2018-06-25  Nathan Sidwell  <nathan@acm.org>

	gcc/c-family/
	* c-lex.c (fe_file_change): Check SYSTEM_IMPLICIT_EXTERN_C not
	NO_IMPLICIT_EXTERN_C.

	gcc/cp/
	* cp/decl.c (decls_match): Check SYSTEM_IMPLICIT_EXTERN_C not
	NO_IMPLICIT_EXTERN_C.
	* cp/parser.c (cp_parser_parameter_declaration_clause): Likewise.

	gcc/
	Replace NO_IMPLICIT_EXTERN_C with SYSTEM_IMPLICIT_EXTERN_C.
	* doc/cpp.texi: Update comment.
	* doc/tm.texi: Rebuilt.
	* doc/tm.texi.in (NO_IMPLICIT_EXTERN_C): Replace with ...
	(SYSTEM_IMPLICIT_EXTERN_C): ... this, opposite sense.
	* doc/extend.texi (Backwards Compatibility): Clarify it is system
	headers affected by extern "C".
	* system.h: Poison NO_IMPLICIT_EXTERN_C.
	* config/alpha/alpha.h, config/arm/uclinux-elf.h,
	config/bfin/elf.h, config/cris/cris.h, config/darwin.h,
	config/dragonfly.h, config/freebsd.h, config/gnu-user.h,
	config/i386/cygming.h, config/i386/djgpp.h, config/i386/nto.h,
	config/ia64/hpux.h, config/lm32/lm32.h, config/lm32/uclinux-elf.h,
	config/lynx.h, config/mips/elf.h, config/mmix/mmix.h,
	config/netbsd.h, config/pa/pa-hpux.h, config/powerpcspe/sysv4.h,
	config/riscv/elf.h, config/rs6000/sysv4.h, config/rtems.h,
	config/s390/tpf.h, config/sh/newlib.h, config/sol2.h,
	config/sparc/openbsd64.h, config/sparc/sp-elf.h,
	config/sparc/sp64-elf.h, config/spu/spu.h,
	config/stormy16/stormy16.h, config/v850/v850.h,
	config/visium/visium.h, config/vx-common.h, config/xtensa/elf.h: Don't
	define NO_IMPLICIT_EXTERN_C.
	* config/rs6000/aix.h: Set SYSTEM_IMPLICIT_EXTERN_C.

Index: c-family/c-lex.c
===================================================================
--- c-family/c-lex.c	(revision 262020)
+++ c-family/c-lex.c	(working copy)
@@ -206,7 +206,7 @@ fe_file_change (const line_map_ordinary
 
 	  input_location = new_map->start_location;
 	  (*debug_hooks->start_source_file) (line, LINEMAP_FILE (new_map));
-#ifndef NO_IMPLICIT_EXTERN_C
+#ifdef SYSTEM_IMPLICIT_EXTERN_C
 	  if (c_header_level)
 	    ++c_header_level;
 	  else if (LINEMAP_SYSP (new_map) == 2)
@@ -219,7 +219,7 @@ fe_file_change (const line_map_ordinary
     }
   else if (new_map->reason == LC_LEAVE)
     {
-#ifndef NO_IMPLICIT_EXTERN_C
+#ifdef SYSTEM_IMPLICIT_EXTERN_C
       if (c_header_level && --c_header_level == 0)
 	{
 	  if (LINEMAP_SYSP (new_map) == 2)
Index: config/alpha/alpha.h
===================================================================
--- config/alpha/alpha.h	(revision 262020)
+++ config/alpha/alpha.h	(working copy)
@@ -922,7 +922,4 @@ extern long alpha_auto_offset;
 /* By default, turn on GDB extensions.  */
 #define DEFAULT_GDB_EXTENSIONS 1
 
-/* The system headers under Alpha systems are generally C++-aware.  */
-#define NO_IMPLICIT_EXTERN_C
-
 #define TARGET_SUPPORTS_WIDE_INT 1
Index: config/arm/uclinux-elf.h
===================================================================
--- config/arm/uclinux-elf.h	(revision 262020)
+++ config/arm/uclinux-elf.h	(working copy)
@@ -48,9 +48,6 @@
     }						\
   while (false)
 
