[PATCH] c++: Accept C++11 attribute-definition [PR101582]

[Jakub Jelinek <jakub@redhat.com>]

Hi!

As the following testcase shows, we don't parse properly
C++11 attribute-declaration:
https://eel.is/c++draft/dcl.dcl#nt:attribute-declaration

cp_parser_toplevel_declaration just handles empty-declaration parsing
(with diagnostics for C++98) and otherwise calls cp_parser_declaration
which on it calls cp_parser_simple_declaration and rejects it with
"does not declare anything" permerror.

The following patch instead handles it in cp_parser_toplevel_declaration
by parsing the attributes (standard ones only, we've never supported
__attribute__((...)); at namespace scope, so I'm not sure we need to
introduce that), which for C++98 emits the needed diagnostics, and then
warning if there are any attributes that we throw away on the floor.

I'll need this later for OpenMP directives at namespace scope, e.g.
[[omp::directive (requires, atomic_default_mem_order(seq_cst))]];
should be valid at namespace scope (and many other directives).

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2021-07-23  Jakub Jelinek  <jakub@redhat.com>

	PR c++/101582
	* parser.c (cp_parser_skip_std_attribute_spec_seq): Add a forward
	declaration.
	(cp_parser_toplevel_declaration): Parse attribute-declaration.

	* g++.dg/cpp0x/gen-attrs-45.C: Expect a warning about ignored
	attributes instead of error.
	* g++.dg/cpp0x/gen-attrs-75.C: New test.

--- gcc/cp/parser.c.jj	2021-07-22 17:47:26.025761491 +0200
+++ gcc/cp/parser.c	2021-07-22 19:09:28.487513184 +0200
@@ -2507,6 +2507,8 @@ static tree cp_parser_std_attribute_spec
   (cp_parser *);
 static tree cp_parser_std_attribute_spec_seq
   (cp_parser *);
+static size_t cp_parser_skip_std_attribute_spec_seq
+  (cp_parser *, size_t);
 static size_t cp_parser_skip_attributes_opt
   (cp_parser *, size_t);
 static bool cp_parser_extension_opt
@@ -14547,6 +14549,20 @@ cp_parser_toplevel_declaration (cp_parse
       if (cxx_dialect < cxx11)
 	pedwarn (input_location, OPT_Wpedantic, "extra %<;%>");
     }
+  else if (cp_lexer_nth_token_is (parser->lexer,
+				  cp_parser_skip_std_attribute_spec_seq (parser,
+									 1),
+				  CPP_SEMICOLON))
+    {
+      location_t attrs_loc = token->location;
+      tree std_attrs = cp_parser_std_attribute_spec_seq (parser);
+      if (std_attrs != NULL_TREE)
+	warning_at (make_location (attrs_loc, attrs_loc, parser->lexer),
+		    OPT_Wattributes,
+		    "attributes in attribute declaration are ignored");
+      if (cp_lexer_next_token_is (parser->lexer, CPP_SEMICOLON))
+	cp_lexer_consume_token (parser->lexer);
+    }
   else
     /* Parse the declaration itself.  */
     cp_parser_declaration (parser, NULL_TREE);
--- gcc/testsuite/g++.dg/cpp0x/gen-attrs-45.C.jj	2020-01-12 11:54:37.072403466 +0100
+++ gcc/testsuite/g++.dg/cpp0x/gen-attrs-45.C	2021-07-22 19:14:38.250222344 +0200
@@ -1,4 +1,4 @@
 // PR c++/52906
 // { dg-do compile { target c++11 } }
 
-[[gnu::deprecated]]; // { dg-error "does not declare anything" }
+[[gnu::deprecated]]; // { dg-warning "attributes in attribute declaration are ignored" }
--- gcc/testsuite/g++.dg/cpp0x/gen-attrs-75.C.jj	2021-07-22 19:14:58.438942693 +0200
+++ gcc/testsuite/g++.dg/cpp0x/gen-attrs-75.C	2021-07-22 19:12:18.442158972 +0200
@@ -0,0 +1,8 @@
+// PR c++/101582
+// { dg-do compile }
+// { dg-options "" }
+
+;
+[[]] [[]] [[]];	// { dg-warning "attributes only available with" "" { target c++98_only } }
+[[foobar]];	// { dg-warning "attributes in attribute declaration are ignored" }
+// { dg-warning "attributes only available with" "" { target c++98_only } .-1 }

	Jakub