Re: [C++ Patch] PR 50810

[Jason Merrill <jason at redhat dot com>]

I'm checking in this variant of your patch; mostly it just adds 
-Wno-narrowing to the warning options used to build GCC, though there 
are a few wording tweaks as well.

Tested x86_64-pc-linux-gnu, applying to trunk.
commit 6b5380306428a3e618027b3fc7a319ae2c520b35
Author: Jason Merrill <jason@redhat.com>
Date:   Tue Nov 1 11:56:30 2011 -0400

    	PR c++/50810
    gcc/c-family
    	* c-opts.c (c_common_handle_option): Enable -Wnarrowing as part
    	of -Wall; include -Wnarrowing in -Wc++0x-compat; adjust default
    	Wnarrowing for C++0x and C++98.
    	* c.opt ([Wnarrowing]): Update.
    gcc/cp
    	* typeck2.c (check_narrowing): Adjust OPT_Wnarrowing diagnostics.
    	(digest_init_r): Call check_narrowing irrespective of the C++ dialect.
    	* decl.c (check_initializer): Likewise.
    	* semantics.c (finish_compound_literal): Likewise.
    gcc/
    	* configure.ac: Add -Wno-narrowing to warning options.
    libcpp/
    	* configure.ac: Add -Wno-narrowing to warning options.

diff --git a/gcc/c-family/c-opts.c b/gcc/c-family/c-opts.c
index b56aec7..465bce3 100644
--- a/gcc/c-family/c-opts.c
+++ b/gcc/c-family/c-opts.c
@@ -406,6 +406,7 @@ c_common_handle_option (size_t scode, const char *arg, int value,
 	  warn_reorder = value;
           warn_cxx0x_compat = value;
           warn_delnonvdtor = value;
+	  warn_narrowing = value;
 	}
 
       cpp_opts->warn_trigraphs = value;
@@ -436,6 +437,10 @@ c_common_handle_option (size_t scode, const char *arg, int value,
       cpp_opts->warn_cxx_operator_names = value;
       break;
 
+    case OPT_Wc__0x_compat:
+      warn_narrowing = value;
+      break;
+
     case OPT_Wdeprecated:
       cpp_opts->cpp_warn_deprecated = value;
       break;
@@ -997,10 +1002,17 @@ c_common_post_options (const char **pfilename)
   if (warn_implicit_function_declaration == -1)
     warn_implicit_function_declaration = flag_isoc99;
 
-  /* If we're allowing C++0x constructs, don't warn about C++0x
-     compatibility problems.  */
   if (cxx_dialect == cxx0x)
-    warn_cxx0x_compat = 0;
+    {
+      /* If we're allowing C++0x constructs, don't warn about C++98
+	 identifiers which are keywords in C++0x.  */
+      warn_cxx0x_compat = 0;
+
+      if (warn_narrowing == -1)
+	warn_narrowing = 1;
+    }
+  else if (warn_narrowing == -1)
+    warn_narrowing = 0;
 
   if (flag_preprocess_only)
     {
diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index 438b8b0..0d7dc88 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -494,8 +494,8 @@ C ObjC C++ ObjC++ Warning
 Warn about use of multi-character character constants
 
 Wnarrowing
-C ObjC C++ ObjC++ Warning Var(warn_narrowing) Init(1)
--Wno-narrowing	  In C++0x mode, ignore ill-formed narrowing conversions within { }
+C ObjC C++ ObjC++ Warning Var(warn_narrowing) Init(-1)
+Warn about narrowing conversions within { } that are ill-formed in C++11
 
 Wnested-externs
 C ObjC Var(warn_nested_externs) Warning
diff --git a/gcc/configure b/gcc/configure
index 3b0b39b..35dbdd8 100755
--- a/gcc/configure
+++ b/gcc/configure
@@ -6394,11 +6394,12 @@ fi
 # * 'long long'
 # * variadic macros
 # * overlong strings
+# * C++11 narrowing conversions in { }
 # So, we only use -pedantic if we can disable those warnings.
 