-/* Do not assume anything about header files.  */
-#define NO_IMPLICIT_EXTERN_C
-
 /* The GNU C++ standard library requires that these macros be defined.  */
 #undef CPLUSPLUS_CPP_SPEC
 #define CPLUSPLUS_CPP_SPEC "-D_GNU_SOURCE %(cpp)"
Index: config/bfin/elf.h
===================================================================
--- config/bfin/elf.h	(revision 262020)
+++ config/bfin/elf.h	(working copy)
@@ -70,5 +70,3 @@ asm (TEXT_SECTION_ASM_OP);
 #undef SUBTARGET_DRIVER_SELF_SPECS
 #define SUBTARGET_DRIVER_SELF_SPECS \
      "%{mfdpic:-msim} %{mid-shared-library:-msim}"
-
-#define NO_IMPLICIT_EXTERN_C
Index: config/cris/cris.h
===================================================================
--- config/cris/cris.h	(revision 262020)
+++ config/cris/cris.h	(working copy)
@@ -1030,8 +1030,6 @@ enum cris_symbol_type
 
 #define FUNCTION_MODE QImode
 
-#define NO_IMPLICIT_EXTERN_C
-
 /*
  * Local variables:
  * eval: (c-set-style "gnu")
Index: config/darwin.h
===================================================================
--- config/darwin.h	(revision 262020)
+++ config/darwin.h	(working copy)
@@ -43,9 +43,6 @@ see the files COPYING3 and COPYING.RUNTI
 #define DARWIN_X86 0
 #define DARWIN_PPC 0
 
-/* Don't assume anything about the header files.  */
-#define NO_IMPLICIT_EXTERN_C
-
 /* Suppress g++ attempt to link in the math library automatically. */
 #define MATH_LIBRARY ""
 
Index: config/dragonfly.h
===================================================================
--- config/dragonfly.h	(revision 262020)
+++ config/dragonfly.h	(working copy)
@@ -104,10 +104,6 @@ see the files COPYING3 and COPYING.RUNTI
 #undef  OBJECT_FORMAT_ELF
 #define OBJECT_FORMAT_ELF
 
-/* Don't assume anything about the header files.  */
-#undef  NO_IMPLICIT_EXTERN_C
-#define NO_IMPLICIT_EXTERN_C	1
-
 /* Follow DragonFly's standard headers (<machine/stdint.h>, etc...).  */
 
 #undef  WCHAR_TYPE
Index: config/freebsd.h
===================================================================
--- config/freebsd.h	(revision 262020)
+++ config/freebsd.h	(working copy)
@@ -68,10 +68,6 @@ along with GCC; see the file COPYING3.
 #undef  OBJECT_FORMAT_ELF
 #define OBJECT_FORMAT_ELF
 
-/* Don't assume anything about the header files.  */
-#undef  NO_IMPLICIT_EXTERN_C
-#define NO_IMPLICIT_EXTERN_C	1
-
 /* Follow FreeBSD's standard headers (<sys/_types.h> etc...).  */
 
 #undef  WCHAR_TYPE
Index: config/gnu-user.h
===================================================================
--- config/gnu-user.h	(revision 262020)
+++ config/gnu-user.h	(working copy)
@@ -26,9 +26,6 @@ a copy of the GCC Runtime Library Except
 see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
 <http://www.gnu.org/licenses/>.  */
 
-/* Don't assume anything about the header files.  */
-#define NO_IMPLICIT_EXTERN_C
-
 #undef ASM_APP_ON
 #define ASM_APP_ON "#APP\n"
 
Index: config/i386/cygming.h
===================================================================
--- config/i386/cygming.h	(revision 262020)
+++ config/i386/cygming.h	(working copy)
@@ -378,9 +378,6 @@ do {						\
 #endif
 #endif
 
-/* Don't assume anything about the header files.  */
-#define NO_IMPLICIT_EXTERN_C
-
 #undef PROFILE_HOOK
 #define PROFILE_HOOK(LABEL)						\
   if (MAIN_NAME_P (DECL_NAME (current_function_decl)))			\
Index: config/i386/djgpp.h
===================================================================
--- config/i386/djgpp.h	(revision 262020)
+++ config/i386/djgpp.h	(working copy)
@@ -23,9 +23,6 @@ along with GCC; see the file COPYING3.
 #undef PREFERRED_DEBUGGING_TYPE
 #define PREFERRED_DEBUGGING_TYPE DWARF2_DEBUG
 
