c++: Implement C++26 P3176R1 - The Oxford variadic comma

While we are already in stage3, I wonder if implementing this small paper
wouldn't be useful even for GCC 15, so that we have in the GCC world one
extra year of deprecation of variadic ellipsis without preceding comma.

The paper just deprecates something, I'd hope most of the C++ code in the
wild when it uses variadic functions at all uses the comma before the
ellipsis.

2024-11-30  Jakub Jelinek  <jakub@redhat.com>

gcc/c-family/
	* c.opt (Wdeprecated-variadic-comma-omission): New option.
	* c.opt.urls: Regenerate.
	* c-opts.cc (c_common_post_options): Default to
	-Wdeprecated-variadic-comma-omission for C++26 or -Wpedantic.
gcc/cp/
	* parser.cc: Implement C++26 P3176R1 - The Oxford variadic comma.
	(cp_parser_parameter_declaration_clause): Emit
	-Wdeprecated-variadic-comma-omission warnings.
gcc/
	* doc/invoke.texi (-Wdeprecated-variadic-comma-omission): Document.
gcc/testsuite/
	* g++.dg/cpp26/variadic-comma1.C: New test.
	* g++.dg/cpp26/variadic-comma2.C: New test.
	* g++.dg/cpp26/variadic-comma3.C: New test.
	* g++.dg/cpp26/variadic-comma4.C: New test.
	* g++.dg/cpp26/variadic-comma5.C: New test.
	* g++.dg/cpp1z/fold10.C: Expect a warning for C++26.
	* g++.dg/ext/attrib33.C: Likewise.
	* g++.dg/cpp1y/lambda-generic-variadic19.C: Likewise.
	* g++.dg/cpp2a/lambda-generic10.C: Likewise.
	* g++.dg/cpp0x/lambda/lambda-const3.C: Likewise.
	* g++.dg/cpp0x/variadic164.C: Likewise.
	* g++.dg/cpp0x/variadic17.C: Likewise.
	* g++.dg/cpp0x/udlit-args-neg.C: Likewise.
	* g++.dg/cpp0x/variadic28.C: Likewise.
	* g++.dg/cpp0x/gen-attrs-33.C: Likewise.
	* g++.dg/cpp23/explicit-obj-diagnostics3.C: Likewise.
	* g++.old-deja/g++.law/operators15.C: Likewise.
	* g++.old-deja/g++.mike/p811.C: Likewise.
	* g++.old-deja/g++.mike/p12306.C (printf): Add , before ... .
	* g++.dg/analyzer/fd-bind-pr107783.C (bind): Likewise.
	* g++.dg/cpp0x/vt-65790.C (printf): Likewise.
libstdc++-v3/
	* include/std/functional (_Bind_check_arity): Add , before ... .
	* include/bits/refwrap.h (_Mem_fn_traits, _Weak_result_type_impl):
	Likewise.
	* include/tr1/type_traits (is_function): Likewise.

1 files changed, 19 insertions, 2 deletions

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 18a5754..fa2532f 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -4069,6 +4069,22 @@ string operator "" _i18n(const char*, std::size_t); // deprecated
 string operator ""_i18n(const char*, std::size_t); // preferred
 @end smallexample
 
+@opindex Wdeprecated-variadic-comma-omission
+@opindex Wno-deprecated-variadic-comma-omission
+@item -Wdeprecated-variadic-comma-omission @r{(C++ and Objective-C++ only)}
+Warn that omitting a comma before the varargs @code{...} at the end of
+a function parameter list is deprecated.  This warning is enabled by
+default in C++26, or with explicit @option{-Wdeprecated}.
+
+@smallexample
+void f1(int...); // deprecated
+void f1(int, ...); // preferred
+template <typename ...T>
+void f2(T...); // ok
+template <typename ...T>
+void f3(T......); // deprecated
+@end smallexample
+
 @opindex Welaborated-enum-base
 @opindex Wno-elaborated-enum-base
 @item -Wno-elaborated-enum-base
@@ -10348,8 +10364,9 @@ In C++, explicitly specifying @option{-Wdeprecated} also enables
 warnings about some features that are deprecated in later language
 standards, specifically @option{-Wcomma-subscript},
 @option{-Wvolatile}, @option{-Wdeprecated-enum-float-conversion},
-@option{-Wdeprecated-enum-enum-conversion}, and
-@option{-Wdeprecated-literal-operator}.
+@option{-Wdeprecated-enum-enum-conversion},
+@option{-Wdeprecated-literal-operator}, and
+@option{-Wdeprecated-variadic-comma-omission}.
 
 @opindex Wno-deprecated-declarations
 @opindex Wdeprecated-declarations