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, 95 insertions, 28 deletions

diff --git a/gcc/c/c-errors.cc b/gcc/c/c-errors.cc
index 7aa5e0d..f36e7f9 100644
--- a/gcc/c/c-errors.cc
+++ b/gcc/c/c-errors.cc
@@ -25,6 +25,45 @@ along with GCC; see the file COPYING3.  If not see
 #include "c-tree.h"
 #include "opts.h"
 
+/* Issue an ISO C23 pedantic warning MSGID if -pedantic outside C2Y mode,
+   otherwise issue warning MSGID if -Wc23-c2y-compat is specified.
+   This function is supposed to be used for matters that are allowed in
+   ISO C2Y but not supported in ISO C23, thus we explicitly don't pedwarn
+   when C2Y is specified.  */
+
+bool
+pedwarn_c23 (location_t location, int opt, const char *gmsgid, ...)
+{
+  diagnostic_info diagnostic;
+  va_list ap;
+  bool warned = false;
+  rich_location richloc (line_table, location);
+
+  va_start (ap, gmsgid);
+  /* If desired, issue the C23/C2Y compat warning, which is more specific
+     than -pedantic.  */
+  if (warn_c23_c2y_compat > 0)
+    {
+      diagnostic_set_info (&diagnostic, gmsgid, &ap, &richloc,
+			   (pedantic && !flag_isoc2y)
+			   ? DK_PEDWARN : DK_WARNING);
+      diagnostic.option_index = OPT_Wc23_c2y_compat;
+      warned = diagnostic_report_diagnostic (global_dc, &diagnostic);
+    }
+  /* -Wno-c23-c2y-compat suppresses even the pedwarns.  */
+  else if (warn_c23_c2y_compat == 0)
+    ;
+  /* For -pedantic outside C2Y, issue a pedwarn.  */
+  else if (pedantic && !flag_isoc2y)
+    {
+      diagnostic_set_info (&diagnostic, gmsgid, &ap, &richloc, DK_PEDWARN);
+      diagnostic.option_index = opt;
+      warned = diagnostic_report_diagnostic (global_dc, &diagnostic);
+    }
+  va_end (ap);
+  return warned;
+}
+
 /* Issue an ISO C11 pedantic warning MSGID if -pedantic outside C23 mode,
    otherwise issue warning MSGID if -Wc11-c23-compat is specified.
    This function is supposed to be used for matters that are allowed in
diff --git a/gcc/c/c-parser.cc b/gcc/c/c-parser.cc
index 0c50181..e83e9c6 100644
--- a/gcc/c/c-parser.cc
+++ b/gcc/c/c-parser.cc
@@ -1534,6 +1534,9 @@ disable_extension_diagnostics (void)
 	     /* Similarly for warn_c11_c23_compat.  */
 	     | ((warn_c11_c23_compat == 1) << 11)
 	     | ((warn_c11_c23_compat == -1) << 12)
+	     /* Similarly for warn_c23_c2y_compat.  */
+	     | ((warn_c23_c2y_compat == 1) << 13)
+	     | ((warn_c23_c2y_compat == -1) << 14)
 	     );
   cpp_opts->cpp_pedantic = pedantic = 0;
   warn_pointer_arith = 0;
@@ -1545,6 +1548,7 @@ disable_extension_diagnostics (void)
   warn_c90_c99_compat = 0;
   warn_c99_c11_compat = 0;
   warn_c11_c23_compat = 0;
+  warn_c23_c2y_compat = 0;
   return ret;
 }
 
@@ -1565,6 +1569,7 @@ restore_extension_diagnostics (int flags)
   warn_c90_c99_compat = (flags >> 7) & 1 ? 1 : ((flags >> 8) & 1 ? -1 : 0);
   warn_c99_c11_compat = (flags >> 9) & 1 ? 1 : ((flags >> 10) & 1 ? -1 : 0);
   warn_c11_c23_compat = (flags >> 11) & 1 ? 1 : ((flags >> 12) & 1 ? -1 : 0);
+  warn_c23_c2y_compat = (flags >> 13) & 1 ? 1 : ((flags >> 14) & 1 ? -1 : 0);
 }
 
 /* Helper data structure for parsing #pragma acc routine.  */
@@ -10273,8 +10278,14 @@ struct c_generic_association
 /* Parse a generic-selection.  (C11 6.5.1.1).
    
    generic-selection:
-     _Generic ( assignment-expression , generic-assoc-list )
-     
+     _Generic ( generic-controlling-operand , generic-assoc-list )
+
+  generic-controlling-operand:
+    assignment-expression
+    type-name
+
+  (The use of a type-name is new in C2Y.)
+
    generic-assoc-list:
      generic-association
      generic-assoc-list , generic-association
@@ -10314,30 +10325,43 @@ c_parser_generic_selection (c_parser *parser)
   if (!parens.require_open (parser))
     return error_expr;
 
