c: Add -std=c2y, -std=gnu2y, -Wc23-c2y-compat, C2Y _Generic with type operand

The first new C2Y feature, _Generic where the controlling operand is a
type name rather than an expression (as defined in N3260), was voted
into C2Y today.  (In particular, this form of _Generic allows
distinguishing qualified and unqualified versions of a type.)  This
feature also includes allowing the generic associations to specify
incomplete and function types.

Add this feature to GCC, along with the -std=c2y, -std=gnu2y and
-Wc23-c2y-compat options to control when and how it is diagnosed.  As
usual, the feature is allowed by default in older standards modes,
subject to diagnosis with -pedantic, -pedantic-errors or
-Wc23-c2y-compat.

Bootstrapped with no regressions on x86_64-pc-linux-gnu.

gcc/
	* doc/cpp.texi (__STDC_VERSION__): Document C2Y handling.
	* doc/invoke.texi (-Wc23-c2y-compat, -std=c2y, -std=gnu2y):
	Document options.
	(-std=gnu23): Update documentation.
	* doc/standards.texi (C Language): Document C2Y.  Update C23
	description.
	* config/rl78/rl78.cc (rl78_option_override): Handle "GNU C2Y"
	language name.
	* dwarf2out.cc (highest_c_language, gen_compile_unit_die):
	Likewise.

gcc/c-family/
	* c-common.cc (flag_isoc2y): New.
	(flag_isoc99, flag_isoc11, flag_isoc23): Update comments.
	* c-common.h (flag_isoc2y): New.
	(clk_c, flag_isoc23): Update comments.
	* c-opts.cc (set_std_c2y): New.
	(c_common_handle_option): Handle OPT_std_c2y and OPT_std_gnu2y.
	(set_std_c89, set_std_c99, set_std_c11, set_std_c17, set_std_c23):
	Set flag_isoc2y.
	(set_std_c23): Update comment.
	* c.opt (Wc23-c2y-compat, std=c2y, std=gnu2y): New.
	* c.opt.urls: Regenerate.

gcc/c/
	* c-errors.cc (pedwarn_c23): New.
	* c-parser.cc (disable_extension_diagnostics)
	(restore_extension_diagnostics): Save and restore
	warn_c23_c2y_compat.
	(c_parser_generic_selection): Handle type name as controlling
	operand.  Allow incomplete and function types subject to
	pedwarn_c23 calls.
	* c-tree.h (pedwarn_c23): New.

gcc/testsuite/
	* gcc.dg/c23-generic-1.c, gcc.dg/c23-generic-2.c,
	gcc.dg/c23-generic-3.c, gcc.dg/c23-generic-4.c,
	gcc.dg/c2y-generic-1.c, gcc.dg/c2y-generic-2.c,
	gcc.dg/c2y-generic-3.c, gcc.dg/gnu2y-generic-1.c: New tests.
	* gcc.dg/c23-tag-6.c: Use -pedantic-errors.

libcpp/
	* include/cpplib.h (CLK_GNUC2Y, CLK_STDC2Y): New.
	* init.cc (lang_defaults): Add GNUC2Y and STDC2Y entries.
	(cpp_init_builtins): Define __STDC_VERSION__ to 202500L for GNUC2Y
	and STDC2Y.

3 files changed, 32 insertions, 3 deletions

diff --git a/gcc/doc/cpp.texi b/gcc/doc/cpp.texi
index 3f38ca3..a99c8df 100644
--- a/gcc/doc/cpp.texi
+++ b/gcc/doc/cpp.texi
@@ -1887,7 +1887,9 @@ signifies the 2011 revision of the C standard; the value
 @code{201710L} signifies the 2017 revision of the C standard (which is
 otherwise identical to the 2011 version apart from correction of
 defects).  The value @code{202311L} is used for the experimental
-@option{-std=c23} and @option{-std=gnu23} modes.
+@option{-std=c23} and @option{-std=gnu23} modes.  An unspecified value
+larger than @code{202311L} is used for the experimental
+@option{-std=c2y} and @option{-std=gnu2y} modes.
 
