Introduce o3tl::optional as an alias for std::optional

...with a boost::optional fallback for Xcode < 10 (as std::optional is only
available starting with Xcode 10 according to
<https://en.cppreference.com/w/cpp/compiler_support>, and our baseline for iOS
and macOS is still Xcode 9.3 according to README.md).  And mechanically rewrite
all code to use o3tl::optional instead of boost::optional.

One immediate benefit is that disabling -Wmaybe-uninitialized for GCC as per
fed7c3deb3f4ec81f78967c2d7f3c4554398cb9d "Slience bogus
-Werror=maybe-uninitialized" should no longer be necessary (and whose check
happened to no longer trigger for GCC 10 trunk, even though that compiler would
still emit bogus -Wmaybe-uninitialized for uses of boost::optional under
--enable-optimized, which made me ponder whether this switch from
boost::optional to std::optional would be a useful thing to do; I keep that
configure.ac check for now, though, and will only remove it in a follow up
commit).

Another longer-term benefit is that the code is now already in good shape for an
eventual switch to std::optional (a switch we would have done anyway once we no
longer need to support Xcode < 10).

Only desktop/qa/desktop_lib/test_desktop_lib.cxx heavily uses
boost::property_tree::ptree::get_child_optional returning boost::optional, so
let it keep using boost::optional for now.

After a number of preceding commits have paved the way for this change, this
commit is completely mechanical, done with

> git ls-files -z | grep -vz -e '^bin/find-unneeded-includes$' -e '^configure.ac$' -e '^desktop/qa/desktop_lib/test_desktop_lib.cxx$' -e '^dictionaries$' -e '^external/' -e '^helpcontent2$' -e '^include/IwyuFilter_include.yaml$' -e '^sc/IwyuFilter_sc.yaml$' -e '^solenv/gdb/boost/optional.py$' -e '^solenv/vs/LibreOffice.natvis$' -e '^translations$' -e '\.svg$' | xargs -0 sed -i -E -e 's|\<boost(/optional)?/optional\.hpp\>|o3tl/optional.hxx|g' -e 's/\<boost(\s*)::(\s*)(make_)?optional\>/o3tl\1::\2\3optional/g' -e 's/\<boost(\s*)::(\s*)none\>/o3tl\1::\2nullopt/g'

(before committing include/o3tl/optional.hxx, and relying on some GNU features).
It excludes some files where mention of boost::optional et al should apparently
not be changed (and the sub-repo directory stubs).  It turned out that all uses
of boost::none across the code base were in combination with boost::optional, so
had all to be rewritten as o3tl::nullopt.

Change-Id: Ibfd9f4b3d5a8aee6e6eed310b988c4e5ffd8b11b
Reviewed-on: https://gerrit.libreoffice.org/84128
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

2 files changed, 3 insertions, 3 deletions

diff --git a/forms/inc/pch/precompiled_frm.hxx b/forms/inc/pch/precompiled_frm.hxx
index ed407615a977..d68969d9d794 100644
--- a/forms/inc/pch/precompiled_frm.hxx
+++ b/forms/inc/pch/precompiled_frm.hxx
@@ -33,7 +33,7 @@
 #include <utility>
 #include <vector>
 #include <boost/lexical_cast.hpp>
-#include <boost/optional.hpp>
+#include <o3tl/optional.hxx>
 #endif // PCH_LEVEL >= 1
 #if PCH_LEVEL >= 2
 #include <osl/diagnose.h>
diff --git a/forms/source/component/ListBox.cxx b/forms/source/component/ListBox.cxx
index 7c51f7c7a1b4..e98c7883f3c9 100644
--- a/forms/source/component/ListBox.cxx
+++ b/forms/source/component/ListBox.cxx
@@ -55,7 +55,7 @@
 #include <unotools/sharedunocomponent.hxx>
 #include <vcl/svapp.hxx>
 
-#include <boost/optional.hpp>
+#include <o3tl/optional.hxx>
 
 #include <algorithm>
 #include <iterator>
@@ -739,7 +739,7 @@ namespace frm
             return;
         }
 
-        ::boost::optional< sal_Int16 > aBoundColumn(boost::none);
+        ::o3tl::optional< sal_Int16 > aBoundColumn(o3tl::nullopt);
         if ( m_aBoundColumn.getValueType().getTypeClass() == TypeClass_SHORT )
         {
             sal_Int16 nBoundColumn( 0 );