C++ PATCH to add -Wignored-attributes (PR c++/69884)

[Marek Polacek <polacek at redhat dot com>]

This PR is a request to add a way how to suppress the warning about
ignored attributes.  We concluded that -Wattributes isn't the right
warning for this case, so I went ahead with Richi's suggestion to
add -Wignored-attributes.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2016-03-23  Marek Polacek  <polacek@redhat.com>

	PR c++/69884
	* c.opt (Wignored-attributes): New option.

	* pt.c (canonicalize_type_argument): Use OPT_Wignored_attributes.

	* doc/invoke.texi: Document -Wignored-attributes.

	* g++.dg/warn/Wignored-attributes-1.C: New test.
	* g++.dg/warn/Wignored-attributes-2.C: New test.

diff --git gcc/c-family/c.opt gcc/c-family/c.opt
index 7c5f6c7..4f86876 100644
--- gcc/c-family/c.opt
+++ gcc/c-family/c.opt
@@ -482,6 +482,10 @@ Wignored-qualifiers
 C C++ Var(warn_ignored_qualifiers) Warning EnabledBy(Wextra)
 Warn whenever type qualifiers are ignored.
 
+Wignored-attributes
+C C++ Var(warn_ignored_attributes) Init(1) Warning
+Warn whenever attributes are ignored.
+
 Wincompatible-pointer-types
 C ObjC Var(warn_incompatible_pointer_types) Init(1) Warning
 Warn when there is a conversion between pointers that have incompatible types.
diff --git gcc/cp/pt.c gcc/cp/pt.c
index 45cd1ea..6837438 100644
--- gcc/cp/pt.c
+++ gcc/cp/pt.c
@@ -6950,7 +6950,8 @@ canonicalize_type_argument (tree arg, tsubst_flags_t complain)
   tree canon = strip_typedefs (arg, &removed_attributes);
   if (removed_attributes
       && (complain & tf_warning))
-    warning (0, "ignoring attributes on template argument %qT", arg);
+    warning (OPT_Wignored_attributes,
+	     "ignoring attributes on template argument %qT", arg);
   return canon;
 }
 
diff --git gcc/doc/invoke.texi gcc/doc/invoke.texi
index 99ac11b..9e54bb7 100644
--- gcc/doc/invoke.texi
+++ gcc/doc/invoke.texi
@@ -267,7 +267,7 @@ Objective-C and Objective-C++ Dialects}.
 -Wno-format-contains-nul -Wno-format-extra-args -Wformat-nonliteral @gol
 -Wformat-security  -Wformat-signedness  -Wformat-y2k -Wframe-address @gol
 -Wframe-larger-than=@var{len} -Wno-free-nonheap-object -Wjump-misses-init @gol
--Wignored-qualifiers  -Wincompatible-pointer-types @gol
+-Wignored-qualifiers  -Wignored-attributes  -Wincompatible-pointer-types @gol
 -Wimplicit  -Wimplicit-function-declaration  -Wimplicit-int @gol
 -Winit-self  -Winline  -Wno-int-conversion @gol
 -Wno-int-to-pointer-cast -Winvalid-memory-model -Wno-invalid-offsetof @gol
@@ -3886,6 +3886,14 @@ even without this option.
 
 This warning is also enabled by @option{-Wextra}.
 
+@item -Wignored-attributes @r{(C and C++ only)}
+@opindex Wignored-attributes
+@opindex Wno-ignored-attributes
+Warn when an attribute is ignored.  This is different from the
+@option{-Wattributes} option in that it warns whenever the compiler decides
+to drop an attribute, not that the attribute is either unknown, used in a
+wrong place, etc.  This warning is enabled by default.
+
 @item -Wmain
 @opindex Wmain
 @opindex Wno-main
diff --git gcc/testsuite/g++.dg/warn/Wignored-attributes-1.C gcc/testsuite/g++.dg/warn/Wignored-attributes-1.C
index e69de29..46ccc4a 100644
--- gcc/testsuite/g++.dg/warn/Wignored-attributes-1.C
+++ gcc/testsuite/g++.dg/warn/Wignored-attributes-1.C
@@ -0,0 +1,6 @@
+// PR c++/69884
+// { dg-do compile }
+
+typedef float __m128 __attribute__((__vector_size__(16), __may_alias__));
+template <typename> struct A;
+template <> struct A<__m128>; // { dg-warning "ignoring attributes on template argument" }
diff --git gcc/testsuite/g++.dg/warn/Wignored-attributes-2.C gcc/testsuite/g++.dg/warn/Wignored-attributes-2.C
index e69de29..6431607 100644
--- gcc/testsuite/g++.dg/warn/Wignored-attributes-2.C
+++ gcc/testsuite/g++.dg/warn/Wignored-attributes-2.C
@@ -0,0 +1,7 @@
+// PR c++/69884
+// { dg-do compile }
+// { dg-options "-Wno-ignored-attributes" }
+
+typedef float __m128 __attribute__((__vector_size__(16), __may_alias__));
+template <typename> struct A;
+template <> struct A<__m128>;

	Marek