c++: Fix up parsing of attributes for using-directive

As I've said earlier and added xfails in gen-attrs-76.C test,
https://eel.is/c++draft/namespace.udir#nt:using-directive
has attribute-specifier-seq[opt] at the start, not at the end before ;
as gcc is expecting.
IMHO we should continue parsing at the end the GNU attributes
because using namespace N __attribute__((strong));, while not supported
anymore, used to be supported in the past, but my code searches for
using namespace N [[gnu::strong]]; didn't reveal anything at all.

2021-08-12  Jakub Jelinek  <jakub@redhat.com>

	* parser.c (cp_parser_block_declaration): Call
	cp_parser_using_directive for C++11 attributes followed by
	using namespace tokens.
	(cp_parser_using_directive): Parse C++11 attributes at the start
	of the directive rather than at the end, only parse GNU attributes
	at the end.

	* g++.dg/lookup/strong-using.C: Add test using [[gnu::strong]]
	as well.
	* g++.dg/lookup/strong-using2.C: Likewise.
	* g++.dg/cpp0x/gen-attrs-58.C: Move alignas(int) before
	using namespace.
	* g++.dg/cpp0x/gen-attrs-59.C: Move alignas(X) before
	using namespace, add tests for alignas before semicolon.
	* g++.dg/cpp0x/gen-attrs-76.C: Remove xfails.  Add test for
	C++11 attributes on using directive before semicolon.

1 files changed, 26 insertions, 5 deletions

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index d564e3b..ec885d7 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -14853,6 +14853,7 @@ cp_parser_block_declaration (cp_parser *parser,
   /* Peek at the next token to figure out which kind of declaration is
      present.  */
   cp_token *token1 = cp_lexer_peek_token (parser->lexer);
+  size_t attr_idx;
 
   /* If the next keyword is `asm', we have an asm-definition.  */
   if (token1->keyword == RID_ASM)
@@ -14906,6 +14907,18 @@ cp_parser_block_declaration (cp_parser *parser,
   /* If the next token is `static_assert' we have a static assertion.  */
   else if (token1->keyword == RID_STATIC_ASSERT)
     cp_parser_static_assert (parser, /*member_p=*/false);
+  /* If the next tokens after attributes is `using namespace', then we have
+     a using-directive.  */
+  else if ((attr_idx = cp_parser_skip_std_attribute_spec_seq (parser, 1)) != 1
+	   && cp_lexer_nth_token_is_keyword (parser->lexer, attr_idx,
+					     RID_USING)
+	   && cp_lexer_nth_token_is_keyword (parser->lexer, attr_idx + 1,
+					     RID_NAMESPACE))
+    {
+      if (statement_p)
+	cp_parser_commit_to_tentative_parse (parser);
+      cp_parser_using_directive (parser);
+    }
   /* Anything else must be a simple-declaration.  */
   else
     cp_parser_simple_declaration (parser, !statement_p,
@@ -21609,14 +21622,21 @@ cp_parser_alias_declaration (cp_parser* parser)
 /* Parse a using-directive.
 
    using-directive:
-     using namespace :: [opt] nested-name-specifier [opt]
-       namespace-name ;  */
+     attribute-specifier-seq [opt] using namespace :: [opt]
+       nested-name-specifier [opt] namespace-name ;  */
 
 static void
 cp_parser_using_directive (cp_parser* parser)
 {
   tree namespace_decl;
-  tree attribs;
+  tree attribs = cp_parser_std_attribute_spec_seq (parser);
+  if (cp_lexer_next_token_is (parser->lexer, CPP_SEMICOLON))
+    {
+      /* Error during attribute parsing that resulted in skipping
+	 to next semicolon.  */
+      cp_parser_require (parser, CPP_SEMICOLON, RT_SEMICOLON);
+      return;
+    }
 
   /* Look for the `using' keyword.  */
   cp_parser_require_keyword (parser, RID_USING, RT_USING);
@@ -21633,8 +21653,9 @@ cp_parser_using_directive (cp_parser* parser)
   /* Get the namespace being used.  */
   namespace_decl = cp_parser_namespace_name (parser);
   cp_warn_deprecated_use_scopes (namespace_decl);
-  /* And any specified attributes.  */
-  attribs = cp_parser_attributes_opt (parser);
+  /* And any specified GNU attributes.  */
+  if (cp_next_tokens_can_be_gnu_attribute_p (parser))
+    attribs = chainon (attribs, cp_parser_gnu_attributes_opt (parser));
 
   /* Update the symbol table.  */
   finish_using_directive (namespace_decl, attribs);