Drop HAVE_GCC_ATTRIBUTE_WARN_UNUSED_STL

For one, loplugin:unusedvariablecheck does not merely check for unused variables
with types from the standard library since
fe2164949b "teach unusedvariablecheck plugin about
SfxPoolItem subclasses", so disabling loplugin:unusedvariablecheck based on
HAVE_GCC_ATTRIBUTE_WARN_UNUSED_STL is wrong.

For another, I have seen no standard library implementation that decorates its
types with such "warn-if-unused" attributes, and
<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0600r0.pdf>
"[[nodiscard]] in the Library" (which proposes to add the corresponding C++17
attribute to just a few select functions and no types at all) makes it appear
unlikely that will happen.

Change-Id: I0a7759e1caf3e3137057c9689080948a4d6747e0

compilerplugins/clang/unusedvariablecheck.cxx

@@ -11,11 +11,6 @@
 
 #include <config_global.h>
 
-// If there is support for warn_unused attribute even in STL classes, then there's
-// no point in having this check enabled, otherwise keep it at least for STL
-// (LO classes won't get duplicated warnings, as the attribute is different).
-#if !HAVE_GCC_ATTRIBUTE_WARN_UNUSED_STL
-
 #include "compat.hxx"
 #include "check.hxx"
 #include "unusedvariablecheck.hxx"
@@ -81,6 +76,4 @@ static Plugin::Registration< UnusedVariableCheck > X( "unusedvariablecheck" );
 
 } // namespace
 
-#endif
-
 /* vim:set shiftwidth=4 softtabstop=4 expandtab: */

config_host/config_global.h.in

@@ -22,7 +22,5 @@ Any change in this header will cause a rebuild of almost everything.
 #define HAVE_SYSLOG_H 0
 /* Compiler supports __attribute__((warn_unused)). */
 #define HAVE_GCC_ATTRIBUTE_WARN_UNUSED 0
-/* C++ library uses __attribute__((warn_unused)) for basic types like std::string. */
-#define HAVE_GCC_ATTRIBUTE_WARN_UNUSED_STL 0
 
 #endif

configure.ac

@@ -5904,21 +5904,6 @@ if test "$GCC" = "yes" -o "$COM_IS_CLANG" = TRUE; then
         ], [AC_MSG_RESULT([no])])
     AC_LANG_POP([C++])
 
-    AC_MSG_CHECKING([whether STL uses __attribute__((warn_unused))])
-    AC_LANG_PUSH([C++])
-    save_CXXFLAGS=$CXXFLAGS
-    CXXFLAGS="$CXXFLAGS -Werror -Wunused"
-    AC_COMPILE_IFELSE([AC_LANG_SOURCE([
-            #include <string>
-            void f() { std::string s; }
-        ])], [
-            AC_MSG_RESULT([no])
-        ], [
-            AC_DEFINE([HAVE_GCC_ATTRIBUTE_WARN_UNUSED_STL],[1])
-            AC_MSG_RESULT([yes])])
-    CXXFLAGS=$save_CXXFLAGS
-    AC_LANG_POP([C++])
-
     AC_MSG_CHECKING([whether $CXX supports __attribute__((warn_unused))])
     AC_LANG_PUSH([C++])
     save_CXXFLAGS=$CXXFLAGS