-/* Don't assume anything about the header files.  */
-#define NO_IMPLICIT_EXTERN_C
-
 #undef BSS_SECTION_ASM_OP
 #define BSS_SECTION_ASM_OP "\t.section\t.bss"
 
Index: config/i386/nto.h
===================================================================
--- config/i386/nto.h	(revision 262020)
+++ config/i386/nto.h	(working copy)
@@ -98,8 +98,6 @@ crti.o%s \
 #undef WCHAR_TYPE_SIZE
 #define WCHAR_TYPE_SIZE BITS_PER_WORD
 
-#define NO_IMPLICIT_EXTERN_C 1
-
 #define TARGET_POSIX_IO
 
 #undef DBX_REGISTER_NUMBER
Index: config/ia64/hpux.h
===================================================================
--- config/ia64/hpux.h	(revision 262020)
+++ config/ia64/hpux.h	(working copy)
@@ -185,9 +185,6 @@ do {								\
 
 #define FLOAT_LIB_COMPARE_RETURNS_BOOL(MODE, COMPARISON) ((MODE) == TFmode)
 
-/* HP-UX headers are C++-compatible.  */
-#define NO_IMPLICIT_EXTERN_C
-
 /* HP-UX uses PROFILE_HOOK instead of FUNCTION_PROFILER but we need a
    FUNCTION_PROFILER defined because its use is not ifdefed.  When using
    PROFILE_HOOK, the profile call comes after the prologue.  */
Index: config/lm32/lm32.h
===================================================================
--- config/lm32/lm32.h	(revision 262020)
+++ config/lm32/lm32.h	(working copy)
@@ -516,8 +516,4 @@ do {
 
 #define FUNCTION_MODE SImode
 
-#ifndef NO_IMPLICIT_EXTERN_C
-#define NO_IMPLICIT_EXTERN_C
-#endif
-
 #define STORE_FLAG_VALUE 1
Index: config/lm32/uclinux-elf.h
===================================================================
--- config/lm32/uclinux-elf.h	(revision 262020)
+++ config/lm32/uclinux-elf.h	(working copy)
@@ -21,10 +21,6 @@
 /* elfos.h should have already been included.  Now just override
    any conflicting definitions and add any extras.  */
 
-/* Do not assume anything about header files.  */
-#undef NO_IMPLICIT_EXTERN_C
-#define NO_IMPLICIT_EXTERN_C
-
 /* The GNU C++ standard library requires that these macros be defined.  */
 #undef CPLUSPLUS_CPP_SPEC
 #define CPLUSPLUS_CPP_SPEC "-D_GNU_SOURCE %(cpp)"
Index: config/lynx.h
===================================================================
--- config/lynx.h	(revision 262020)
+++ config/lynx.h	(working copy)
@@ -165,12 +165,6 @@ along with GCC; see the file COPYING3.
 # define PREFERRED_DEBUGGING_TYPE DBX_DEBUG
 #endif
 
-/* We have C++ support in our system headers.  */
-
-#ifndef NO_IMPLICIT_EXTERN_C
-# define NO_IMPLICIT_EXTERN_C
-#endif
-
 #ifndef TARGET_POSIX_IO
 # define TARGET_POSIX_IO
 #endif
Index: config/mips/elf.h
===================================================================
--- config/mips/elf.h	(revision 262020)
+++ config/mips/elf.h	(working copy)
@@ -46,5 +46,3 @@ along with GCC; see the file COPYING3.
 
