HAVE_CPP_ATTRIBUTE_NODISCARD is always true now

...but for safety, leave the configure.ac check in for some longer.

Also, save removing now-redundant SAL_WARN_UNUSED_RESULT in internal code for a
follow-up commit.

Change-Id: Ibe30b51c5cc4abc270f955c7c40b59f268986672
Reviewed-on: https://gerrit.libreoffice.org/64771
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

config_host/config_global.h.in

@@ -21,9 +21,6 @@ Any change in this header will cause a rebuild of almost everything.
 /* Compiler supports __attribute__((warn_unused)). */
 #define HAVE_GCC_ATTRIBUTE_WARN_UNUSED 0
 
-/* [[nodiscard]] (C++17), __has_cpp_attribute(nodiscard) (C++2a): */
-#define HAVE_CPP_ATTRIBUTE_NODISCARD 0
-
 /* Guaranteed copy elision (C++17), __cpp_guaranteed_copy_elision (C++2a): */
 #define HAVE_CPP_GUARANTEED_COPY_ELISION 0
 

configure.ac

@@ -6447,7 +6447,8 @@ AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
 CXXFLAGS=$save_CXXFLAGS
 AC_LANG_POP([C++])
 
-AC_MSG_CHECKING([[whether $CXX supports [[nodiscard]]]])
+dnl This check can eventually be removed completely (e.g., after libreoffice-6-3 branch off):
+AC_MSG_CHECKING([[that $CXX supports [[nodiscard]]]])
 AC_LANG_PUSH([C++])
 save_CXXFLAGS=$CXXFLAGS
 CXXFLAGS="$CXXFLAGS $CXXFLAGS_CXX11"
@@ -6467,10 +6468,8 @@ AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
         #else
         [[nodiscard]] int f();
         #endif
-    ]])], [
-        AC_DEFINE([HAVE_CPP_ATTRIBUTE_NODISCARD],[1])
-        AC_MSG_RESULT([yes])
-    ], [AC_MSG_RESULT([no])])
+    ]])],
+    AC_MSG_RESULT([yes]), AC_MSG_ERROR([$CXX lacks required [[nodiscard]] support]))
 CXXFLAGS=$save_CXXFLAGS
 AC_LANG_POP([C++])
 

include/sal/types.h

@@ -292,7 +292,7 @@ typedef void *                   sal_Handle;
     Compilers that support a construct of this nature will emit a compile
     time warning on unchecked return value.
 */
-#if defined __cplusplus && HAVE_CPP_ATTRIBUTE_NODISCARD
+#if defined LIBO_INTERNAL_ONLY && defined __cplusplus
 #define SAL_WARN_UNUSED_RESULT [[nodiscard]]
 #elif (defined __GNUC__ \
      && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 1))) \