 This macro is not defined if the @option{-traditional-cpp} option is
 used, nor when compiling C++ or Objective-C@.
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index a0b3756..26e6a34 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -340,7 +340,7 @@ Objective-C and Objective-C++ Dialects}.
 -Wbool-compare  -Wbool-operation
 -Wno-builtin-declaration-mismatch
 -Wno-builtin-macro-redefined  -Wc90-c99-compat  -Wc99-c11-compat
--Wc11-c23-compat
+-Wc11-c23-compat  -Wc23-c2y-compat
 -Wc++-compat  -Wc++11-compat  -Wc++14-compat  -Wc++17-compat
 -Wc++20-compat
 -Wno-c++11-extensions  -Wno-c++14-extensions -Wno-c++17-extensions
@@ -2472,6 +2472,10 @@ ISO C23, the 2023 revision of the ISO C standard (expected to be
 published in 2024).  The support for this version is experimental and
 incomplete.  The name @samp{c2x} is deprecated.
 
+@item c2y
+The next version of the ISO C standard, still under development.  The
+support for this version is experimental and incomplete.
+
 @item gnu90
 @itemx gnu89
 GNU dialect of ISO C90 (including some C99 features).
@@ -2491,6 +2495,10 @@ GNU dialect of ISO C17.  This is the default for C code.
 
 @item gnu23
 @itemx gnu2x
+GNU dialect of ISO C23.  The support for this version is experimental
+and incomplete.  The name @samp{gnu2x} is deprecated.
+
+@item gnu2y
 The next version of the ISO C standard, still under development, plus
 GNU extensions.  The support for this version is experimental and
 incomplete.  The name @samp{gnu2x} is deprecated.
@@ -9326,6 +9334,19 @@ deprecated.
 When not compiling in C23 mode, these warnings are upgraded to errors
 by @option{-pedantic-errors}.
 
+@opindex Wc23-c2y-compat
+@opindex Wno-c23-c2y-compat
+@item -Wc23-c2y-compat @r{(C and Objective-C only)}
+@itemx -Wc23-c2y-compat @r{(C and Objective-C only)}
+Warn about features not present in ISO C23, but present in ISO C2Y.
+For instance, warn about @code{_Generic} selecting with a type name
+instead of an expression.  This option is independent of the standards
+mode.  Warnings are disabled in the expression that follows
+@code{__extension__}.
+
+When not compiling in C2Y mode, these warnings are upgraded to errors
+by @option{-pedantic-errors}.
+
 @opindex Wc++-compat
 @opindex Wno-c++-compat
 @item -Wc++-compat @r{(C and Objective-C only)}
diff --git a/gcc/doc/standards.texi b/gcc/doc/standards.texi
index 586835b..484fbb1 100644
--- a/gcc/doc/standards.texi
+++ b/gcc/doc/standards.texi
@@ -42,6 +42,8 @@ with some exceptions, and possibly with some extensions.
 @cindex C23
 @cindex ISO C2X
 @cindex C2X
+@cindex ISO C2Y
+@cindex C2Y
 @cindex Technical Corrigenda
 @cindex TC1
 @cindex Technical Corrigendum 1
@@ -113,12 +115,16 @@ known as @dfn{C17} and is supported with @option{-std=c17} or
 @option{-std=c11}, and the only difference between the options is the
 value of @code{__STDC_VERSION__}.
 
-A further version of the C standard, known as @dfn{C23}, is under
+A fifth version of the C standard, known as @dfn{C23}, is under
 development and expected to be published in 2024 as ISO/IEC 9899:2024.
 (While in development, drafts of this standard version were referred
 to as @dfn{C2X}.)  Experimental and incomplete support for this is
 enabled with @option{-std=c23} or @option{-std=iso9899:2024}.
 
+A further version of the C standard, known as @dfn{C2Y}, is under
+development; experimental and incomplete support for this is enabled
+with @option{-std=c2y}.
+
 By default, GCC provides some extensions to the C language that, on
 rare occasions conflict with the C standard.  @xref{C
 Extensions,,Extensions to the C Language Family}.