c++: Pedwarn on [[]]; at class scope [PR110345]

For C++ 26 P2552R3 I went through all the spots (except modules) where
attribute-specifier-seq appears in the grammar and tried to construct
a testcase in all those spots, for now for [[deprecated]] attribute.

The fourth issue is that we just emit (when enabled) -Wextra-semi warning
not just for lone semicolon at class scope (correct), but also for
[[]]; or [[whatever]]; there too.
While just semicolon is valid in C++11 and newer,
https://eel.is/c++draft/class.mem#nt:member-declaration
allows empty-declaration, unlike namespace scope or block scope
something like attribute-declaration or empty statement with attributes
applied for it aren't supported.
While syntactically it matches
attribute-specifier-seq [opt] decl-specifier-seq [opt] member-declarator-list [opt] ;
with the latter two omitted, there is
https://eel.is/c++draft/class.mem#general-3
which says that is not valid.

So, the following patch emits a pedwarn in that case.

2024-08-16  Jakub Jelinek  <jakub@redhat.com>

	PR c++/110345
	* parser.cc (cp_parser_member_declaration): Call maybe_warn_extra_semi
	only if it is empty-declaration, if there are some tokens like
	attribute, pedwarn that the declaration doesn't declare anything.

	* g++.dg/cpp0x/gen-attrs-84.C: New test.

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 852efe45076f6d5f9967ad05491d8f65aa461508..c9654cfff9d2483de00844b24addaa976a0bbf86 100644 (file)
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -28259,7 +28259,11 @@ cp_parser_member_declaration (cp_parser* parser)
       if (!decl_specifiers.any_specifiers_p)
        {
          cp_token *token = cp_lexer_peek_token (parser->lexer);
-         maybe_warn_extra_semi (token->location, extra_semi_kind::member);
+         if (decl_spec_token_start == token)
+           maybe_warn_extra_semi (token->location, extra_semi_kind::member);
+         else
+           pedwarn (decl_spec_token_start->location, OPT_Wpedantic,
+                    "declaration does not declare anything");
        }
       else
        {

diff --git a/gcc/testsuite/g++.dg/cpp0x/gen-attrs-84.C b/gcc/testsuite/g++.dg/cpp0x/gen-attrs-84.C
new file mode 100644 (file)
index 0000000..c573918
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/gen-attrs-84.C
@@ -0,0 +1,8 @@
+// { dg-do compile { target c++11 } }
+
+struct A {
+  [[]];                        // { dg-error "declaration does not declare anything" }
+};
+struct B {
+  [[gnu::deprecated]]; // { dg-error "declaration does not declare anything" }
+};