c++: Fix up final handling in C++98 [PR120628]

The following patch is on top of the
https://gcc.gnu.org/pipermail/gcc-patches/2025-June/686210.html
patch which stopped treating override as conditional keyword in
class properties.
This PR mentions another problem; we emit a bogus warning on code like
struct C {}; struct C final = {};
in C++98.  In this case we parse final as conditional keyword in C++
(including pedwarn) but the caller then immediately aborts the tentative
parse because it isn't followed by { nor (in some cases) : .
I think we certainly shouldn't pedwarn on it, but I think we even shouldn't
warn for it say for -Wc++11-compat, because we don't actually treat the
identifier as conditional keyword even in C++11 and later.
The patch only does this if final is the only class property conditional
keyword, if one uses
struct S __final final __final = {};
one gets the warning and duplicate diagnostics and later parsing errors.

2025-07-10  Jakub Jelinek  <jakub@redhat.com>

	PR c++/120628
	* parser.cc (cp_parser_elaborated_type_specifier): Use
	cp_parser_nth_token_starts_class_definition_p with extra argument 1
	instead of cp_parser_next_token_starts_class_definition_p.
	(cp_parser_class_property_specifier_seq_opt): For final conditional
	keyword in C++98 check if the token after it isn't
	cp_parser_nth_token_starts_class_definition_p nor CPP_NAME and in
	that case break without consuming it nor warning.
	(cp_parser_class_head): Use
	cp_parser_nth_token_starts_class_definition_p with extra argument 1
	instead of cp_parser_next_token_starts_class_definition_p.
	(cp_parser_next_token_starts_class_definition_p): Renamed to ...
	(cp_parser_nth_token_starts_class_definition_p): ... this.  Add N
	argument.  Use cp_lexer_peek_nth_token instead of cp_lexer_peek_token.

	* g++.dg/cpp0x/final1.C: New test.
	* g++.dg/cpp0x/final2.C: New test.
	* g++.dg/cpp0x/override6.C: New test.

3 files changed, 63 insertions, 0 deletions

diff --git a/gcc/testsuite/g++.dg/cpp0x/final1.C b/gcc/testsuite/g++.dg/cpp0x/final1.C
new file mode 100644
index 0000000..1d64095
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/final1.C
@@ -0,0 +1,11 @@
+// PR c++/120628
+// { dg-do compile { target c++98_only } }
+
+namespace A {
+  struct B {};
+  struct B final = {};
+}
+namespace C {
+  struct D { D (int, int); };
+  struct D final (42, 0);
+}
diff --git a/gcc/testsuite/g++.dg/cpp0x/final2.C b/gcc/testsuite/g++.dg/cpp0x/final2.C
new file mode 100644
index 0000000..d8d5866
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/final2.C
@@ -0,0 +1,26 @@
+// PR c++/120628
+// { dg-do compile }
+// { dg-options "" }
+// { dg-additional-options "-pedantic" { target c++14 } }
+
+namespace U {
+  struct A {};
+  struct A final = {};
+}
+namespace V {
+  template <int N>
+  struct B {};
+  template <int N>
+  struct B<N> final = {};	// { dg-warning "variable templates only available with" "" { target c++11_down } }
+}
+struct C {
+  struct D {};
+  static D foo ();
+  struct D final = foo ();	// { dg-warning "non-static data member initializers only available with" "" { target c++98_only } }
+};
+namespace W {
+  struct E { struct F {}; };
+  struct E::F final = {};
+}
+template <int N>
+struct V::B<N> final = {};	// { dg-warning "variable templates only available with" "" { target c++11_down } }
diff --git a/gcc/testsuite/g++.dg/cpp0x/override6.C b/gcc/testsuite/g++.dg/cpp0x/override6.C
new file mode 100644
index 0000000..601e91d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/override6.C
@@ -0,0 +1,26 @@
+// PR c++/120628
+// { dg-do compile }
+// { dg-options "" }
+// { dg-additional-options "-pedantic" { target c++14 } }
+
+namespace U {
+  struct A {};
+  struct A override = {};
+}
+namespace V {
+  template <int N>
+  struct B {};
+  template <int N>
+  struct B<N> override = {};	// { dg-warning "variable templates only available with" "" { target c++11_down } }
+}
+struct C {
+  struct D {};
+  static D foo ();
+  struct D override = foo ();	// { dg-warning "non-static data member initializers only available with" "" { target c++98_only } }
+};
+namespace W {
+  struct E { struct F {}; };
+  struct E::F override = {};
+}
+template <int N>
+struct V::B<N> override = {};	// { dg-warning "variable templates only available with" "" { target c++11_down } }