Add strftime format checking support for C2x %OB and %Ob (bug 82752)

Index: gcc/c-family/c-format.c
===================================================================
--- gcc/c-family/c-format.c	(revision 276588)
+++ gcc/c-family/c-format.c	(working copy)
@@ -404,9 +404,11 @@  decode_format_attr (const_tree fntype, tree atname
 /* The C standard version we are checking formats against when pedantic.  */
 #define C_STD_VER		((int) (c_dialect_cxx ()		   \
 				 ? CPLUSPLUS_STD_VER			   \
-				 : (flag_isoc99				   \
-				    ? STD_C99				   \
-				    : (flag_isoc94 ? STD_C94 : STD_C89))))
+				 : (flag_isoc2x				   \
+				    ? STD_C2X				   \
+				    : (flag_isoc99			   \
+				       ? STD_C99			   \
+				       : (flag_isoc94 ? STD_C94 : STD_C89)))))
 /* The name to give to the standard version we are warning about when
    pedantic.  FEATURE_VER is the version in which the feature warned out
    appeared, which is higher than C_STD_VER.  */
@@ -415,7 +417,9 @@  decode_format_attr (const_tree fntype, tree atname
 				    : "ISO C++11")		\
 				 : ((FEATURE_VER) == STD_EXT	\
 				    ? "ISO C"			\
-				    : "ISO C90"))
+				    : ((FEATURE_VER) == STD_C2X	\
+				       ? "ISO C17"		\
+				       : "ISO C90")))
 /* Adjust a C standard version, which may be STD_C9L, to account for
    -Wno-long-long.  Returns other standard versions unchanged.  */
 #define ADJ_STD(VER)		((int) ((VER) == STD_C9L		      \
@@ -653,6 +657,7 @@  static const format_flag_spec strftime_flag_specs[
   { 'E', 0,   0, 0, N_("'E' modifier"), N_("the 'E' strftime modifier"),      STD_C99 },
   { 'O', 0,   0, 0, N_("'O' modifier"), N_("the 'O' strftime modifier"),      STD_C99 },
   { 'O', 'o', 0, 0, NULL,               N_("the 'O' modifier"),               STD_EXT },
+  { 'O', 'p', 0, 0, NULL,               N_("the 'O' modifier"),               STD_C2X },
   { 0, 0, 0, 0, NULL, NULL, STD_C89 }
 };
 
@@ -887,7 +892,8 @@  static const format_char_info scan_char_table[] =
 static const format_char_info time_char_table[] =
 {
   /* C89 conversion specifiers.  */
-  { "ABZab",		0, STD_C89, NOLENGTHS, "^#",     "",   NULL },
+  { "AZa",		0, STD_C89, NOLENGTHS, "^#",     "",   NULL },
+  { "Bb",		0, STD_C89, NOLENGTHS, "O^#",    "p",  NULL },
   { "cx",		0, STD_C89, NOLENGTHS, "E",      "3",  NULL },
   { "HIMSUWdmw",	0, STD_C89, NOLENGTHS, "-_0Ow",  "",   NULL },
   { "j",		0, STD_C89, NOLENGTHS, "-_0Ow",  "o",  NULL },
Index: gcc/c-family/c-format.h
===================================================================
--- gcc/c-family/c-format.h	(revision 276588)
+++ gcc/c-family/c-format.h	(working copy)
@@ -48,6 +48,7 @@  enum format_std_version
   STD_C94,
   STD_C9L, /* C99, but treat as C89 if -Wno-long-long.  */
   STD_C99,
+  STD_C2X,
   STD_EXT
 };
 
@@ -149,6 +150,7 @@  struct format_char_info
      two digit year formats, "3" for strftime formats giving two digit
      years in some locales, "4" for "2" which becomes "3" with an "E" modifier,
      "o" if use of strftime "O" is a GNU extension beyond C99,
+     "p" if use of strftime "O" is a C2x feature,
      "W" if the argument is a pointer which is dereferenced and written into,
      "R" if the argument is a pointer which is dereferenced and read from,
      "i" for printf integer formats where the '0' flag is ignored with
Index: gcc/testsuite/gcc.dg/format/c2x-strftime-1.c
===================================================================
--- gcc/testsuite/gcc.dg/format/c2x-strftime-1.c	(nonexistent)
+++ gcc/testsuite/gcc.dg/format/c2x-strftime-1.c	(working copy)
@@ -0,0 +1,15 @@ 
+/* Test for strftime formats.  Formats using C2x features.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c2x -pedantic -Wformat" } */
+
+#include "format.h"
+
+void
+foo (char *s, size_t m, const struct tm *tp)
+{
+  strftime (s, m, "%Ob", tp);
+  strftime (s, m, "%OB", tp);
+  /* It's not clear that %h equivalence to %b means %Oh is equivalent
+     to %Ob; here we expect %Oh to be diagnosed.  */
+  strftime (s, m, "%Oh", tp); /* { dg-warning "flag|modifier" "bad %Oh" } */
+}