c++: Reject attributes without arguments used as pack expansion [PR109756]

c++: Reject attributes without arguments used as pack expansion [PR109756]

The following testcase shows we silently accept (and ignore) attributes without
arguments used as pack expansions.  This is because we call
make_pack_expansion and that starts with
  if (!arg || arg == error_mark_node)
    return arg;
Now, an attribute without arguments like [[noreturn...]] is IMHO always
invalid, in this case for 2 reasons; one is that as it has no arguments,
no pack can be present and second is that the standard says that
attributes need to specially permit uses of parameter pack and doesn't
explicitly permit it for any of the standard attributes (except for alignas?
which has different syntax).
If an attribute has some arguments but doesn't contain packs in those
arguments, make_pack_expansion will already diagnose it.

The patch also changes cp_parser_std_attribute, such that for attributes unknown
to the compiler (or perhaps registered just for -Wno-attributes=) we differentiate
between the attribute having no arguments (in that case we want to diagnose them
when followed by ellipsis even if they are unknown, as they can't contain a pack
in that case) and the case where they do have arguments but we've just skipped over
those arguments because we don't know how to parse them (except that they are
a balanced token sequence) - in that case we really don't know if they contain
packs or not.

2023-05-10  Jakub Jelinek  <jakub@redhat.com>

	PR c++/109756
	* parser.cc (cp_parser_std_attribute): For unknown attributes with
	arguments set TREE_VALUE (attribute) to error_mark_node after skipping
	the balanced tokens.
	(cp_parser_std_attribute_list): If ... is used after attribute without
	arguments, diagnose it and return error_mark_node.  If
	TREE_VALUE (attribute) is error_mark_node, don't call
	make_pack_expansion nor return early error_mark_node.

	* g++.dg/cpp0x/gen-attrs-78.C: New test.