 #undef  ENDFILE_SPEC
 #define ENDFILE_SPEC "crtend%O%s crtn%O%s"
-
-#define NO_IMPLICIT_EXTERN_C 1
Index: config/mmix/mmix.h
===================================================================
--- config/mmix/mmix.h	(revision 262020)
+++ config/mmix/mmix.h	(working copy)
@@ -789,8 +789,6 @@ typedef struct { int regs; int lib; } CU
 
 #define FUNCTION_MODE QImode
 
-#define NO_IMPLICIT_EXTERN_C
-
 /* mmix-knuth-mmixware target has no support of C99 runtime */
 #undef TARGET_LIBC_HAS_FUNCTION
 #define TARGET_LIBC_HAS_FUNCTION no_c99_libc_has_function
Index: config/netbsd.h
===================================================================
--- config/netbsd.h	(revision 262020)
+++ config/netbsd.h	(working copy)
@@ -149,10 +149,6 @@ along with GCC; see the file COPYING3.
 #undef TARGET_POSIX_IO
 #define TARGET_POSIX_IO
 
-/* Don't assume anything about the header files.  */
-#undef  NO_IMPLICIT_EXTERN_C
-#define NO_IMPLICIT_EXTERN_C    1
-
 /* Define some types that are the same on all NetBSD platforms,
    making them agree with <machine/ansi.h>.  */
 
Index: config/pa/pa-hpux.h
===================================================================
--- config/pa/pa-hpux.h	(revision 262020)
+++ config/pa/pa-hpux.h	(working copy)
@@ -102,10 +102,6 @@ along with GCC; see the file COPYING3.
   "%{mlinker-opt:-O} %{!shared:-u main} %{static:-a archive} %{g*:-a archive} %{shared:-b}"
 #endif
 
-/* hpux8 and later have C++ compatible include files, so do not
-   pretend they are `extern "C"'.  */
-#define NO_IMPLICIT_EXTERN_C
-
 /* hpux11 and earlier don't have fputc_unlocked, so we must inhibit the
    transformation of fputs_unlocked and fprintf_unlocked to fputc_unlocked.  */
 #define DONT_HAVE_FPUTC_UNLOCKED
Index: config/powerpcspe/sysv4.h
===================================================================
--- config/powerpcspe/sysv4.h	(revision 262020)
+++ config/powerpcspe/sysv4.h	(working copy)
@@ -23,10 +23,6 @@
    see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
    <http://www.gnu.org/licenses/>.  */
 
-/* Header files should be C++ aware in general.  */
-#undef  NO_IMPLICIT_EXTERN_C
-#define NO_IMPLICIT_EXTERN_C
-
 /* Yes!  We are ELF.  */
 #define	TARGET_OBJECT_FORMAT OBJECT_ELF
 
Index: config/riscv/elf.h
===================================================================
--- config/riscv/elf.h	(revision 262020)
+++ config/riscv/elf.h	(working copy)
@@ -32,5 +32,3 @@ along with GCC; see the file COPYING3.
 
 #undef  ENDFILE_SPEC
 #define ENDFILE_SPEC "crtend%O%s"
-
-#define NO_IMPLICIT_EXTERN_C 1
Index: config/rs6000/aix.h
===================================================================
--- config/rs6000/aix.h	(revision 262020)
+++ config/rs6000/aix.h	(working copy)
@@ -23,6 +23,9 @@
 #undef  TARGET_AIX
 #define TARGET_AIX 1
 
+/* System headers are not C++-aware.  */
+#define SYSTEM_IMPLICIT_EXTERN_C 1
+
 /* Linux64.h wants to redefine TARGET_AIX based on -m64, but it can't be used
    in the #if conditional in options-default.h, so provide another macro.  */
 #undef  TARGET_AIX_OS
Index: config/rs6000/sysv4.h
===================================================================
--- config/rs6000/sysv4.h	(revision 262020)
+++ config/rs6000/sysv4.h	(working copy)
@@ -23,10 +23,6 @@
    see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
    <http://www.gnu.org/licenses/>.  */
 
-/* Header files should be C++ aware in general.  */
-#undef  NO_IMPLICIT_EXTERN_C
-#define NO_IMPLICIT_EXTERN_C
-
 /* Yes!  We are ELF.  */
 #define	TARGET_OBJECT_FORMAT OBJECT_ELF
 
Index: config/rtems.h
===================================================================
--- config/rtems.h	(revision 262020)
+++ config/rtems.h	(working copy)
@@ -22,10 +22,6 @@
    see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
    <http://www.gnu.org/licenses/>.  */
 