 loose_warn=
 save_CFLAGS="$CFLAGS"
-for option in -W -Wall -Wwrite-strings -Wcast-qual; do
+for option in -W -Wall -Wno-narrowing -Wwrite-strings -Wcast-qual; do
   as_acx_Woption=`$as_echo "acx_cv_prog_cc_warning_$option" | $as_tr_sh`
 
   { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether $CC supports $option" >&5
diff --git a/gcc/configure.ac b/gcc/configure.ac
index dd6cf2f..9196996 100644
--- a/gcc/configure.ac
+++ b/gcc/configure.ac
@@ -329,10 +329,11 @@ GCC_STDINT_TYPES
 # * 'long long'
 # * variadic macros
 # * overlong strings
+# * C++11 narrowing conversions in { }
 # So, we only use -pedantic if we can disable those warnings.
 
 ACX_PROG_CC_WARNING_OPTS(
-	m4_quote(m4_do([-W -Wall -Wwrite-strings -Wcast-qual])), [loose_warn])
+	m4_quote(m4_do([-W -Wall -Wno-narrowing -Wwrite-strings -Wcast-qual])), [loose_warn])
 ACX_PROG_CC_WARNING_OPTS(
 	m4_quote(m4_do([-Wstrict-prototypes -Wmissing-prototypes])),
 	[c_loose_warn])
diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 860556c..edbc783 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -5538,7 +5538,7 @@ check_initializer (tree decl, tree init, int flags, tree *cleanup)
 	  else
 	    {
 	      init = reshape_init (type, init, tf_warning_or_error);
-	      if (cxx_dialect >= cxx0x && SCALAR_TYPE_P (type))
+	      if (SCALAR_TYPE_P (type))
 		check_narrowing (type, init);
 	    }
 	}
diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index d76df51..a80aec6 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -2369,7 +2369,7 @@ finish_compound_literal (tree type, tree compound_literal,
       && check_array_initializer (NULL_TREE, type, compound_literal))
     return error_mark_node;
   compound_literal = reshape_init (type, compound_literal, complain);
-  if (cxx_dialect >= cxx0x && SCALAR_TYPE_P (type)
+  if (SCALAR_TYPE_P (type)
       && !BRACE_ENCLOSED_INITIALIZER_P (compound_literal))
     check_narrowing (type, compound_literal);
   if (TREE_CODE (type) == ARRAY_TYPE
diff --git a/gcc/cp/typeck2.c b/gcc/cp/typeck2.c
index 57cd5e0..1b43449 100644
--- a/gcc/cp/typeck2.c
+++ b/gcc/cp/typeck2.c
@@ -804,8 +804,16 @@ check_narrowing (tree type, tree init)
     }
 
   if (!ok)
-    pedwarn (input_location, OPT_Wnarrowing, "narrowing conversion of %qE "
-	     "from %qT to %qT inside { }", init, ftype, type);
+    {
+      if (cxx_dialect >= cxx0x)
+	pedwarn (EXPR_LOC_OR_HERE (init), OPT_Wnarrowing,
+		 "narrowing conversion of %qE from %qT to %qT inside { }",
+		 init, ftype, type);
+      else
+	warning_at (EXPR_LOC_OR_HERE (init), OPT_Wnarrowing,
+		    "narrowing conversion of %qE from %qT to %qT inside { } "
+		    "is ill-formed in C++11", init, ftype, type);
+    }
 }
 
 /* Process the initializer INIT for a variable of type TYPE, emitting
@@ -902,7 +910,7 @@ digest_init_r (tree type, tree init, bool nested, int flags,
     {
       tree *exp;
 
-      if (cxx_dialect != cxx98 && nested)
+      if (nested)
 	check_narrowing (type, init);
       init = convert_for_initialization (0, type, init, flags,
 					 ICR_INIT, NULL_TREE, 0,
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index b68f607..d6c1faf 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -2363,17 +2363,22 @@ an instance of a derived class through a pointer to a base class if the
 base class does not have a virtual destructor.  This warning is enabled
 by @option{-Wall}.
 
-@item -Wno-narrowing @r{(C++ and Objective-C++ only)}
+@item -Wnarrowing @r{(C++ and Objective-C++ only)}
 @opindex Wnarrowing
 @opindex Wno-narrowing
-With -std=c++11, suppress the diagnostic required by the standard for
-narrowing conversions within @samp{@{ @}}, e.g.
+Warn when a narrowing conversion prohibited by C++11 occurs within
+@samp{@{ @}}, e.g.
 
