GCC only supports inline variables since GCC 7

(see also discussion at <https://gerrit.libreoffice.org/#/c/59204/11> "new
loplugin:conststringfield" about its changes to registry/source/regimpl.cxx)

Change-Id: Id2743adbfeb4d7c42105a65ba8400d7051da2f03
Reviewed-on: https://gerrit.libreoffice.org/59873
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

config_host/config_global.h.in

@@ -26,6 +26,9 @@ Any change in this header will cause a rebuild of almost everything.
 /* Guaranteed copy elision (C++17), __cpp_guaranteed_copy_elision (C++2a): */
 #define HAVE_CPP_GUARANTEED_COPY_ELISION 0
 
+/* Inline variables (C++17), __cpp_inline_variables (C++2a): */
+#define HAVE_CPP_INLINE_VARIABLES 0
+
 /* Fix for <http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1579> "Return by converting
    move constructor": */
 #define HAVE_CXX_CWG1579_FIX 0

configure.ac

@@ -6429,6 +6429,25 @@ AC_COMPILE_IFELSE([AC_LANG_SOURCE([
 CXXFLAGS=$save_CXXFLAGS
 AC_LANG_POP([C++])
 
+AC_MSG_CHECKING([whether $CXX supports inline variables])
+AC_LANG_PUSH([C++])
+save_CXXFLAGS=$CXXFLAGS
+CXXFLAGS="$CXXFLAGS $CXXFLAGS_CXX11"
+AC_LINK_IFELSE([AC_LANG_PROGRAM([[
+        #if !defined __cpp_inline_variables
+        // This tests for one specific aspect of inline variables that is actually used by by
+        // ORegistry::ROOT (registry/source/regimpl.cxx):
+        struct S { constexpr S() {} };
+        struct T { static constexpr S s{}; };
+        S const * f() { return &T::s; }
+        #endif
+    ]])], [
+        AC_DEFINE([HAVE_CPP_INLINE_VARIABLES],[1])
+        AC_MSG_RESULT([yes])
+    ], [AC_MSG_RESULT([no])])
+CXXFLAGS=$save_CXXFLAGS
+AC_LANG_POP([C++])
+
 AC_MSG_CHECKING([whether $CXX has a fix for CWG1579])
 AC_LANG_PUSH([C++])
 save_CXXFLAGS=$CXXFLAGS

registry/source/regimpl.cxx

@@ -41,6 +41,7 @@
 #include "reflcnst.hxx"
 #include "keyimpl.hxx"
 
+#include <config_global.h>
 #include <osl/thread.h>
 #include <rtl/alloc.h>
 #include <rtl/ustring.hxx>
@@ -420,7 +421,7 @@ void dumpType(typereg::Reader const & reader, OString const & indent) {
 
 }
 
-#if __cplusplus <= 201402
+#if !HAVE_CPP_INLINE_VARIABLES
 constexpr OUStringLiteral ORegistry::ROOT;
 #endif
 

sw/source/filter/html/svxcss1.cxx

@@ -20,6 +20,7 @@
 #include <memory>
 #include <stdlib.h>
 
+#include <config_global.h>
 #include <svx/svxids.hrc>
 #include <i18nlangtag/languagetag.hxx>
 #include <svtools/ctrltool.hxx>
@@ -357,7 +358,7 @@ void SvxCSS1BorderInfo::SetBorderLine( SvxBoxItemLine nLine, SvxBoxItem &rBoxIte
     rBoxItem.SetLine( &aBorderLine, nLine );
 }
 
-#if __cplusplus <= 201402
+#if !HAVE_CPP_INLINE_VARIABLES
 constexpr sal_uInt16 SvxCSS1PropertyInfo::UNSET_BORDER_DISTANCE;
 #endif
 

sw/source/filter/xml/xmltbli.cxx

@@ -23,6 +23,7 @@
 #include <com/sun/star/lang/XMultiServiceFactory.hpp>
 #include <com/sun/star/text/XTextTable.hpp>
 #include <com/sun/star/table/XCellRange.hpp>
+#include <config_global.h>
 #include <o3tl/numeric.hxx>
 #include <o3tl/make_unique.hxx>
 #include <o3tl/safeint.hxx>
@@ -1226,7 +1227,7 @@ public:
     }
 };
 
-#if __cplusplus <= 201402 || (defined __GNUC__ && __GNUC__ <= 6 && !defined __clang__)
+#if !HAVE_CPP_INLINE_VARIABLES
 constexpr sal_Int32 SwXMLTableContext::MAX_WIDTH;
 #endif