Index: gcc/c-family/c-opts.c
===================================================================
--- gcc/c-family/c-opts.c	(revision 239276)
+++ gcc/c-family/c-opts.c	(working copy)
@@ -1256,6 +1256,10 @@ sanitize_cpp_opts (void)
   cpp_opts->unsigned_char = !flag_signed_char;
   cpp_opts->stdc_0_in_system_headers = STDC_0_IN_SYSTEM_HEADERS;
 
+  cpp_opts->warn_expansion_to_defined = cpp_warn_expansion_to_defined;
+  if (cpp_warn_expansion_to_defined == -1)
+    cpp_warn_expansion_to_defined = pedantic || extra_warnings;
+
   /* Wlong-long is disabled by default. It is enabled by:
       [-Wpedantic | -Wtraditional] -std=[gnu|c]++98 ; or
       [-Wpedantic | -Wtraditional] -std=non-c99 
Index: gcc/c-family/c.opt
===================================================================
--- gcc/c-family/c.opt	(revision 239276)
+++ gcc/c-family/c.opt	(working copy)
@@ -506,6 +506,10 @@ Wdouble-promotion
 C ObjC C++ ObjC++ Var(warn_double_promotion) Warning
 Warn about implicit conversions from \"float\" to \"double\".
 
+Wexpansion-to-defined
+C ObjC C++ ObjC++ CppReason(CPP_W_EXPANSION_TO_DEFINED) Var(cpp_warn_expansion_to_defined) Init(-1) Warning
+Warn if an undefined macro is used in an #if directive.
+
 Wimplicit-function-declaration
 C ObjC Var(warn_implicit_function_declaration) Init(-1) Warning LangEnabledBy(C ObjC,Wimplicit)
 Warn about implicit function declarations.
Index: gcc/doc/cpp.texi
===================================================================
--- gcc/doc/cpp.texi	(revision 239276)
+++ gcc/doc/cpp.texi	(working copy)
@@ -3342,7 +3342,8 @@ If the @code{defined} operator appears as a result
 the C standard says the behavior is undefined.  GNU cpp treats it as a
 genuine @code{defined} operator and evaluates it normally.  It will warn
 wherever your code uses this feature if you use the command-line option
-@option{-pedantic}, since other compilers may handle it differently.
+@option{-pedantic}, since other compilers may handle it differently.  The
+warning can also be enabled individually with @option{-Wexpansion-to-defined}.
 
 @node Else
 @subsection Else
Index: gcc/doc/cppopts.texi
===================================================================
--- gcc/doc/cppopts.texi	(revision 239276)
+++ gcc/doc/cppopts.texi	(working copy)
@@ -120,6 +120,12 @@ Warn whenever an identifier which is not a macro i
 @samp{#if} directive, outside of @samp{defined}.  Such identifiers are
 replaced with zero.
 
+@item -Wexpansion-to-defined
+@opindex Wexpansion-to-defined
+Warn whenever @samp{defined} is encountered in the expansion of a macro
+(including the case where the macro is expanded by an @samp{#if} directive).
+Such usage is not portable.
+
 @item -Wunused-macros
 @opindex Wunused-macros
 Warn about macros defined in the main file that are unused.  A macro
Index: gcc/doc/invoke.texi
===================================================================
--- gcc/doc/invoke.texi	(revision 239276)
+++ gcc/doc/invoke.texi	(working copy)
@@ -4914,6 +4914,11 @@ Warn when a declaration is found after a statement
 construct, known from C++, was introduced with ISO C99 and is by default
 allowed in GCC@.  It is not supported by ISO C90.  @xref{Mixed Declarations}.
 
+@item -Wexpansion-to-defined
+@opindex Wexpansion-to-defined
+Warn whenever @samp{defined} is encountered in the expansion of a macro.
+This warning is also enabled by @option{-Wpedantic} and @option{-Wextra}.
+
 @item -Wundef
 @opindex Wundef
 @opindex Wno-undef
Index: gcc/system.h
===================================================================
--- gcc/system.h	(revision 239276)
+++ gcc/system.h	(working copy)
@@ -577,16 +577,22 @@ extern int vsnprintf (char *, size_t, const char *
 
 /* 1 if we have C99 designated initializers.  */
 #if !defined(HAVE_DESIGNATED_INITIALIZERS)
+#ifdef __cplusplus
+#define HAVE_DESIGNATED_INITIALIZERS 0
+#else
 #define HAVE_DESIGNATED_INITIALIZERS \
-  (((GCC_VERSION >= 2007) || (__STDC_VERSION__ >= 199901L)) \
-   && !defined(__cplusplus))
+  (((GCC_VERSION >= 2007) || (__STDC_VERSION__ >= 199901L))
 #endif
+#endif
 