-/* The system headers under RTEMS are C++-aware.  */
-#undef NO_IMPLICIT_EXTERN_C
-#define NO_IMPLICIT_EXTERN_C
-
 /*
  * Dummy start/end specification to let linker work as
  * needed by autoconf scripts using this compiler.
Index: config/s390/tpf.h
===================================================================
--- config/s390/tpf.h	(revision 262020)
+++ config/s390/tpf.h	(working copy)
@@ -30,7 +30,6 @@ along with GCC; see the file COPYING3.
 #define ASM_APP_ON "#APP\n"
 #undef ASM_APP_OFF
 #define ASM_APP_OFF "#NO_APP\n"
-#define NO_IMPLICIT_EXTERN_C
 #define TARGET_POSIX_IO
 
 #undef  SIZE_TYPE
Index: config/sh/newlib.h
===================================================================
--- config/sh/newlib.h	(revision 262020)
+++ config/sh/newlib.h	(working copy)
@@ -23,7 +23,3 @@ along with GCC; see the file COPYING3.
 
 #undef LIB_SPEC
 #define LIB_SPEC "-lc -lgloss"
-
-#undef  NO_IMPLICIT_EXTERN_C
-#define NO_IMPLICIT_EXTERN_C 1
-
Index: config/sol2.h
===================================================================
--- config/sol2.h	(revision 262020)
+++ config/sol2.h	(working copy)
@@ -423,9 +423,6 @@ along with GCC; see the file COPYING3.
    produce the same format.  */
 #define NM_FLAGS "-png"
 
-/* The system headers under Solaris 2 are C++-aware since 2.0.  */
-#define NO_IMPLICIT_EXTERN_C
-
 #define STDC_0_IN_SYSTEM_HEADERS 1
 
 /* Support Solaris-specific format checking for cmn_err.  */
Index: config/sparc/openbsd64.h
===================================================================
--- config/sparc/openbsd64.h	(revision 262020)
+++ config/sparc/openbsd64.h	(working copy)
@@ -41,9 +41,6 @@ along with GCC; see the file COPYING3.
     }						\
   while (0)
 
-/* Inherited from sp64-elf.  */
-#undef NO_IMPLICIT_EXTERN_C
-
 #undef ASM_SPEC
 #define ASM_SPEC "\
 -s %{" FPIE_OR_FPIC_SPEC ":-K PIC} \
Index: config/sparc/sp-elf.h
===================================================================
--- config/sparc/sp-elf.h	(revision 262020)
+++ config/sparc/sp-elf.h	(working copy)
@@ -18,9 +18,6 @@ You should have received a copy of the G
 along with GCC; see the file COPYING3.  If not see
 <http://www.gnu.org/licenses/>.  */
 
-/* Don't assume anything about the header files.  */
-#define NO_IMPLICIT_EXTERN_C
-
 /* It's safe to pass -s always, even if -g is not used.  */
 #undef ASM_SPEC
 #define ASM_SPEC \
Index: config/sparc/sp64-elf.h
===================================================================
--- config/sparc/sp64-elf.h	(revision 262020)
+++ config/sparc/sp64-elf.h	(working copy)
@@ -27,9 +27,6 @@ along with GCC; see the file COPYING3.
 #undef SPARC_DEFAULT_CMODEL
 #define SPARC_DEFAULT_CMODEL CM_EMBMEDANY
 
-/* Don't assume anything about the header files.  */
-#define NO_IMPLICIT_EXTERN_C
-
 #undef ASM_SPEC
 #define ASM_SPEC "\
 -s %{" FPIE_OR_FPIC_SPEC ":-K PIC} \
Index: config/spu/spu.h
===================================================================
--- config/spu/spu.h	(revision 262020)
+++ config/spu/spu.h	(working copy)
@@ -483,8 +483,6 @@ do {									\
 
 #define FUNCTION_MODE QImode
 
-#define NO_IMPLICIT_EXTERN_C 1
-
 
 /* Address spaces.  */
 #define ADDR_SPACE_EA	1
Index: config/stormy16/stormy16.h
===================================================================
--- config/stormy16/stormy16.h	(revision 262020)
+++ config/stormy16/stormy16.h	(working copy)
@@ -476,5 +476,3 @@ enum reg_class
 #define Pmode HImode
 
 #define FUNCTION_MODE HImode
-
-#define NO_IMPLICIT_EXTERN_C
Index: config/v850/v850.h
===================================================================
--- config/v850/v850.h	(revision 262020)
+++ config/v850/v850.h	(working copy)
@@ -851,12 +851,6 @@ extern const char * GHS_current_section_
 
