[libcxx] Mark do_open, do_get and do_close parameters unused when catopen is missing

[phosek]

Summary

When catopen is missing, do_open, do_get and do_close end up being
no-op, and as such their parameters will be unused which triggers a
warning when building with -Wunused-parameter.

[mclow.lists]

libcxx/include/locale

3565 ↗

(On Diff #179351)

Looking at this, I frowned. It seems like a lot of text just to remove a parameter name.

I went looking in the rest of libc++, and I found places in <unordered_map> where we say ((void)__p) and in <stdexcept> where we say ((void)__msg) rather than conditionally remove the parameter name.

This makes me think that a _LIBCPP_UNUSED_VAR macro might be useful; it would make it clear what we're trying to accomplish: #define _LIBCPP_UNUSED_VAR(x) ((void)(x))

[mclow.lists]

I like this much better now; making a note to myself to go back to <unordered_map> and <stdexcept> and use this there.

libcxx/trunk/include/__config

 #    if defined(_DLL)
 #      pragma comment(lib, "c++.lib")
 #    else
 #      pragma comment(lib, "libc++.lib")
 #    endif
 #  endif // defined(_LIBCPP_ABI_MICROSOFT) && !defined(_LIBCPP_BUILDING_LIBRARY)
 #endif // _LIBCPP_NO_AUTO_LINK
 
+#define _LIBCPP_UNUSED_VAR(x) ((void)(x))
+
 #endif // __cplusplus
 
 #endif // _LIBCPP_CONFIG

libcxx/trunk/include/locale

 messages<_CharT>::do_open(const basic_string<char>& __nm, const locale&) const
 {
 #ifdef _LIBCPP_HAS_CATOPEN
     catalog __cat = (catalog)catopen(__nm.c_str(), NL_CAT_LOCALE);
     if (__cat != -1)
         __cat = static_cast<catalog>((static_cast<size_t>(__cat) >> 1));
     return __cat;
 #else // !_LIBCPP_HAS_CATOPEN
+    _LIBCPP_UNUSED_VAR(__nm);
     return -1;
 #endif // _LIBCPP_HAS_CATOPEN
 }
 
 template <class _CharT>
 typename messages<_CharT>::string_type
 messages<_CharT>::do_get(catalog __c, int __set, int __msgid,
                          const string_type& __dflt) const
 {
 #ifdef _LIBCPP_HAS_CATOPEN
     string __ndflt;
     __narrow_to_utf8<sizeof(char_type)*__CHAR_BIT__>()(back_inserter(__ndflt),
                                                        __dflt.c_str(),
                                                        __dflt.c_str() + __dflt.size());
     if (__c != -1)
         __c <<= 1;
     nl_catd __cat = (nl_catd)__c;
     char* __n = catgets(__cat, __set, __msgid, __ndflt.c_str());
     string_type __w;
     __widen_from_utf8<sizeof(char_type)*__CHAR_BIT__>()(back_inserter(__w),
                                                         __n, __n + strlen(__n));
     return __w;
 #else // !_LIBCPP_HAS_CATOPEN
+    _LIBCPP_UNUSED_VAR(__c);
+    _LIBCPP_UNUSED_VAR(__set);
+    _LIBCPP_UNUSED_VAR(__msgid);
     return __dflt;
 #endif // _LIBCPP_HAS_CATOPEN
 }
 
 template <class _CharT>
 void
 messages<_CharT>::do_close(catalog __c) const
 {
 #ifdef _LIBCPP_HAS_CATOPEN
     if (__c != -1)
         __c <<= 1;
     nl_catd __cat = (nl_catd)__c;
     catclose(__cat);
+#else // !_LIBCPP_HAS_CATOPEN
+    _LIBCPP_UNUSED_VAR(__c);
 #endif // _LIBCPP_HAS_CATOPEN
 }
 
 _LIBCPP_EXTERN_TEMPLATE2(class _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS messages<char>)
 _LIBCPP_EXTERN_TEMPLATE2(class _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS messages<wchar_t>)
 
 template <class _CharT>
 class _LIBCPP_TEMPLATE_VIS messages_byname