 #if !defined(HAVE_DESIGNATED_UNION_INITIALIZERS)
+#ifdef __cplusplus
+#define HAVE_DESIGNATED_UNION_INITIALIZERS (GCC_VERSION >= 4007)
+#else
 #define HAVE_DESIGNATED_UNION_INITIALIZERS \
-  (((GCC_VERSION >= 2007) || (__STDC_VERSION__ >= 199901L)) \
-   && (!defined(__cplusplus) || (GCC_VERSION >= 4007)))
+  (((GCC_VERSION >= 2007) || (__STDC_VERSION__ >= 199901L))
 #endif
+#endif
 
 #if HAVE_SYS_STAT_H
 # include <sys/stat.h>
Index: gcc/testsuite/gcc.dg/cpp/defined-Wexpansion-to-defined.c
===================================================================
--- gcc/testsuite/gcc.dg/cpp/defined-Wexpansion-to-defined.c	(revision 0)
+++ gcc/testsuite/gcc.dg/cpp/defined-Wexpansion-to-defined.c	(working copy)
@@ -0,0 +1,30 @@
+/* Copyright (C) 2000 Free Software Foundation, Inc.  */
+
+/* { dg-do preprocess } */
+/* { dg-options "-Wexpansion-to-defined" } */
+
+/* Use of defined in different contexts.  */
+
+/*  Source: Neil Booth, 29 Oct 2000, Zack Weinberg 11 Dec 2000.  */
+
+#define Z
+
+#define bad0 defined Z
+#if !bad0			/* { dg-warning "may not be portable" } */
+#error Z is defined
+#endif 
+
+#define bad1 defined
+#if !bad1 Z			/* { dg-warning "may not be portable" } */
+#error Z is defined
+#endif 
+
+#if !bad1 (Z)			/* { dg-warning "may not be portable" } */
+#error Z is defined
+#endif 
+
+#define bad2 defined (Z
+#if !bad2)			/* { dg-warning "may not be portable" } */
+#error Z is defined
+#endif 
+
Index: gcc/testsuite/gcc.dg/cpp/defined-Wextra-Wno-expansion-to-defined.c
===================================================================
--- gcc/testsuite/gcc.dg/cpp/defined-Wextra-Wno-expansion-to-defined.c	(revision 0)
+++ gcc/testsuite/gcc.dg/cpp/defined-Wextra-Wno-expansion-to-defined.c	(working copy)
@@ -0,0 +1,30 @@
+/* Copyright (C) 2000 Free Software Foundation, Inc.  */
+
+/* { dg-do preprocess } */
+/* { dg-options "-Wextra -Wno-expansion-to-defined" } */
+
+/* Use of defined in different contexts.  */
+
+/*  Source: Neil Booth, 29 Oct 2000, Zack Weinberg 11 Dec 2000.  */
+
+#define Z
+
+#define bad0 defined Z
+#if !bad0                       /* { dg-bogus "may not be portable" } */
+#error Z is defined
+#endif
+
+#define bad1 defined
+#if !bad1 Z			/* { dg-bogus "may not be portable" } */
+#error Z is defined
+#endif 
+
+#if !bad1 (Z)			/* { dg-bogus "may not be portable" } */
+#error Z is defined
+#endif 
+
+#define bad2 defined (Z
+#if !bad2)			/* { dg-bogus "may not be portable" } */
+#error Z is defined
+#endif 
+
Index: gcc/testsuite/gcc.dg/cpp/defined-Wextra.c
===================================================================
--- gcc/testsuite/gcc.dg/cpp/defined-Wextra.c	(revision 0)
+++ gcc/testsuite/gcc.dg/cpp/defined-Wextra.c	(working copy)
@@ -0,0 +1,30 @@
+/* Copyright (C) 2000 Free Software Foundation, Inc.  */
+
+/* { dg-do preprocess } */
+/* { dg-options "-Wextra" } */
+
+/* Use of defined in different contexts.  */
+
+/*  Source: Neil Booth, 29 Oct 2000, Zack Weinberg 11 Dec 2000.  */
+
+#define Z
+
+#define bad0 defined Z
+#if !bad0                       /* { dg-warning "may not be portable" } */
+#error Z is defined
+#endif
+
+#define bad1 defined
+#if !bad1 Z			/* { dg-warning "may not be portable" } */
+#error Z is defined
+#endif 
+
+#if !bad1 (Z)			/* { dg-warning "may not be portable" } */
+#error Z is defined
+#endif 
+
+#define bad2 defined (Z
+#if !bad2)			/* { dg-warning "may not be portable" } */
+#error Z is defined
+#endif 
+
Index: gcc/testsuite/gcc.dg/cpp/defined-Wno-expansion-to-defined.c
===================================================================
--- gcc/testsuite/gcc.dg/cpp/defined-Wno-expansion-to-defined.c	(revision 0)
+++ gcc/testsuite/gcc.dg/cpp/defined-Wno-expansion-to-defined.c	(working copy)
@@ -0,0 +1,30 @@
+/* Copyright (C) 2000 Free Software Foundation, Inc.  */
+
+/* { dg-do preprocess } */
+/* { dg-options "-ansi -pedantic-errors -Wno-expansion-to-defined" } */
+
+/* Use of defined in different contexts.  */
+
+/*  Source: Neil Booth, 29 Oct 2000, Zack Weinberg 11 Dec 2000.  */
+
+#define Z
+
+#define bad0 defined Z
+#if !bad0                       /* { dg-bogus "may not be portable" } */
+#error Z is defined
+#endif
+
+#define bad1 defined
+#if !bad1 Z			/* { dg-bogus "may not be portable" } */
+#error Z is defined
+#endif 
+
+#if !bad1 (Z)			/* { dg-bogus "may not be portable" } */
+#error Z is defined
+#endif 
+
+#define bad2 defined (Z
+#if !bad2)			/* { dg-bogus "may not be portable" } */
+#error Z is defined
+#endif 
+
Index: gcc/testsuite/gcc.dg/cpp/defined.c
===================================================================
--- gcc/testsuite/gcc.dg/cpp/defined.c	(revision 239276)
+++ gcc/testsuite/gcc.dg/cpp/defined.c	(working copy)
@@ -21,7 +21,7 @@
 