 #define TARGET_ASM_INIT_SECTIONS v850_asm_init_sections
 
-/* Define this so that the cc1plus will not think that system header files
-   need an implicit 'extern "C" { ... }' assumed.  This breaks testing C++
-   in a build directory where the libstdc++ header files are found via a
-   -isystem <path-to-build-dir>.  */
-#define NO_IMPLICIT_EXTERN_C
-
 #define ADJUST_INSN_LENGTH(INSN, LENGTH) \
   ((LENGTH) = v850_adjust_insn_length ((INSN), (LENGTH)))
 
Index: config/visium/visium.h
===================================================================
--- config/visium/visium.h	(revision 262020)
+++ config/visium/visium.h	(working copy)
@@ -1211,14 +1211,6 @@ do									\
    machines this should be `QImode'. */
 #define FUNCTION_MODE SImode
 
-/* `NO_IMPLICIT_EXTERN_C'
-
-   Define this macro if the system header files support C++ as well as
-   C.  This macro inhibits the usual method of using system header
-   files in C++, which is to pretend that the file's contents are
-   enclosed in `extern "C" {...}'. */
-#define NO_IMPLICIT_EXTERN_C
-
 /* Dividing the Output into Sections (Texts, Data, ...)
 
    An object file is divided into sections containing different types
Index: config/vx-common.h
===================================================================
--- config/vx-common.h	(revision 262020)
+++ config/vx-common.h	(working copy)
@@ -18,10 +18,6 @@ You should have received a copy of the G
 along with GCC; see the file COPYING3.  If not see
 <http://www.gnu.org/licenses/>.  */
 
-/* VxWorks headers are C++-aware.  */
-#undef  NO_IMPLICIT_EXTERN_C
-#define NO_IMPLICIT_EXTERN_C
-
 /* Most of these will probably be overridden by subsequent headers.  We
    undefine them here just in case, and define VXWORKS_ versions of each,
    to be used in port-specific vxworks.h.  */
Index: config/xtensa/elf.h
===================================================================
--- config/xtensa/elf.h	(revision 262020)
+++ config/xtensa/elf.h	(working copy)
@@ -20,9 +20,6 @@ along with GCC; see the file COPYING3.
 
 #define TARGET_SECTION_TYPE_FLAGS xtensa_multibss_section_type_flags
 
-/* Don't assume anything about the header files.  */
-#define NO_IMPLICIT_EXTERN_C
-
 #undef ASM_APP_ON
 #define ASM_APP_ON "#APP\n"
 
Index: cp/decl.c
===================================================================
--- cp/decl.c	(revision 262020)
+++ cp/decl.c	(working copy)
@@ -967,7 +967,7 @@ decls_match (tree newdecl, tree olddecl,
 	{
 	  if (!prototype_p (f2) && DECL_EXTERN_C_P (olddecl)
 	      && (DECL_BUILT_IN (olddecl)
-#ifndef NO_IMPLICIT_EXTERN_C
+#ifdef SYSTEM_IMPLICIT_EXTERN_C
 		  || (DECL_IN_SYSTEM_HEADER (newdecl) && !DECL_CLASS_SCOPE_P (newdecl))
 		  || (DECL_IN_SYSTEM_HEADER (olddecl) && !DECL_CLASS_SCOPE_P (olddecl))
 #endif
@@ -977,7 +977,7 @@ decls_match (tree newdecl, tree olddecl,
 	      if (p1 == void_list_node)
 		TREE_TYPE (newdecl) = TREE_TYPE (olddecl);
 	    }
-#ifndef NO_IMPLICIT_EXTERN_C
+#ifdef SYSTEM_IMPLICIT_EXTERN_C
 	  else if (!prototype_p (f1)
 		   && (DECL_EXTERN_C_P (olddecl)
 		       && DECL_IN_SYSTEM_HEADER (olddecl)
Index: cp/parser.c
===================================================================
--- cp/parser.c	(revision 262020)
+++ cp/parser.c	(working copy)
@@ -21349,7 +21349,7 @@ cp_parser_parameter_declaration_clause (
   else if (token->type == CPP_CLOSE_PAREN)
     /* There are no parameters.  */
     {
-#ifndef NO_IMPLICIT_EXTERN_C
+#ifdef SYSTEM_IMPLICIT_EXTERN_C
       if (in_system_header_at (input_location)
 	  && current_class_type == NULL
 	  && current_lang_name == lang_name_c)
Index: doc/cpp.texi
===================================================================
--- doc/cpp.texi	(revision 262020)
+++ doc/cpp.texi	(working copy)
@@ -3735,7 +3735,7 @@ so certain warnings should be suppressed
 @item 4
 This indicates that the following text should be treated as being
 wrapped in an implicit @code{extern "C"} block.
-@c maybe cross reference NO_IMPLICIT_EXTERN_C
+@c maybe cross reference SYSTEM_IMPLICIT_EXTERN_C
 @end table
 