 @smallexample
 int i = @{ 2.2 @}; // error: narrowing from double to int
 @end smallexample
 
-This flag can be useful for compiling valid C++98 code in C++11 mode.
+This flag is included in @option{-Wall} and @option{-Wc++11-compat}.
+
+With -std=c++11, @option{-Wno-narrowing} suppresses the diagnostic
+required by the standard.  Note that this does not affect the meaning
+of well-formed code; narrowing conversions are still considered
+ill-formed in SFINAE context.
 
 @item -Wnoexcept @r{(C++ and Objective-C++ only)}
 @opindex Wnoexcept
@@ -4062,9 +4067,10 @@ ISO C and ISO C++, e.g.@: request for implicit conversion from
 @code{void *} to a pointer to non-@code{void} type.
 
 @item -Wc++11-compat @r{(C++ and Objective-C++ only)}
-Warn about C++ constructs whose meaning differs between ISO C++ 1998 and
-ISO C++ 2011, e.g., identifiers in ISO C++ 1998 that will become keywords
-in ISO C++ 2011.  This warning is enabled by @option{-Wall}.
+Warn about C++ constructs whose meaning differs between ISO C++ 1998
+and ISO C++ 2011, e.g., identifiers in ISO C++ 1998 that are keywords
+in ISO C++ 2011.  This warning turns on @option{-Wnarrowing} and is
+enabled by @option{-Wall}.
 
 @item -Wcast-qual
 @opindex Wcast-qual
diff --git a/gcc/testsuite/g++.dg/cpp0x/warn_cxx0x2.C b/gcc/testsuite/g++.dg/cpp0x/warn_cxx0x2.C
new file mode 100644
index 0000000..116b233
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/warn_cxx0x2.C
@@ -0,0 +1,4 @@
+// PR c++/50810
+// { dg-options "-std=gnu++98 -Wc++11-compat" }
+
+signed char data[] = { 0xff }; // { dg-warning "narrowing" }
diff --git a/gcc/testsuite/g++.dg/cpp0x/warn_cxx0x3.C b/gcc/testsuite/g++.dg/cpp0x/warn_cxx0x3.C
new file mode 100644
index 0000000..c3df9d9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/warn_cxx0x3.C
@@ -0,0 +1,4 @@
+// PR c++/50810
+// { dg-options "-std=gnu++98 -Wc++11-compat -Wno-narrowing" }
+
+signed char data[] = { 0xff };
diff --git a/libcpp/configure b/libcpp/configure
index c400d23..a26a477 100755
--- a/libcpp/configure
+++ b/libcpp/configure
@@ -4687,7 +4687,7 @@ test -n "$AUTOHEADER" || AUTOHEADER="$MISSING autoheader"
 
 warn=
 save_CFLAGS="$CFLAGS"
-for option in -W -Wall -Wwrite-strings \
+for option in -W -Wall -Wno-narrowing -Wwrite-strings \
 			  -Wmissing-format-attribute; do
   as_acx_Woption=`$as_echo "acx_cv_prog_cc_warning_$option" | $as_tr_sh`
 
diff --git a/libcpp/configure.ac b/libcpp/configure.ac
index e1d8851..e843768 100644
--- a/libcpp/configure.ac
+++ b/libcpp/configure.ac
@@ -33,7 +33,7 @@ AC_CHECK_PROGS([AUTOHEADER], [autoheader], [$MISSING autoheader])
 # Figure out what compiler warnings we can enable.
 # See config/warnings.m4 for details.
 
-ACX_PROG_CC_WARNING_OPTS([-W -Wall -Wwrite-strings \
+ACX_PROG_CC_WARNING_OPTS([-W -Wall -Wno-narrowing -Wwrite-strings \
 			  -Wmissing-format-attribute], [warn])
 ACX_PROG_CC_WARNING_OPTS([-Wstrict-prototypes -Wmissing-prototypes \
 			  -Wold-style-definition -Wc++-compat], [c_warn])