 /* The behavior of "defined" when it comes from a macro expansion is
    now documented.  */
-#if is_Z_defined
+#if is_Z_defined                /* { dg-warning "may not be portable" } */
 #error Macro expanding into defined operator test 1
 #endif
 
@@ -31,7 +31,7 @@
 #error Z is defined
 #endif
 
-#if !is_Z_defined
+#if !is_Z_defined               /* { dg-warning "may not be portable" } */
 #error Macro expanding into defined operator test 2
 #endif
 
@@ -53,7 +53,7 @@
 
 /* The behavior of "defined" when it comes from a macro expansion is
    now documented.  */
-#if is_Z_defined
+#if is_Z_defined                /* { dg-warning "may not be portable" } */
 #error Macro expanding into defined operator test 1
 #endif
 
@@ -63,7 +63,7 @@
 #error Z is defined
 #endif
 
-#if !is_Z_defined
+#if !is_Z_defined               /* { dg-warning "may not be portable" } */
 #error Macro expanding into defined operator test 2
 #endif
 
Index: libcpp/expr.c
===================================================================
--- libcpp/expr.c	(revision 239276)
+++ libcpp/expr.c	(working copy)
@@ -947,9 +947,11 @@ parse_defined (cpp_reader *pfile)
 
   if (node)
     {
-      if (pfile->context != initial_context && CPP_PEDANTIC (pfile))
-	cpp_error (pfile, CPP_DL_WARNING,
-		   "this use of \"defined\" may not be portable");
+      if ((pfile->context != initial_context
+	   || initial_context != &pfile->base_context)
+	  && CPP_OPTION (pfile, warn_expansion_to_defined))
+        cpp_warning (pfile, CPP_W_EXPANSION_TO_DEFINED,
+		     "this use of \"defined\" may not be portable");
 
       _cpp_mark_macro_used (node);
       if (!(node->flags & NODE_USED))
Index: libcpp/include/cpplib.h
===================================================================
--- libcpp/include/cpplib.h	(revision 239276)
+++ libcpp/include/cpplib.h	(working copy)
@@ -410,6 +410,10 @@ struct cpp_options
   /* Nonzero means warn if undefined identifiers are evaluated in an #if.  */
   unsigned char warn_undef;
 
+  /* Nonzero means warn if "defined" is encountered in a place other than
+     an #if.  */
+  unsigned char warn_expansion_to_defined;
+
   /* Nonzero means warn of unused macros from the main file.  */
   unsigned char warn_unused_macros;
 
@@ -1025,7 +1029,8 @@ enum {
   CPP_W_DATE_TIME,
   CPP_W_PEDANTIC,
   CPP_W_C90_C99_COMPAT,
-  CPP_W_CXX11_COMPAT
+  CPP_W_CXX11_COMPAT,
+  CPP_W_EXPANSION_TO_DEFINED
 };
 
 /* Output a diagnostic of some kind.  */
Index: libcpp/system.h
===================================================================
--- libcpp/system.h	(revision 239276)
+++ libcpp/system.h	(working copy)
@@ -375,10 +375,13 @@ extern void abort (void);
    ??? C99 designated initializers are not supported by most C++
    compilers, including G++.  -- gdr, 2005-05-18  */
 #if !defined(HAVE_DESIGNATED_INITIALIZERS)
+#ifdef __cplusplus
+#define HAVE_DESIGNATED_INITIALIZERS 0
+#else
 #define HAVE_DESIGNATED_INITIALIZERS \
-  (!defined(__cplusplus) \
-   && ((GCC_VERSION >= 2007) || (__STDC_VERSION__ >= 199901L)))
+   ((GCC_VERSION >= 2007) || (__STDC_VERSION__ >= 199901L))
 #endif
+#endif
 
 #ifndef offsetof
 #define offsetof(TYPE, MEMBER)	((size_t) &((TYPE *) 0)->MEMBER)
