c++: Fix up attribute rollbacks in cp_parser_statement

During the OpenMP directives using C++ attribute syntax work, I've noticed
that cp_parser_statement when parsing various block declarations that do
not allow attribute-specifier-seq at the start rolls back the attributes
only if std_attrs is non-NULL (i.e. some attributes have been parsed),
but doesn't roll back if some tokens were parsed as attribute-specifier-seq,
but didn't yield any attributes (e.g. [[]][[]][[]][[]]), which means
we accept those empty attributes even in places where they don't appear
in the grammar.

The following patch fixes that by instead checking if there are any
tokens to roll back.  This makes the parsing handle the first
function the same as the second one (where some attribute appears).

The testcase contains two xfails, using namespace ... apparently
allows attributes at the start and the attributes shall appeartain to
using in that case.  To be fixed incrementally.

2021-07-30  Jakub Jelinek  <jakub@redhat.com>

	* parser.c (cp_parser_statement): Rollback attributes not just
	when std_attrs is non-NULL, but whenever
	cp_parser_std_attribute_spec_seq parsed any tokens.

	* g++.dg/cpp0x/gen-attrs-76.C: New test.

2 files changed, 38 insertions, 4 deletions

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index d960d38..97078f9 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -11904,6 +11904,7 @@ cp_parser_statement (cp_parser* parser, tree in_statement_expr,
   cp_token *token;
   location_t statement_location, attrs_loc;
   bool in_omp_attribute_pragma = parser->lexer->in_omp_attribute_pragma;
+  bool has_std_attrs;
 
  restart:
   if (if_p != NULL)
@@ -11912,7 +11913,8 @@ cp_parser_statement (cp_parser* parser, tree in_statement_expr,
   statement = NULL_TREE;
 
   saved_token_sentinel saved_tokens (parser->lexer);
-  attrs_loc = cp_lexer_peek_token (parser->lexer)->location;
+  token = cp_lexer_peek_token (parser->lexer);
+  attrs_loc = token->location;
   if (c_dialect_objc ())
     /* In obj-c++, seeing '[[' might be the either the beginning of
        c++11 attributes, or a nested objc-message-expression.  So
@@ -11926,6 +11928,7 @@ cp_parser_statement (cp_parser* parser, tree in_statement_expr,
       if (!cp_parser_parse_definitely (parser))
 	std_attrs = NULL_TREE;
     }
+  has_std_attrs = cp_lexer_peek_token (parser->lexer) != token;
 
   if (std_attrs && (flag_openmp || flag_openmp_simd))
     std_attrs = cp_parser_handle_statement_omp_attributes (parser, std_attrs);
@@ -11994,7 +11997,7 @@ cp_parser_statement (cp_parser* parser, tree in_statement_expr,
 
 	case RID_NAMESPACE:
 	  /* This must be a namespace alias definition.  */
-	  if (std_attrs != NULL_TREE)
+	  if (has_std_attrs)
 	    {
 	      /* Attributes should be parsed as part of the
 		 declaration, so let's un-parse them.  */
@@ -12099,7 +12102,7 @@ cp_parser_statement (cp_parser* parser, tree in_statement_expr,
     {
       if (cp_lexer_next_token_is_not (parser->lexer, CPP_SEMICOLON))
 	{
-	  if (std_attrs != NULL_TREE)
+	  if (has_std_attrs)
 	    /* Attributes should be parsed as part of the declaration,
 	       so let's un-parse them.  */
 	    saved_tokens.rollback();
@@ -12111,7 +12114,7 @@ cp_parser_statement (cp_parser* parser, tree in_statement_expr,
 	  if (cp_parser_parse_definitely (parser))
 	    return;
 	  /* It didn't work, restore the post-attribute position.  */
-	  if (std_attrs)
+	  if (has_std_attrs)
 	    cp_lexer_set_token_position (parser->lexer, statement_token);
 	}
       /* All preceding labels have been parsed at this point.  */
diff --git a/gcc/testsuite/g++.dg/cpp0x/gen-attrs-76.C b/gcc/testsuite/g++.dg/cpp0x/gen-attrs-76.C
new file mode 100644
index 0000000..cbda8de
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/gen-attrs-76.C
@@ -0,0 +1,31 @@
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wno-attributes" }
+
+namespace N {}
+namespace O { typedef int T; };
+
+void
+foo ()
+{
+  [[]] asm ("");				// { dg-error "expected" }
+  [[]] __extension__ asm ("");			// { dg-error "expected" }
+  __extension__ [[]] asm ("");			// { dg-error "expected" }
+  [[]] namespace M = ::N;			// { dg-error "expected" }
+  [[]] using namespace N;			// { dg-bogus "expected" "" { xfail *-*-* } }
+  [[]] using O::T;				// { dg-error "expected" }
+  [[]] __label__ foo;				// { dg-error "expected" }
+  [[]] static_assert (true, "");		// { dg-error "expected" }
+}
+
+void
+bar ()
+{
+  [[gnu::unused]] asm ("");			// { dg-error "expected" }
+  [[gnu::unused]] __extension__ asm ("");	// { dg-error "expected" }
+  __extension__ [[gnu::unused]] asm ("");	// { dg-error "expected" }
+  [[gnu::unused]] namespace M = ::N;		// { dg-error "expected" }
+  [[gnu::unused]] using namespace N;		// { dg-bogus "expected" "" { xfail *-*-* } }
+  [[gnu::unused]] using O::T;			// { dg-error "expected" }
+  [[gnu::unused]] __label__ foo;		// { dg-error "expected" }
+  [[gnu::unused]] static_assert (true, "");	// { dg-error "expected" }
+}