adjust for upstreaming of warn_unused attribute

The warn_unused attribute has been upstream to GCC and Clang, so use it if present.
Still warn about STL types if those do not use it yet (which is the status as of now).

Change-Id: I3c003e44c08d1d141e23bba38cf92e663a5ac353

4 files changed, 44 insertions, 3 deletions

diff --git a/compilerplugins/clang/unusedvariablecheck.cxx b/compilerplugins/clang/unusedvariablecheck.cxx
index a0763ac2f11d..86f405db0f7c 100644
--- a/compilerplugins/clang/unusedvariablecheck.cxx
+++ b/compilerplugins/clang/unusedvariablecheck.cxx
@@ -8,6 +8,13 @@
  *
  */
 
+#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 "unusedvariablecheck.hxx"
 
 #include <clang/AST/Attr.h>
@@ -101,3 +108,5 @@ bool UnusedVariableCheck::VisitVarDecl( const VarDecl* var )
 static Plugin::Registration< UnusedVariableCheck > X( "unusedvariablecheck" );
 
 } // namespace
+
+#endif
diff --git a/config_host/config_global.h.in b/config_host/config_global.h.in
index 9b1b12a97c63..31f64e6ed54d 100644
--- a/config_host/config_global.h.in
+++ b/config_host/config_global.h.in
@@ -23,5 +23,9 @@ Any change in this header will cause a rebuild of almost everything.
 #define HAVE_GCC_PRAGMA_DIAGNOSTIC_SCOPE 0
 #define HAVE_THREADSAFE_STATICS 0
 #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
diff --git a/configure.ac b/configure.ac
index add4a6bbd8c8..1147e4dddf37 100644
--- a/configure.ac
+++ b/configure.ac
@@ -5668,6 +5668,34 @@ if test "$GCC" = "yes"; then
             AC_MSG_RESULT([yes])
         ], [AC_MSG_RESULT([no])])
     AC_LANG_POP([C++])
+
+    AC_MSG_CHECKING([whether $CXX supports __attribute__((warn_unused))])
+    AC_LANG_PUSH([C++])
+    save_CXXFLAGS=$CXXFLAGS
+    CXXFLAGS="$CFLAGS -Werror -Wunknown-pragmas"
+    AC_COMPILE_IFELSE([AC_LANG_SOURCE([
+            struct __attribute__((warn_unused)) dummy {};
+        ])], [
+            AC_DEFINE([HAVE_GCC_ATTRIBUTE_WARN_UNUSED],[1])
+            AC_MSG_RESULT([yes])
+        ], [AC_MSG_RESULT([no])])
+    CXXFLAGS=$save_CXXFLAGS
+    AC_LANG_POP([C++])
+
+    AC_MSG_CHECKING([whether STL uses __attribute__((warn_unused))])
+    AC_LANG_PUSH([C++])
+    save_CXXFLAGS=$CXXFLAGS
+    CXXFLAGS="$CFLAGS -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++])
 fi
 
 AC_SUBST(HAVE_GCC_NO_LONG_DOUBLE)
diff --git a/include/sal/types.h b/include/sal/types.h
index 070a3f29474a..9ce2cef8aa19 100644
--- a/include/sal/types.h
+++ b/include/sal/types.h
@@ -551,13 +551,13 @@ template< typename T1, typename T2 > inline T1 static_int_cast(T2 n) {
  or external constructors or destructors. Classes marked with SAL_WARN_UNUSED
  will be warned about.
 
- Currently implemented by a Clang compiler plugin.
-
  @since LibreOffice 4.0
 
 */
 
-#if defined __clang__
+#if HAVE_GCC_ATTRIBUTE_WARN_UNUSED
+#define SAL_WARN_UNUSED __attribute__((warn_unused))
+#elif defined __clang__
 #define SAL_WARN_UNUSED __attribute__((annotate("lo_warn_unused")))
 #else
 #define SAL_WARN_UNUSED