[PATCH] Fix unused private field warning in stdexcept after r207695

[Dimitry Andric]

Hi joerg, mclow.lists,

As I mentioned on IRC, r207695 leads to a warning when compiling <stdexcept> (or any other header that includes it), if you use -Wsystem-headers, like we do in FreeBSD:

In file included from test-stdexcept.cpp:1:
In file included from include/array:109:
include/stdexcept:56:17: warning: private field '__imp_' is not used [-Wunused-private-field]
    const char *__imp_;
                ^
1 warning generated.

Apart from just including <__refstring> in stdexcept, which is apparently too costly, the fix with lowest impact would probably be to add __attribute__((__unused__)) to the __imp_ field.

However, there isn't any _LIBCPP_UNUSED define in __config yet, so that would have to be added, like in this proposed change.

Somebody who knows about Microsoft C++ and IBM C++ should fill in the unused attribute syntax appropriate for those compilers, if there is any.

http://reviews.llvm.org/D6836

Files:
  include/__config
  include/stdexcept

Index: include/__config
===================================================================
--- include/__config
+++ include/__config
@@ -259,6 +259,8 @@
 #  define _LIBCPP_NORETURN __attribute__ ((noreturn))
 #endif
 
+#define _LIBCPP_UNUSED __attribute__((__unused__))
+
 #if !(__has_feature(cxx_defaulted_functions))
 #define _LIBCPP_HAS_NO_DEFAULTED_FUNCTIONS
 #endif  // !(__has_feature(cxx_defaulted_functions))
@@ -381,6 +383,8 @@
 
 #define _LIBCPP_NORETURN __attribute__((noreturn))
 
+#define _LIBCPP_UNUSED __attribute__((__unused__))
+
 #if _GNUC_VER >= 407
 #define _LIBCPP_UNDERLYING_TYPE(T) __underlying_type(T)
 #define _LIBCPP_IS_LITERAL(T) __is_literal_type(T)
@@ -478,6 +482,7 @@
 #define _LIBCPP_HAS_NO_DEFAULTED_FUNCTIONS
 #define __alignof__ __alignof
 #define _LIBCPP_NORETURN __declspec(noreturn)
+#define _LIBCPP_UNUSED
 #define _ALIGNAS(x) __declspec(align(x))
 #define _LIBCPP_HAS_NO_VARIADICS
 
@@ -501,6 +506,7 @@
 #define _ALIGNAS_TYPE(x) __attribute__((__aligned__(__alignof(x))))
 #define _ATTRIBUTE(x) __attribute__((x))
 #define _LIBCPP_NORETURN __attribute__((noreturn))
+#define _LIBCPP_UNUSED
 
 #define _NOEXCEPT throw()
 #define _NOEXCEPT_(x)
Index: include/stdexcept
===================================================================
--- include/stdexcept
+++ include/stdexcept
@@ -53,7 +53,7 @@
 #ifndef _LIBCPP___REFSTRING
 _LIBCPP_BEGIN_NAMESPACE_STD
 class _LIBCPP_HIDDEN __libcpp_refstring {
-    const char *__imp_;
+    const char *__imp_ _LIBCPP_UNUSED;
 };
 _LIBCPP_END_NAMESPACE_STD
 #endif

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