will.  Better always return 0
+            for scoped attributes.  */
+         get_token_no_padding (pfile); // Eat first colon.
+         get_token_no_padding (pfile); // Eat second colon.
+         nxt_token = get_token_no_padding (pfile);
+         if (nxt_token->type != CPP_NAME)
+           cpp_error (pfile, CPP_DL_ERROR,
+                      "attribute identifier required after scope");
+         attr_name = NULL_TREE;
+       }
+      else if (nxt_token->type == CPP_SCOPE)
        {
          have_scope = true;
          get_token_no_padding (pfile); // Eat scope.
The drawback of the second patch is that then users in -std=c{89,99,11,17}
modes don't have a way to query whether a certain scoped attribute is supported
in the preprocessor if they are aware that they need to use [[__extension__
vendor::attr]] rather then
[[vendor::attr]].  On the other side, e.g. in -std=gnu11 -pedantic-errors
compilation
we give 1 for __has_c_attribute (gnu::unused), but it is still rejected, just
with -std=c11 it is rejected even without -Wpedantic.

Maybe instead of loose_scope_p we should be using flag_iso && !flag_isoc23 and
accept [[vendor: :attr]] in the -std=c{89,99,11,17} modes too (with pedwarn for
the [[]] use), and on the other side reject [[__extension__ vendor: :attr]] in
-std=c23
or -std=gnu{89,99,11,17} modes, so that people don't feel using 2 colons rather
than a scope is correct.  And then perhaps go with the first patch rather than
second.

Joseph/Richard, your thoughts on this?