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

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 moves the handling of empty-declaration from
cp_parser_toplevel_declaration to cp_parser_declaration and
handles attribute-declaration in cp_parser_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).

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

	PR c++/101582
	* parser.c (cp_parser_skip_std_attribute_spec_seq): Add a forward
	declaration.
	(cp_parser_declaration): Parse empty-declaration and
	attribute-declaration.
	(cp_parser_toplevel_declaration): Don't parse empty-declaration here.

	* 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.
	* g++.dg/modules/pr101582-1.C: New test.

gcc/cp/parser.c

@@ -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
@@ -14405,6 +14407,30 @@ cp_parser_declaration (cp_parser* parser, tree prefix_attrs)
   cp_token *token2 = (token1->type == CPP_EOF
 		      ? token1 : cp_lexer_peek_nth_token (parser->lexer, 2));
 
+  if (token1->type == CPP_SEMICOLON)
+    {
+      cp_lexer_consume_token (parser->lexer);
+      /* A declaration consisting of a single semicolon is invalid
+       * before C++11.  Allow it unless we're being pedantic.  */
+      if (cxx_dialect < cxx11)
+	pedwarn (input_location, OPT_Wpedantic, "extra %<;%>");
+      return;
+    }
+  else if (cp_lexer_nth_token_is (parser->lexer,
+				  cp_parser_skip_std_attribute_spec_seq (parser,
+									 1),
+				  CPP_SEMICOLON))
+    {
+      location_t attrs_loc = token1->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, "attribute ignored");
+      if (cp_lexer_next_token_is (parser->lexer, CPP_SEMICOLON))
+	cp_lexer_consume_token (parser->lexer);
+      return;
+    }
+
   /* Get the high-water mark for the DECLARATOR_OBSTACK.  */
   void *p = obstack_alloc (&declarator_obstack, 0);
 
@@ -14555,14 +14581,6 @@ cp_parser_toplevel_declaration (cp_parser* parser)
        cp_parser_declaration.  (A #pragma at block scope is
        handled in cp_parser_statement.)  */
     cp_parser_pragma (parser, pragma_external, NULL);
-  else if (token->type == CPP_SEMICOLON)
-    {
-      cp_lexer_consume_token (parser->lexer);
-      /* A declaration consisting of a single semicolon is invalid
-       * before C++11.  Allow it unless we're being pedantic.  */
-      if (cxx_dialect < cxx11)
-	pedwarn (input_location, OPT_Wpedantic, "extra %<;%>");
-    }
   else
     /* Parse the declaration itself.  */
     cp_parser_declaration (parser, NULL_TREE);

gcc/testsuite/g++.dg/cpp0x/gen-attrs-45.C

@@ -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 "attribute ignored" }

gcc/testsuite/g++.dg/cpp0x/gen-attrs-75.C

@@ -0,0 +1,35 @@
+// PR c++/101582
+// { dg-do compile }
+// { dg-options "" }
+
+;
+[[]] [[]] [[]];	// { dg-warning "attributes only available with" "" { target c++98_only } }
+[[foobar]];	// { dg-warning "attribute ignored" }
+// { dg-warning "attributes only available with" "" { target c++98_only } .-1 }
+
+extern "C" ;
+extern "C" [[]];	// { dg-warning "attributes only available with" "" { target c++98_only } }
+extern "C" extern "C" ;
+extern "C" extern "C" [[]][[]][[]];	// { dg-warning "attributes only available with" "" { target c++98_only } }
+__extension__ ;
+__extension__ [[]];			// { dg-warning "attributes only available with" "" { target c++98_only } }
+__extension__ __extension__ ;
+__extension__ __extension__ [[]][[]];	// { dg-warning "attributes only available with" "" { target c++98_only } }
+
+namespace N {
+
+;
+[[]] [[]] [[]];	// { dg-warning "attributes only available with" "" { target c++98_only } }
+[[foobar]];	// { dg-warning "attribute ignored" }
+// { dg-warning "attributes only available with" "" { target c++98_only } .-1 }
+
+extern "C" ;
+extern "C" [[]];	// { dg-warning "attributes only available with" "" { target c++98_only } }
+extern "C" extern "C" ;
+extern "C" extern "C" [[]][[]][[]];	// { dg-warning "attributes only available with" "" { target c++98_only } }
+__extension__ ;
+__extension__ [[]];			// { dg-warning "attributes only available with" "" { target c++98_only } }
+__extension__ __extension__ ;
+__extension__ __extension__ [[]][[]];	// { dg-warning "attributes only available with" "" { target c++98_only } }
+
+}

gcc/testsuite/g++.dg/modules/pr101582-1.C

@@ -0,0 +1,9 @@
+// PR c++/101582
+// { dg-additional-options "-fmodules-ts" }
+export module pr101582;
+// { dg-module-cmi "pr101582" }
+export ;			// { dg-error "export declaration does not declare anything" "" { xfail *-*-* } }
+export [[]];			// { dg-error "export declaration does not declare anything" "" { xfail *-*-* } }
+export				// { dg-error "export declaration does not declare anything" "" { xfail *-*-* } }
+{
+}