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/sc/inc/pch/precompiled_sc.hxx b/sc/inc/pch/precompiled_sc.hxx
index bb380644fcdb..04a20a9c8076 100644
--- a/sc/inc/pch/precompiled_sc.hxx
+++ b/sc/inc/pch/precompiled_sc.hxx
@@ -52,8 +52,8 @@
 #include <vector>
 #include <boost/functional/hash.hpp>
 #include <boost/intrusive_ptr.hpp>
-#include <boost/optional.hpp>
-#include <boost/optional/optional.hpp>
+#include <o3tl/optional.hxx>
+#include <o3tl/optional.hxx>
 #include <boost/property_tree/json_parser.hpp>
 #include <boost/property_tree/ptree.hpp>
 #endif // PCH_LEVEL >= 1
diff --git a/sc/inc/pch/precompiled_scfilt.hxx b/sc/inc/pch/precompiled_scfilt.hxx
index aa18ffafa27a..d13e9f4a854a 100644
--- a/sc/inc/pch/precompiled_scfilt.hxx
+++ b/sc/inc/pch/precompiled_scfilt.hxx
@@ -44,7 +44,7 @@
 #include <unordered_set>
 #include <utility>
 #include <vector>
-#include <boost/optional.hpp>
+#include <o3tl/optional.hxx>
 #include <boost/property_tree/ptree.hpp>
 #endif // PCH_LEVEL >= 1
 #if PCH_LEVEL >= 2