-  c_inhibit_evaluation_warnings++;
   selector_loc = c_parser_peek_token (parser)->location;
-  selector = c_parser_expr_no_commas (parser, NULL);
-  selector = default_function_array_conversion (selector_loc, selector);
-  c_inhibit_evaluation_warnings--;
-
-  if (selector.value == error_mark_node)
+  if (c_parser_next_tokens_start_typename (parser, cla_prefer_id))
     {
-      c_parser_skip_until_found (parser, CPP_CLOSE_PAREN, NULL);
-      return selector;
-    }
-  mark_exp_read (selector.value);
-  selector_type = TREE_TYPE (selector.value);
-  /* In ISO C terms, rvalues (including the controlling expression of
-     _Generic) do not have qualified types.  */
-  if (TREE_CODE (selector_type) != ARRAY_TYPE)
-    selector_type = TYPE_MAIN_VARIANT (selector_type);
-  /* In ISO C terms, _Noreturn is not part of the type of expressions
-     such as &abort, but in GCC it is represented internally as a type
-     qualifier.  */
-  if (FUNCTION_POINTER_TYPE_P (selector_type)
-      && TYPE_QUALS (TREE_TYPE (selector_type)) != TYPE_UNQUALIFIED)
-    selector_type
-      = build_pointer_type (TYPE_MAIN_VARIANT (TREE_TYPE (selector_type)));
+      c_inhibit_evaluation_warnings++;
+      pedwarn_c23 (selector_loc, OPT_Wpedantic,
+		   "ISO C does not support use of type name as %<_Generic%> "
+		   "controlling operand before C2Y");
+      struct c_type_name *type = c_parser_type_name (parser);
+      selector_type = groktypename (type, NULL, NULL);
+      c_inhibit_evaluation_warnings--;
+    }
+  else
+    {
+      c_inhibit_evaluation_warnings++;
+      selector = c_parser_expr_no_commas (parser, NULL);
+      selector = default_function_array_conversion (selector_loc, selector);
+      c_inhibit_evaluation_warnings--;
+
+      if (selector.value == error_mark_node)
+	{
+	  c_parser_skip_until_found (parser, CPP_CLOSE_PAREN, NULL);
+	  return selector;
+	}
+      mark_exp_read (selector.value);
+      selector_type = TREE_TYPE (selector.value);
+      /* In ISO C terms, rvalues (including the controlling expression
+	 of _Generic) do not have qualified types.  */
+      if (TREE_CODE (selector_type) != ARRAY_TYPE)
+	selector_type = TYPE_MAIN_VARIANT (selector_type);
+      /* In ISO C terms, _Noreturn is not part of the type of expressions
+	 such as &abort, but in GCC it is represented internally as a type
+	 qualifier.  */
+      if (FUNCTION_POINTER_TYPE_P (selector_type)
+	  && TYPE_QUALS (TREE_TYPE (selector_type)) != TYPE_UNQUALIFIED)
+	selector_type
+	  = build_pointer_type (TYPE_MAIN_VARIANT (TREE_TYPE (selector_type)));
+    }
 
   if (!c_parser_require (parser, CPP_COMMA, "expected %<,%>"))
     {
@@ -10376,11 +10400,13 @@ c_parser_generic_selection (c_parser *parser)
 	    }
 
 	  if (TREE_CODE (assoc.type) == FUNCTION_TYPE)
-	    error_at (assoc.type_location,
-		      "%<_Generic%> association has function type");
+	    pedwarn_c23 (assoc.type_location, OPT_Wpedantic,
+			 "ISO C does not support %<_Generic%> association with "
+			 "function type before C2Y");
 	  else if (!COMPLETE_TYPE_P (assoc.type))
-	    error_at (assoc.type_location,
-		      "%<_Generic%> association has incomplete type");
+	    pedwarn_c23 (assoc.type_location, OPT_Wpedantic,
+			 "ISO C does not support %<_Generic%> association with "
+			 "incomplete type before C2Y");
 
 	  if (c_type_variably_modified_p (assoc.type))
 	    error_at (assoc.type_location,
diff --git a/gcc/c/c-tree.h b/gcc/c/c-tree.h
index 56a33b8..15da875 100644
--- a/gcc/c/c-tree.h
+++ b/gcc/c/c-tree.h
@@ -905,6 +905,8 @@ extern bool pedwarn_c99 (location_t, int opt, const char *, ...)
     ATTRIBUTE_GCC_DIAG(3,4);
 extern bool pedwarn_c11 (location_t, int opt, const char *, ...)
     ATTRIBUTE_GCC_DIAG(3,4);
+extern bool pedwarn_c23 (location_t, int opt, const char *, ...)
+    ATTRIBUTE_GCC_DIAG(3,4);
 
 extern void
 set_c_expr_source_range (c_expr *expr,