 As an extension, the preprocessor accepts linemarkers in non-assembler
Index: doc/extend.texi
===================================================================
--- doc/extend.texi	(revision 262020)
+++ doc/extend.texi	(working copy)
@@ -23450,7 +23450,7 @@ deprecated.   @xref{Deprecated Features}
 
 @item Implicit C language
 Old C system header files did not contain an @code{extern "C" @{@dots{}@}}
-scope to set the language.  On such systems, all header files are
+scope to set the language.  On such systems, all system header files are
 implicitly scoped inside a C language scope.  Also, an empty prototype
 @code{()} is treated as an unspecified number of arguments, rather
 than no arguments, as C++ demands.
Index: doc/tm.texi
===================================================================
--- doc/tm.texi	(revision 262020)
+++ doc/tm.texi	(working copy)
@@ -11149,11 +11149,10 @@ Define this hook to return the name of a
 Define this hook to add target-specific C++ implicit extern C functions. If this function returns true for the name of a file-scope function, that function implicitly gets extern "C" linkage rather than whatever language linkage the declaration would normally have.  An example of such function is WinMain on Win32 targets.
 @end deftypefn
 
-@defmac NO_IMPLICIT_EXTERN_C
-Define this macro if the system header files support C++ as well as C@.
-This macro inhibits the usual method of using system header files in
-C++, which is to pretend that the file's contents are enclosed in
-@samp{extern "C" @{@dots{}@}}.
+@defmac SYSTEM_IMPLICIT_EXTERN_C
+Define this macro if the system header files do not support C++@.
+This macro handles system header files by pretending that system
+header files are enclosed in @samp{extern "C" @{@dots{}@}}.
 @end defmac
 
 @findex #pragma
Index: doc/tm.texi.in
===================================================================
--- doc/tm.texi.in	(revision 262020)
+++ doc/tm.texi.in	(working copy)
@@ -7657,11 +7657,10 @@ files @code{__STDC__} will always expand
 
 @hook TARGET_CXX_IMPLICIT_EXTERN_C
 
-@defmac NO_IMPLICIT_EXTERN_C
-Define this macro if the system header files support C++ as well as C@.
-This macro inhibits the usual method of using system header files in
-C++, which is to pretend that the file's contents are enclosed in
-@samp{extern "C" @{@dots{}@}}.
+@defmac SYSTEM_IMPLICIT_EXTERN_C
+Define this macro if the system header files do not support C++@.
+This macro handles system header files by pretending that system
+header files are enclosed in @samp{extern "C" @{@dots{}@}}.
 @end defmac
 
 @findex #pragma
Index: system.h
===================================================================
--- system.h	(revision 262020)
+++ system.h	(working copy)
@@ -1025,7 +1025,7 @@ extern void fancy_abort (const char *, i
 	LIBGCC2_LONG_DOUBLE_TYPE_SIZE STRUCT_VALUE			   \
 	EH_FRAME_IN_DATA_SECTION TARGET_FLT_EVAL_METHOD_NON_DEFAULT	   \
 	JCR_SECTION_NAME TARGET_USE_JCR_SECTION SDB_DEBUGGING_INFO	   \
-	SDB_DEBUG
+	SDB_DEBUG NO_IMPLICIT_EXTERN_C
 
 /* Hooks that are no longer used.  */
  #pragma GCC poison LANG_HOOKS_FUNCTION_MARK LANG_HOOKS_FUNCTION_FREE	\
